On the Assessment of Induction- Heated Adhesive Bonding

Master of Science Thesis
On the Assessment of InductionHeated Adhesive Bonding
An Experimental Investigation of its Abilities as a
Green, Next-Generation Manufacturing Technique for
the Aerospace Industry
C. Severijns
Faculty of Aerospace Engineering
·
Delft University of Technology
On the Assessment of
Induction-Heated Adhesive Bonding
An Experimental Investigation of its Abilities as a Green,
Next-Generation Manufacturing Technique for the Aerospace
Industry
Master of Science Thesis
For obtaining the degree of Master of Science in Aerospace Engineering
at Delft University of Technology
C. Severijns
29th April 2016
Faculty of Aerospace Engineering · Delft University of Technology
c C. Severijns
Copyright All rights reserved.
Delft University of Technology
Faculty of Aerospace Engineering
Department of Aerospace Structures and Materials
GRADUATION COMMITTEE
Dated: 29th April 2016
Chair holder:
Prof.dr.ir. R.Benedictus
Committee members:
Dr. J.A.Poulis
Dr.ir. S.Teixeira de Freitas
Dr.ir. M.B.Zaaijer
Ir. M.Bosman
Abstract
The autoclave/oven curing process is known to be the current manufacturing technique that
provides the best quality of composite laminates and bonded joints. However, this process is
expensive, because of its big initial investment. It also has a large ecological foot print, since
it involves a high energy consumption. Furthermore, with the current complete composite
fuselages, it is infeasible to use autoclave/oven curing processes to assemble large sections of
an aircraft.
Therefore, new manufacturing solutions must be developed in order to make composites
and composite bonding cost- attractive and feasible to large assembly lines. This research
addresses the challenge to explore an out- of- autoclave process for bonded joints, that it is
cost- attractive, low- energy- consuming and that delivers at least the same product quality
as the current autoclave/oven processes.
This research project focusses on induction heating as a possible out- of- oven curing process
for bonded structures. Ferromagnetic susceptor particles are mixed into the adhesive and
thermal curing is achieved by hysteresis- heating of the particles when the material is exposed
to an electro- magnetic field.
The aim of the project is first to understand which manufacturing parameters are most
important to guarantee a feasible and reliable induction heating curing process. Secondly, the
project investigates the effect of susceptor- assisted induction heating on the curing behaviour,
mechanical performance and energy consumption of adhesively bonded lap- shear joints. At
the end, the project aims to deliver a proof of concept for susceptor- assisted, inductionheated adhesive bonding and to compare the process’ performance against conventional ovencuring.
Firstly, an assessment was made on the heat- generating properties of different susceptor
materials. The conclusion was made that Iron- based particles perform the best as susceptor
material. Secondly, the influence of different induction heating process parameters, such
as field strength, coupling distance and bond layer thickness, was covered. Additionally, the
induction- heating process was modelled in order to understand better the influence of process
parameters which could not be changed on the experimental set- up, such as the magnetic
field’s frequency.
After obtaining the optimized process parameters, lap shear specimens were manufactured to
assess the effect on the mechanical performance. Identical specimens were manufactured using
viii
Abstract
conventional oven- curing. Samples cured by induction showed a slight increase in lap- shear
strength of 6%. Parallel to this, an optimized cure cycle was developed for the adhesive in
scope in order to decrease the processing time as much as possible without damaging the bond
quality. By increasing the curing temperature from 65◦ C (manufacturer recommendation) to
110◦ C, the required curing time of the EC9323 adhesive could be reduced from two hours to
47 minutes, while reducing the adhesive’s lap- shear strength by not more than 5%.
A final benchmark between oven- and induction- curing was performed in terms of energy
consumption, investment cost and processing time. Induction- curing is more energy- efficient
than oven- curing for large- scale applications, which have a relatively small bonded area.
An induction- curing set- up, able to cure surfaces of up to 50 cm2 , has an equal energy
consumption as an oven of approximately 0.8 m3 . The investment cost of an induction
equipment remains however more expensive than that of oven set- ups, even for mediumsized applications. Lastly, the cycle time for the adhesive to obtain a full cure is considered
not to be influenced significantly by the type of curing process used.
Summary
This report shows the major findings of a nine- month experimental project on assessing the
potential of induction- heated adhesive bonding as an alternative out- of- oven/autoclave manufacturing process for the aerospace industry. It consists of four major parts, each focussing
on a different aspect of induction- heated adhesive bonding.
Introduction
Adhesives have been used extensively for structural bonding in aerospace applications [1].
They are incorporated both as manufacturing- and repair strategy because of its advantages
over other joining techniques [2]. Within the manufacturing industry, ecological concerns have
gained some importance over the past years. This trend is also noted for adhesively bonded
joints, a production technique characterized by its high energy consumption, through the
use of oven/autoclave- curing systems. Additional concerns for such systems are their high
acquisition cost and the unavoidable size constraints they imply on the structure’s external dimensions, as it has to fit within the oven/autoclave. Shifting towards out- of- autoclave/oven
processes would result in enormous cost- and energy savings, which would add to the attractiveness of adhesive bonding.
Among several alternative curing methods (E.g. microwave-, ultraviolet- or electron beam
curing), induction heating has been selected as the main focus of this project for several
reasons. First, as an energy transfer mechanism, induction heating is often referred to as an
energy efficient, quick and clean process. Secondly, induction heating equipment is considered
in literature to be more cost- effective than any of the other alternative curing methods
considered [3]. Additionally, only limited research has been focussing yet on induction- heated
adhesive bonding, which provides interesting research opportunities within the context of a
master thesis project.
The principles of induction theory have been established by Michael Faraday [4]. By exposing
a material to an alternating electromagnetic field, heat is generated through either one of
two mechanisms. When working with conductive materials, so- called Eddy- currents are
generated within the material, which generates heat through the Joule heating principle. If
the material possesses ferromagnetic properties, its grains will try to align themselves with
the alternating magnetic field, which generates heat through the hysteresis effect.
x
Summary
As paste adhesives do not show either conductive or magnetic properties, strategies have to
be developed in order to apply induction heating on adhesively bonded joints. When working
with conductive adherends, such as aluminium or carbon fibre reinforced plastic (CFRP),
heat can be generated by induction within the adherend and then conductively transferred to
the bond layer. This is called susceptorless induction heating. If non- conductive adherends
are used however, the adhesive itself has to be modified in order to generate heat by induction,
so- called susceptor- assisted induction heating. A possible strategy in such a case is to mix
ferromagnetic nanoparticles within the adhesive.
As mentioned already before, only limited research has been performed yet on inductionheated adhesive bonding. Most research has only focussed on the effect of different processand material parameters on the induction heating process, and the achievable temperature
by a certain set- up [5] [6] [7]. Non of those projects has however extended the project’s
scope towards assessing the effect of induction heating on the cure behaviour and mechanical
performance of adhesively bonded joints.
Sanchez Cebrian A. and his research group at ETH Zurich have been the first to perform
several experiments on induction- heated adhesive bonding [8] [9] [10] [11]. On one side,
they established experiments to assess the effect of different susceptor particles on inductionheated adhesive bonding of CFRP and GFRP adherends. On the other hand, a separate
study was performed to simulate and test the void formation during induction heating and
to optimize the void elimination by multi- stage curing processes. Initial findings on the
susceptorless CFRP set- up showed a reduction in energy consumption of approximately
25%, compared to conventional oven- curing. No published conclusions were found however
on the susceptor- assisted experiments. The performance of susceptor- assisted inductionheated adhesive bonding in terms of mechanical performance and energy consumption were
therefore identified as a major scientific gap of interest, to be taken into account for this
master thesis.
The purpose of this experimental research project is to develop additional insights in the field
of susceptor- assisted, induction- heated adhesive bonding. The project aims at delivering
a proof of concept for susceptor- assisted, induction- heated adhesive bonding. Finally, advantages, disadvantages and limitations of induction- heated adhesive bonding compared to
conventional oven/autoclave curing will be drawn.
This report is split into five major parts. The first part analyses different susceptor materials, their influence on the induction heating process and the influence of different process
parameters. The second part focusses on modelling the induction heating process. The next
chapter assesses the cure characteristics of the adhesive in scope and proposes an optimized
cure cycle. The fourth part evaluates the mechanical performance of induction- cured adhesives. The last chapter focusses on the actual comparison between oven- cured and inductioncured samples.
xi
Susceptor Analysis
The objective of the first experimental phase was to assess the heat- generating properties of
different materials. Based on literature, several types of Iron-, Nickel- and Graphene- based
susceptor particles have been selected. Tests were performed on 10w% and 2v% samples, at a
coil current of 400 A and a coupling distance of 1 mm. Among all tested susceptor particles,
Iron powder has been found to be the most effective heat- generator, explained by its higher
magnetic permeability than that of Nickel. The Graphene Oxide particles used in this project
did not show any heat- generation at the specified conditions.
Additional tests were performed on susceptorless induction heating of CFRP material. By
having very good conductive properties, induction heating of CFRP was found to be several
times more effective than that of any of the tested susceptor particles. The strength of the
produced magnetic field had to be reduced in order not to overheat the CFRP material. For
the set- up used and the susceptor particles used, hysteresis heating in the susceptor particles
was not triggered yet. The conclusion was therefore made that a combined set- up of CFRP
and susceptor particles could be considered as being infeasible. The decision was made to
proceed with non- conductive glass fibre reinforced plastic (GFRP) adherends for the further
progress of the project.
The effect of major process parameters has also been established in this part of the project.
The coupling distance, coil current and volume- percentage of susceptor particles have a
significant influence on the amount of heat generated. It is a combination of those three
parameters that mainly drives the amount of generated heat. As example, due to limitations
concerning the cooling unit of the induction set- up in the lab, only coil currents of 175 A
can be used on continuous heating processes, in order not to overheat the equipment. This
can be compensated by increasing the amount of susceptor particles added to the adhesive.
An additional parameter is the coil geometry. Both a one- turn coil and a pancake coil have
been evaluated. The pancake coil, due to its higher number of windings, produces a stronger
magnetic field and therefore enhances the heating process of the susceptor- assisted set- up.
Modelling of the Induction Heating Process
In order to understand better the experimentally obtained results, a model was made of
the induction heating process in COMSOL Multiphysics. The build- in induction heating
interface of COMSOL does, however, not take hysteresis losses into account when calculating
the amount of heat generated. An additional partial differential equation (PDE) solver had
therefore to be added, based on the Jiles- Atherton theory. In order not to make the model
too complex, the sample existing out of adhesive and Iron particles was modelled as a single,
uniform material, having homogeneous properties. Properties of the combined material were
taken as a "rule- of- mixture" between the properties of the adhesive and the Iron particles,
with a mixing ratio defined by the volume- percentage of each material.
Validation of the model was performed by comparing the model’s output against experimentally obtained results from the previous chapter. According to those tests, the model’s
accuracy was considered to be within a 15%- range. General trends for each of the varied
parameters were however found to be correct.
xii
Summary
The model was thereafter used to assess the effect of two additional parameters, which could
not be varied on the actual set- up in the lab: the field frequency and the conductivity of
the adhesive. Increasing the field frequency showed an unbound increase in heat generation.
The consideration has to be taken into account however that additional phenomena start to
act due to wave absorption at higher field frequencies. Increasing the adhesive’s conductivity
was found to be a very efficient method to increase its heat- generating characteristics. More
detailed modelling of the induction heating process is however considered to be essential if
further conclusions have to be made.
Adhesive Characterization
In parallel with the suscepor analysis, an evaluation was made of the physical properties of the
adhesive used in this project, EC9323 produced by 3M. A series of TGA and DSC experiments
were performed in order to assess the curing chemistry of the adhesive, its stability at different
curing temperatures and the effect of adding Iron powder.
The EC9323 adhesive was found to be stable up to a curing temperature of 110◦ C, its onset
temperature of degassing. Exceeding this onset temperature would lead to excessive gas
formation, resulting in an increased void content.
DSC experiments were used to assess the cure chemistry of the adhesive. From this analysis,
the conclusion was made that adding Iron powder to the adhesive does not influence the
curing chemistry of the epoxy. A second objective of the DSC experiments was to investigate
the impact of an increased curing temperature on the required process time to reach a fullcure. The cure time could be reduced from two hours, the time required at 65◦ C, down to
only 47 minutes by increasing the curing temperature to 110◦ C.
Mechanical Testing
The mechanical performance of the adhesive was evaluated by single lap- shear tests. The first
test consisted of samples, cured in the oven at different temperatures, in order to complete
the cure cycle assessment started in the previous section of this project.
Curing temperatures exceeding the manufacturer’s recommended cure temperature of 65◦ C by
15◦ C resulted in a significant reduction in mechanical performance of about 15%. Increasing
the curing temperature further, up to the onset temperature of degassing (110◦ C), resulted in
a recovery of the adhesive’s lap- shear strength. Curing at a temperature of 110◦ C for example,
resulted in only a very small decrease in lap- shear strength of 5%, compared to the samples
cured at 65◦ C. Exceeding the onset temperature of degassing resulted in a significant drop of
mechanical performance of approximately 25%, considered not to be acceptable for structural
applications. The conclusion was therefore made that increasing the curing temperature of
EC9323 from 65◦ C to 110◦ C might be beneficial from a process time point of view, while
having no significant impact on the adhesive’s mechanical performance. Unfortunately, it
was not possible to test this cure cycle on the induction set- up in the lab, as the required
coil currents would exceed the cooling equipment’s capacity for a 47- minute heating cycle.
xiii
Secondly, the impact of Iron powder on the lap- shear strength was assessed. An Iron powder
content as low as 0.5v% already resulted in a significant drop in mechanical performance of
about 15%. Additional tests were performed at increased volume- percentages of up to 7.5v%,
which did not result in a significant additional decrease in lap- shear strength.
Lastly, the effect of induction curing was assessed by comparing the lap- shear strength of
oven- and induction- cured samples, both containing 7.5v% of Iron powder. This relatively
high particle content was required in order to achieve a bond layer temperature of 65◦ C at
a coil current as low as 175 A, in order not to overheat the equipment during a two- hour
heating cycle. The induction- cured samples showed a slight increase in lap- shear strength
of approximately 6%, compared to the oven- cured ones.
Induction vs. Oven Benchmark
In a last phase, the project has focussed on comparing induction curing and oven- curing in
terms of energy consumption, processing time and investment cost. The case study used for
this assessment was developed with the experimental data obtained during the production of
single lap- shear joints. The findings of this benchmark are found in table 1.
Table 1: Comparison on the production of single lap- shear joints by oven- and induction
curing
Energy consumption [kWh]
Process time
Investment cost [Euro]
Induction
2.92
2h
24 510
Oven
0.212
2h10’
1 884
Delta
+1350%
-8%
+1300%
As can be seen, induction curing of single lap- shear joints is more energy consuming and more
expensive than oven- curing, while the reduction is processing time is only very moderate.
However, increasing the size of the adherends while keeping the area to be bonded relatively
small can result in the induction process to become more energy efficient. An induction setup able to cure area’s of up to 50 cm2 consumes, for a curing process of two hours at 65◦ C, the
same amount of energy as an oven of approximately 0.8 m3 . The cycle time for the adhesive
to obtain a full cure is considered not to be influenced significantly by the type of curing
process used.
xiv
Summary
Conclusion & Recommendations
This research has lead to some interesting insights on susceptor- assisted, induction- heated
adhesive bonding. The most important findings of this project could be summarized as follows:
• For the set- up and susceptor particles used in this project, combining CFRP- and
susceptor particles in one set- up is not considered to enhance the induction heating
process, as either CFRP material will overheat at higher coil currents, or the susceptor
particles will not contribute at lower current set- ups
• Mixing- in Iron powder does not impact the curing chemistry of the adhesive
• By increasing the curing temperature from 65◦ C (manufacturer recommendation) to
110◦ C, the required curing time of the EC9323 adhesive can be reduced from two hours
to 47 minutes, while having no significant impact on the adhesive’s lap- shear strength
• Adding Iron powder to the adhesive results in a reduction of lap- shear strength by
15%-20%, even at volume- percentages as low as 0.5v%
• Curing from inside the adhesive, as done in susceptor- assisted induction heating, results
in a slight increase in lap- shear strength (6%), compared to oven- cured samples (curing
from the outside) for the same amount of susceptor particles
• The production of standard, single lap- shear joints through induction curing is both
more expensive and more energy consuming than oven- curing. Joints having a bonded
area of less than 50 cm2 , but requiring an oven- volume of more than 0.8 m3 can be
joined more efficiently by induction, resulting in a lower energy consumption
• The limited ability to heat large surfaces at the same time by induction is considered
to be the technique’s major limitation for large- scale applications
• The investment cost for an induction set- up is significantly higher than that of smalland medium scaled ovens of up to 15 m3
The following recommendations are stated for further research in order to extend the knowledge on induction- heated adhesive bonding:
• Evaluate the impact of material properties, such as grain size- and shape, remanence
and coercivity on the induction heating process
• Assess the durability of susceptor- assisted adhesively bonded joints
• Develop a paste adhesive with sufficient conductive properties in order to heat- up the
adhesive through the Joule heating mechanism
• Reducing the required curing time of aerospace- grade adhesives could have a significant
impact on the attractiveness of induction curing for large- scale assemblies
Table of Contents
Abstract
vii
Summary
ix
Preface
xxvii
Acknowledgments
xxix
1 Introduction
1.1 Latest Developments in Adhesive Bonding . . . . . . . . . . . . . . . . . . . . .
1
1
1.2
Alternative, Out- of- Autoclave / Oven- Curing Techniques . . . . . . . . . . . .
2
1.3
1.4
The Induction Heating Process . . . . . . . . . . . . . . . . . . . . . . . . . . .
Project Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
7
2 Susceptor Analysis
2.1 Introduction . . . . . . . . . . . . . .
2.2 Materials & Specimen . . . . . . . . .
2.2.1 Susceptor-Assisted Heating . .
2.2.2 Susceptorless Heating . . . . .
2.3 Experimental Set- Up & Methodology
2.3.1 The Induction System . . . . .
2.3.2 Temperature Measurements . .
2.3.3 Test Plan . . . . . . . . . . .
2.4 Results & Discussion . . . . . . . . .
2.4.1 Susceptor- Assisted Set- Up . .
2.4.2 Susceptorless Set- Up . . . . .
2.5 Preliminary Conclusions . . . . . . . .
2.5.1 Impact on the Further Progress
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of the Project
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3 Modelling of the Induction Heating Process
3.1 Introduction . . . . . . . . . . . . . . . . .
3.2 Development of the Model . . . . . . . . .
3.2.1 Geometrical Aspects . . . . . . . . .
3.2.2 Magnetic Field . . . . . . . . . . . .
3.2.3 Induction Heating . . . . . . . . . .
3.2.4 Heat Transfer . . . . . . . . . . . .
3.2.5 Validation of the Model . . . . . . .
3.3 Results . . . . . . . . . . . . . . . . . . . .
3.3.1 Effect of Field Frequency . . . . . .
3.3.2 Effect of the Adhesive’s Conductivity
3.4 Discussion . . . . . . . . . . . . . . . . . .
3.4.1 Model Accuracy . . . . . . . . . . .
3.4.2 Electrical Conductivity . . . . . . .
3.5 Preliminary Conclusions . . . . . . . . . . .
Table of Contents
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4 Adhesive Characterization
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . .
4.2 Experimental Set- up . . . . . . . . . . . . . . . . . .
4.2.1 Thermogravimetric Analysis (TGA) . . . . . . .
4.2.2 Differential Scanning Calorimetry (DSC) . . . .
4.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1 TGA . . . . . . . . . . . . . . . . . . . . . . .
4.3.2 DSC Analysis . . . . . . . . . . . . . . . . . .
4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . .
4.4.1 Effect of Iron Powder on the Curing Chemistry
4.4.2 Effect of Increasing Cure Temperature . . . . .
4.5 Preliminary Conclusions . . . . . . . . . . . . . . . . .
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5 Mechanical Testing
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Materials & Specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2 Specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Experimental Set- Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1 Test Bench . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.2 Test Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.1 Effect of Cure Temperature . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.2 Effect of Susceptor Particle- Content . . . . . . . . . . . . . . . . . . . .
5.4.3 Effect of Induction Curing . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.1 Effect of Cure Temperature & Induction Heating, a Comparison with Sanchez’
Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.2 Effect of Susceptor Particle- Content . . . . . . . . . . . . . . . . . . . .
5.6 Preliminary Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
63
63
64
64
64
69
69
69
71
72
73
75
76
76
78
80
Table of Contents
xvii
6 Comparison of Oven- vs. Induction Curing
81
6.1
6.2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Energy Consumption Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . .
81
82
6.3
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3.1 Energy Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . .
83
83
6.3.2 Process Time . .
6.3.3 Investment Cost .
Discussion . . . . . . . .
6.4.1 Up- Scaling of the
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86
86
87
87
Preliminary Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
91
6.4
6.5
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Bonding
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Process
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7 Conclusion & Recommendations
93
References
98
References
A Appendix A : Detailed Test-Data of Single Lap- Shear Tests
99
103
A.1 Effect of Cure Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
103
A.2 Effect of Particle Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3 Effect of Induction Curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
104
105
B Appendix B
107
xviii
Table of Contents
List of Figures
1.1
Examples of a magnetization curves and their effect on hysteresis heating . . . .
3
1.2
The heating characteristics of different susceptor materials [6] . . . . . . . . . .
4
1.3
The heating characteristics of different susceptor sizes [6] . . . . . . . . . . . . .
5
1.4
The effect of the applied frequency on the heating rate of CFRP plates [12] . . .
6
2.1
SEM image from a 2v% Fe specimen . . . . . . . . . . . . . . . . . . . . . . . .
14
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
SEM detection of Fe elements in a 2v% Fe specimen
Samples used for the susceptor-particle experiments .
The CFRP samples used for the phase I experiments
A one- turn coil . . . . . . . . . . . . . . . . . . . .
A pancake coil . . . . . . . . . . . . . . . . . . . . .
Test set- up for the phase I experiments . . . . . . .
Cross- sectional sketch of the test set- up . . . . . .
Temperature profiles measured both by the IR camera
14
15
16
18
18
21
21
22
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
and the thermocouple system
2.10 Temperature profiles for 10w% samples, 400 A and 1 mm coupling . . . . . . . .
25
2.11 Temperature profiles for 2v% samples, 400 A and 1 mm coupling . . . . . . . . .
25
2.12 Temperature profiles for 1,2 and 5v% of Fe particles, 400 A and 1 mm coupling .
26
2.13 Temperature profiles for 100,200,300,400 and 500 A, 1 mm coupling and 2v% of
Fe particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
2.14 Temperature profiles for a 1,3,5 and 7 mm coupling distance at 400 A and 2v% of
Fe particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.15 Temperature profiles for a one- turn coil and a pancake coil, 400 A, 1 mm coupling
and 2v% of Fe particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.16 The heat- affected area with a one- turn coil . . . . . . . . . . . . . . . . . . . .
2.17 The heat- affected area with a pancake coil . . . . . . . . . . . . . . . . . . . .
2.18 The heat- affected zone for the CFRP sample exposed under a one- turn coil, 60
A and 5 mm coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
28
29
29
30
xx
List of Figures
2.19 Profiles for maximum and average temperature over the heat- affected zone of the
CFRP sample, 60 A 5 mm coupling . . . . . . . . . . . . . . . . . . . . . . . . .
30
2.20 Temperature profiles for the CFRP under different coil currents, 5mm coupling .
31
2.21 Temperature profiles for the CFRP at different coupling distances, 60 A . . . . .
31
2.22 Temperature profiles for the CFRP with different coil geometries, 60 A and 5 mm
coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
2.23 The heat-affected area with a one-turn coil . . . . . . . . . . . . . . . . . . . .
2.24 The heat-affected area with a pancake coil . . . . . . . . . . . . . . . . . . . . .
32
32
2.25 Test specimen of CFRP material + adhesive . . . . . . . . . . . . . . . . . . . .
33
2.26 Temperature profiles for the adhesive + CFRP set-up, 60 A and 3 mm coupling .
33
2.27 Temperature profiles the susceptor- assisted set- up and bare CFRP material, 100
A and 5 mm coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
3.1
Schematic representation of the induction heating model . . . . . . . . . . . . .
38
3.2
The induction heating set- up as modelled for this project in COMSOL Multiphysics 38
3.3
Temperature profile of the validation test for set- up 1 . . . . . . . . . . . . . .
44
3.4
Model prediction of the effect of the field’s frequency on the induction heating
process, one- turn coil, 400 A and 1 mm coupling distance . . . . . . . . . . . .
45
Model prediction of the effect of electrical conductivity on the induction heating
process, one- turn coil, 400 A and 1 mm coupling distance . . . . . . . . . . . .
46
4.1
Empty DSC test cup (left) including adhesive (middle) sealed (right) . . . . . . .
51
4.2
TGA Analysis for EC9323 adhesive . . . . . . . . . . . . . . . . . . . . . . . . .
53
3.5
4.3
A heatflow graph of a sample cured at
65◦ C
65◦ C
for 2 hours . . . . . . . . . . . . . .
54
4.4
Post-cure cycle, 2 hours at
. . . . . . . . . . . . . . . . . . . . . . . . . .
55
4.5
Initial cure cycle (no full-cure), 45 minutes at 65◦ C . . . . . . . . . . . . . . . .
55
4.6
4.7
Post-cure cycle, 45 minutes at
65◦ C
Isolated cure- energy plot, 2 hours at
65◦ C
. . . . . . . . . . . . . . . . . . . . . . . .
55
65◦ C
. . . . . . . . . . . . . . . . . . . . .
56
4.8
Total cure- energy, 2 hours at
. . . . . . . . . . . . . . . . . . . . . . . . .
57
4.9
Normalized cure- energy, 2 hours at 65◦ C . . . . . . . . . . . . . . . . . . . . .
57
4.10 Normalized cure- energy for sample with 2v% Iron powder, 2 hours at
65◦ C
. . .
4.11 Corrected normalized cure- energy for sample with 2v% Iron powder 2 hours at
4.12 Normalized cure- energy for sample with 2v% Iron powder, 1 hour at
95◦ C
65◦ C
. . .
58
58
59
4.13 Corrected normalized cure- energy for sample with 2v% Iron powder 1 hour at 95◦ C 59
4.14 Normalized cure- energy of adhesive without susceptor particles, for cure temperatures of 65◦ C, 80◦ C, 95◦ C and 110◦ C . . . . . . . . . . . . . . . . . . . . . . .
60
5.1
GFRP specimen used for producing the single lap- shear test coupons . . . . . .
64
5.2
Single lap- shear specimen used for mechanical testing [13] . . . . . . . . . . . .
65
5.3
Mould used for assembling individual single lap- shear specimen . . . . . . . . .
66
5.4
Temperature profile for both the oven- cured and induction- cured samples for two
hours at 65◦ C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
List of Figures
5.5
5.6
5.7
xxi
ZWICK 10kN bench including a test specimen . . . . . . . . . . . . . . . . . . .
Examples of load- displacement curves of the tested specimen . . . . . . . . . .
A fracture surface of a tested specimen, including 0.5v% of Iron powder and ovencured for two hours at 65◦ C . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A fracture surface of a tested specimen without Iron powder, oven- cured for two
hours at 65◦ C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lap- shear strength of specimen, oven- cured at different temperatures . . . . . .
Lap- shear strength of specimen, oven- cured for 2 hours at 65◦ C with different
volume-percentages of Iron- powder . . . . . . . . . . . . . . . . . . . . . . . .
Lap- shear strength of oven- and induction- cured for 2 hours at 65◦ C and 7.5v%
of Iron powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Experimental results of Sanchez’ work on assessing the impact of the curing temperature on lap- shear strength of adhesively bonded joints [10] . . . . . . . . . .
This project’s extended results on assessing the impact of curing temperature on
lap- shear strength of adhesively bonded joints . . . . . . . . . . . . . . . . . . .
SEM image from the fracture surface of a single lap- shear specimen, containing
7.5v% of Iron powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SEM image from the fracture surface of a single lap- shear specimen, containing
7.5v% of Iron powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SEM- picture of pure Iron powder (x200), used as susceptor particle for this project
69
71
The measurement equipment used for the energy consumption monitoring . . . .
Energy consumption of the T6030 oven for heating- up and maintaining 65◦ C . .
Graphical representation of the EasyHeat’s energy consumption at different levels
of coil current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Statistical relation between the oven’s volume and energy consumption . . . . .
Energy consumption estimations for oven- curing in function of the oven’s volume
Energy consumption estimation : Break- even estimation for induction- and ovencuring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
82
83
Load- displacement curves of single lap- shear tests, oven- cured, 2h @ 65◦ C . . .
Load- displacement curves of single lap- shear tests, oven- cured, 1h11’ @ 80◦ C .
Load- displacement curves of single lap- shear tests, oven- cured, 51’ @ 95◦ C . .
Load- displacement curves of single lap- shear tests, oven- cured, 47’ @ 110◦ C . .
Load- displacement curves of single lap- shear tests, oven- cured, 2h @ 65◦ C, 0.5v%
Fe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load- displacement curves of single lap- shear tests, oven- cured, 2h @ 65◦ C, 2v%
Fe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load- displacement curves of single lap- shear tests, oven- cured, 2h @ 65◦ C, 5v%
Fe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load- displacement curves of single lap- shear tests, oven- cured, 2h @ 65◦ C, 7.5v%
Fe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load- displacement curves of single lap- shear tests, induction- cured, 2h @ 65◦ C,
7.5v% Fe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
103
103
104
104
B.1 Temperature profile of the validation test for set- up 2 . . . . . . . . . . . . . .
B.2 Temperature profile of the validation test for set- up 3 . . . . . . . . . . . . . .
107
107
5.8
5.9
5.10
5.11
5.12
5.13
5.14
5.15
5.16
6.1
6.2
6.3
6.4
6.5
6.6
A.1
A.2
A.3
A.4
A.5
A.6
A.7
A.8
A.9
72
72
73
74
75
76
77
78
78
79
85
89
89
89
104
104
105
105
105
xxii
List of Figures
List of Tables
1
Comparison on the production of single lap- shear joints by oven- and induction
curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xiii
Process comparison of microwave (MW), resistance (RH), induction (IH), ultraviolet (UV), electron beam (EB) and gamma radiation (GR) heating for adhesive
curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2.1
Susceptor particle properties . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
2.2
Susceptor particle magnetic permeability . . . . . . . . . . . . . . . . . . . . . .
11
2.3
2.4
Adhesive characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
List of all specimen produced . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
13
2.5
Specimen characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
2.6
Test matrix for the susceptor particle analysis . . . . . . . . . . . . . . . . . . .
23
2.7
Test matrix for the assessing the effect of susceptor particle- content, coil current,
coupling distance and coil geometry . . . . . . . . . . . . . . . . . . . . . . . .
23
2.8
Weight- to- volume conversion table . . . . . . . . . . . . . . . . . . . . . . . .
25
3.1
Geometrical parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
3.2
Magnetic field parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
3.3
Joule- heating parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
40
3.4
Hysteresis- heating parameters [14]
. . . . . . . . . . . . . . . . . . . . . . . .
42
3.5
Heat transfer parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
3.6
Description of the validation test set- up . . . . . . . . . . . . . . . . . . . . . .
44
4.1
TGA experimental set- up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
4.2
Cure cycles tested during the DSC analysis . . . . . . . . . . . . . . . . . . . . .
52
4.3
Required curing time for different temperatures to reach a fully cured state . . .
61
1.1
xxiv
5.1
List of Tables
Process parameters used for producing the induction- heated adhesive bonding
single lap- shear specimen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
5.2
Test parameters for Zwick 10 kN tensile bench
. . . . . . . . . . . . . . . . . .
69
5.3
5.4
Test matrix for the mechanical tests . . . . . . . . . . . . . . . . . . . . . . . .
Summary of the lap- shear strength of all tested set- ups . . . . . . . . . . . . .
70
72
5.5
Lap- shear strength of specimen, oven- cured at different temperatures . . . . . .
73
5.6
Lap- shear strength of specimen, oven- cured for 2 hours at 65◦ C with different
volume- percentages of Iron- powder . . . . . . . . . . . . . . . . . . . . . . . .
74
Lap- shear strength of oven- and induction- cured for 2 hours at 65◦ C and 7.5v%
of Iron powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
6.1
Total energy consumption of the oven- curing process at 65◦ C . . . . . . . . . .
84
6.2
Energy consumption of the EasyHeat unit at different levels of coil current . . . .
84
6.3
Energy consumption of the EasyHeat’s cooling unit . . . . . . . . . . . . . . . .
84
6.4
Total energy consumption of the induction curing process at 175A coil current .
85
6.5
Investment cost for the oven- and induction- set- up used in this project . . . . .
86
6.6
Estimation of the total energy consumption of a susceptorless curing process with
CFRP adherends and 60A coil current . . . . . . . . . . . . . . . . . . . . . . .
Results of the energy consumption monitoring experiment performed in the lab .
87
88
5.7
6.7
xxvi
List of Tables
Nomenclature
A
a
AC
B
c
CB
CD
CFRP
CNT
CP
D
DSC
GFRP
H
I
IR
k
k
L
MS
Man
Mirr
SEM
SLS
t
TGA
W
α
r
µr
ρ
σ
ω
Magnetic vector potential
Langevin parameter
Alternating current
Magnetic flux density
Domain rotation loss
Carbon black
Coupling distance
Carbon fibre reinforced plastic
Carbon nano tubes
Specific heat capacity
Coil outer diameter
Differential scanning calorimetry
Glass fibre reinforced plastic
Magnetic field strength
Coil current
Infra- red
Pinning factor
Thermal conductivity
Coil length
Magnetic saturation
Anhysteretic magnetization
Irreversible magnetization
Scanning electron microscopy
Single lap- shear joint
Adhesive thickness
Thermogravimetric analysis
Coil width
Local field factor
Relative permittivity
Relative permeability
Density
Electrical conductivity
Magnetic field frequency
Preface
This report has been written in the context of my master thesis, performed at the faculty
of aerospace engineering of the Technical University of Delft. Its purpose is to demonstrate
the scientific work delivered throughout my thesis and to address the project’s findings. The
project was performed in a nine- month period, including a two- month literature study and
seven months of scientific research.
The topic of this thesis is induction- heated adhesive bonding and assesses its potential as
a future bonding technique for the aerospace industry. The project’s topic was selected in
cooperation with my two supervisors, Dr. J.A.Poulis and Dr. S.Teixeira de Freitas, who both
have a top- notch expertise in the field of adhesive bonding.
This report consists of six chapter, each targeting a different aspect of the master thesis.
Readers who are specifically interested in the process of induction heating are referred to
chapter two, five and six. Those who are more interested in the optimization of an adhesive’s
cure cycle can focus their attention on chapter four. Modelling of the induction heating
process is explained in chapter three.
xxviii
Preface
Acknowledgments
First of all, I would like to thank my two supervisors, Dr. J.A.Poulis and Dr. S.Teixeira
de Freitas for their day- to- day supervision of my thesis. The concept of synergy, in which
1+1>2, was definitely present in their coaching throughout the project. I would also like
to express my gratitude towards F.Oostrum, G.J.Mulder, L.Xue and all other technicians
from the Dutch Aerospace Structures and Materials Laboratory for their guidance during
my experiments in the lab. This thesis would not have achieved the same level of scientific
contribution without their assistance in performing the required experimental research.
Further, I would like to thank Ir. M.Bosman from Fokker for supplying the necessary materials
and equipment, as well as for participating in steering this project towards a relevant outcome
of interest for the industry. I would also like to express my gratitude towards Dr. K.V.Bramon
and Ir. W.Post, both performing research at the department for Aerospace Structures &
Materials, for sharing their experience on induction heating.
Additionally I would like to thank my office- mates, G. Plaisier and M. Roest for their daily
contribution towards a dynamic, constructive and motivating thesis- environment. I would
also like to thank C.Pollier for her assistance in providing this report the needed photography
of all test set- ups.
Last, but definitely not least, I would like to thank my girlfriend Severine, my friends, family
and relatives for motivating me throughout my studies in Delft. All of them have contributed
enormously towards this unforgettable time in the Dutch capital of technology.
Delft, University of Technology
29th April 2016
C. Severijns
xxx
Acknowledgments
“Research is creating new knowledge.”
— Neil Armstrong
Chapter 1
Introduction
Adhesive bonding has a wide history in the aviation industry. Despite having some specific
advantages over other joining methods, some important challenges are still limiting its applicability in today’s aerospace industry. In order to understand the context of this thesis project,
the current chapter elaborates on the challenges as well as the latest developments within the
field of adhesive bonding. This chapter can be considered as a summarized version of the
literature study performed at the beginning of this project. First an overview will be provided
about the most important challenges that exist today in the field of adhesively joined aerospace
structures. The second part provides an overview of the different alternative adhesive bonding
processes. The third section elaborates in more detail on the principles of induction- heated
adhesive bonding. The final part of this chapter will elaborate more on the objectives of this
master thesis.
1.1
Latest Developments in Adhesive Bonding
Adhesives have been used extensively for structural bonding in aerospace applications [1].
They are incorporated today both as manufacturing and repair strategy because of their
many advantages over other joining techniques [2].
When looking at the general technological trends of the aerospace industry, one can conclude
that the next generation of lightweight aerospace products will have an increased presence
of composite and hybrid structures. This shift in materials will also have an influence on
the preferred joining techniques. As joining composite products mechanically will result in
fibre cuts and stress concentrations, different solutions will have to be developed to establish
high- quality joints, with adhesive joining as a promising alternative [15]. Parts to be joined
will also increase in number, complexity and size, which will have its influence on the applied
joining methods.
Another set of challenges for adhesive bonded joints will come from operational requirements
of the manufacturer and end- user, which are often cost- driven. Curing of adhesives requires
the control over three main process parameters: heat and time to complete the cross- linking
2
Introduction
of the adhesive and pressure to control void volume and for product positioning [16]. Today’s
curing of adhesives within the aerospace industry uses most often autoclave or oven systems
to achieve the best reproducible structural performance of the bond. These systems show a
high acquisition- and usage cost and require long cure cycles to guarantee good structural
performance of the joint.
Shifting towards out- of- autoclave and out- of- oven processes would result in enormous
cost-, energy- and time savings, which would add to the attractiveness of adhesive bonding.
Possible methods to get rid of the autoclave are to develop adhesives which cure either at
room temperature, or to develop alternative out- of- autoclave/oven- curing methods. Due to
the significant reduction in mechanical performance of room temperature- cured- adhesives,
today’s focus is mainly on out- of- oven/autoclave- alternative curing methods [15].
1.2
Alternative, Out- of- Autoclave / Oven- Curing Techniques
As mentioned in the previous section, the development of out- of- oven/autoclave curing
methods has been the subject of many research projects in recent history. Microwave-, UV-,
electron beam-, resistance- and induction heating have all been the subject of many research
projects. Those alternative curing methods can be categorized as either thermal curing or
radiation curing.
Microwave-, resistance-, and induction heating can all be classified under thermal curing
processes. The working principle is that the polymerization of the adhesive takes place by
the generated heat. The physics behind each curing method is, however, still different.
Radiation curing is based on the ionization (bond breakage) of radiation sensitive polymers
by the use of high- energy electromagnetic radiation such as gamma rays, ultraviolet or
accelerated electron beams [3].
Table 1.1 shows a direct comparison between different alternative cure techniques, with respect to some of the most important parameters in adhesive bonding. The objective of this
comparison is to give a clear summary of the advantages and disadvantages of each method.
Most of this comparison is based on own insights and a sector review made by D. Abliz. [3].
Table 1.1: Process comparison of microwave (MW), resistance (RH), induction (IH), ultraviolet (UV), electron beam (EB) and gamma radiation (GR) heating for adhesive curing
Investment cost
Operating cost
Adherend limitations
Curing speed
Temperature control
Through-thickness curing
Handling flexibility
Disassembly
Safety
Research performed
MW
++
+
+
+++
++
+
RH
+++
+
+
++
+
-
IH
+
++
+
+
++
+
+
+
-
UV
+
++
++
+
++
++
+
EB
+++
+
+
+
GR
--+
-++
--
1.3 The Induction Heating Process
3
From table 1.1, one can see that each alternative curing method is having its own specific
advantages and disadvantages. When looking specifically at the induction heating process,
one could conclude that it provides an alternative curing process at the lowest possible cost
and the highest adherend flexibility and through- thickness curing. This makes induction
heating a very attractive alternative to be used in large- scale applications within the aviation
industry. In order to succeed however, much more research has to be performed to get a
sufficient understanding of the induction heating process and its impact on the adhesive
bonding process. The development of additional know- how concerning induction- heated
adhesive bonding can be considered to be one of the main objectives of this master thesis
project.
1.3
The Induction Heating Process
The principles of induction theory have been established by Michael Faraday [4]. By exposing
a material to an alternating electromagnetic field, heat is generated through either one of the
following two mechanisms:
• Joule heating: By applying an electromagnetic field to a conductive material, socalled Eddy- currents will be generated within closed loops of the material. Those
Eddy- currents as a result generate heat through the resistance heating principle. Joule
heating depends on the current generated, the resistance of the conductor and its length.
Additional heat is generated by dielectric heating, as molecules collide when changing
direction due to the alternating electromagnetic field.
• Hysteresis heating: If a ferromagnetic material is exposed to an external electromagnetic field H , its grains will try to align themselves with that field, which is called its
internal magnetic flux density B . By alternating the magnetic field, a continuous rotation of the grains will be achieved, which then generates the so- called hysteresis losses.
The easiness of a material’s grains to align themselves with the magnetic field is shown
in the material’s magnetization curve or H- B curve. An example of such magnetization
curve is shown in figure 1.1. One can say that the amount of heat generated through
hysteresis losses is therefore directly proportional to the size of the H- B curve of a
ferromagnetic material.
Figure 1.1: Examples of a magnetization curves and their effect on hysteresis heating
4
Introduction
For those induction heating principles to work, the material has to possess either conductiveor magnetic properties. Two forms of induction heating exist, depending on how the required material properties are achieved, called susceptor- assisted and susceptorless induction
heating. Each of those techniques will be covered in more detail in the following sections.
Susceptor- Assisted Induction Heating
Susceptor- assisted induction heating creates the required conductive- or magnetic properties
within the bondline by adding additional material. Susceptor material can be added to the
bond line either as particles or as a mesh. Mesh- assisted applications have not been covered
further, as they require special considerations because of the difficult adhesion between the
mesh and the adhesive [17].
In theory, all materials possessing either electrical conductive or magnetic properties can be
used as susceptor material. Some research projects have assessed the impact on the heat generation of several conductive, piezoelectric, ferromagnetic, and ferromagnetic powder materials
as susceptor for induction heating [6] [18] [8]. As can be seen from the temperature profiles
generated by Bayerl et al. in Figure 1.2, Iron shows the best heat- generating characteristics of
all tested susceptor materials. Another low cost and widely used susceptor material is Nickel.
Its simple micro- structure and its availability in varying particle sizes makes Nickel very suitable for induction heating. Susceptor materials which only possess conductive properties, such
as carbon black (CB) and carbon nano tubes (CNT) do not generate any heat, as they are not
able to generate the required closed electrical loops within the adhesive at low concentrations.
Figure 1.2: The heating characteristics of different susceptor materials [6]
Susceptor- assisted induction heating has some specific advantages compared to susceptorless
processes [19]:
• Ability to join non- conductive adherends
• Ability to heat very locally, reducing the overall thermal stresses
• When coated in resin, the susceptor can fill voids within the adhesive
1.3 The Induction Heating Process
5
• Automatic temperature control in case of ferromagnetic susceptor material, as the material looses its magnetic properties beyond its Curie temperature
• Inside- out heating as established in susceptor- assisted heating reduces the void content
as gas liberation from the edge is improved [20]
Susceptor- assisted induction heating is a complex mechanism, which depends on many different process parameters. Experiments have shown that different susceptor particles show
better heating characteristics at either low-, medium-, and high- magnetic field strengths [5].
Also the susceptor size- and weight- percentage have a significant impact on the heating
characteristics. In general one can conclude that smaller particle sizes and a higher particle
weight- percentages are beneficial for the heat generation, as can be seen in Figure 1.2 for
the particle content and in Figure 1.3 for the particle size [6] [7]. Only for very small particle
sizes (<25 µm) this relation is not valid any more, as oxidation effects reduce the heating
capability.
Figure 1.3: The heating characteristics of different susceptor sizes [6]
Fokker has recently tried to apply the already established knowledge in a practical experiment
to join their carbon fibre reinforced plastic (CFRP) adherends by susceptor- assisted adhesive
bonding [21]. Because of the good heating characteristics of CFRP, the applied field power
was limited to only 40 A, in order not to overheat the CFRP adherends locally. At this
field strength, the susceptor particles are not yet generating significant amounts of heat. The
conclusion was made that further knowledge has to be gained through extended experimental
testing in order to achieve a ’proof of concept’.
6
Introduction
Susceptorless Induction Heating
Susceptorless induction heating is a more simple and straightforward method, possessing some
specific advantages over susceptor- assisted heating:
• Less system complexity
• Fewer processing steps (no mixing/integration of the susceptor particles)
• Lower process cost
The two main applications of susceptorless induction heating are adhesive bonding of metallic
and CFRP substrates. The working principles are exactly the same as those for susceptorassisted induction heating. The heat generation is primarily based on the Joule heating
mechanism [22] [23]. As fibres are in direct contact at junctions, contact resistance and a
voltage drop will be generated, resulting in additional heat generation, which is called the
junction- heating effect. This effect has to be taken into account to avoid local overheating
of the adherend.
As for susceptor- assisted induction heating, most of the research performed already on susceptorless induction heating has focussed on understanding the impact of different process
parameters [12] [21]. The applied frequency has again a significant impact on the amount of
heat generated, as increasing frequencies show higher heat generation characteristics (Figure
1.4). The amount of heat generated is also increased by applying a higher field strength. This
can be done either through reducing the coupling distance between the induction coil and the
material, or by applying a higher current through the induction system.
Figure 1.4: The effect of the applied frequency on the heating rate of CFRP plates [12]
C. Sanchez has performed a series of experiments to optimize the induction heating cure
cycle of a susceptorless CFRP set- up. By modelling the cure kinetics, Sanchez has been able
to assess the effect of the cure temperature on the formation of voids and the mechanical
performance of the joint [9] [24] [8]. The conclusion was made that void formation of up to
2% has no impact on the mechanical properties of the joint. Based on his initial findings,
Sanchez has been able to develop a methodology for reducing the processing time as much as
87%, compared to oven- curing, while keeping the void formation within acceptable limits [10].
By performing those experiments, Sanchez has been the first to address the notion of bond
quality as a bridging element between the cure characteristics and the joint’s strength.
1.4 Project Scope
7
Addressing the Scientific Gap
As can be seen in the previous sections, only few research projects have focussed already
on induction- heated adhesive bonding. Most of those projects have been focussed towards
susceptorless heating of conductive adherends, such as Aluminium or CFRP.
On the other hand, the field of susceptor- assisted adhesive bonding has not yet been covered
widely. Most of the research on susceptor- assisted induction- heated adhesive bonding has
focussed on the influence of different process- and material properties on the heat generation
within the adhesive.
The primary difference with susceptorless induction heating comes however from the addition
of susceptor particles to the adhesive, which in turn generate heat directly within the bond
layer. First of all, one has to be sure that both the susceptor- particles and the adherend
materials can sucessfully be joined by induction heating, as both might have different heatgenerating characteristics when exposed to induction.
Secondly, additional aspects of susceptor- assisted adhesive bonding are the possible impact
of the susceptor particles on the cure chemistry of the adhesive, the required cure cycle and
the mechanical performance of the joint, both in terms of initial strength as well as durability
aspects.
A final point of interest is a comparison between the performance of induction- heated adhesive bonding and conventional oven- curing. Important performance indicators, such as processing time, energy consumption and mechanical performance should be compared for both
techniques, in order to quantify the advantages, disadvantages and limitations of inductionheated adhesive bonding as a next- generation manufacturing technique for the aerospace
industry.
1.4
Project Scope
The purpose of this research project is to develop additional insights in the field of susceptorassisted induction- heated adhesive bonding. The main research question of this master thesis
could therefore be summarized as followed:
”How does susceptor- assisted induction heating influence the cure cycle and mechanical performance of adhesively bonded joints?”
Linked to this main research question, several project objectives have been developed:
• Assess the compatibility of different susceptor- particles and adherend materials
• Determine the effect of the cure temperature on the required cure time and mechanical
performance of adhesively bonded joints
• Analyse the effect of susceptor particles on the mechanical performance of adhesively
bonded lap- shear joints
• Compare key performance indicators such as process time, energy consumption and
mechanical performance of induction- cured joints against oven- cured specimen
8
Introduction
By completing those research objectives, this project is considered to be of academical standards, addressing specific gaps in today’s knowledge on induction- heated adhesive bonding.
A first important relevance of this project is to provide the aerospace industry with a ’proof
of concept’ of susceptor- assisted, induction- heated adhesive bonding.
Moreover, this project’s aim is to identify the advantages, disadvantages and limitations of
induction- heated adhesive bonding. The project’s findings will be provided as recommendations towards industrial partners of the Delft University of Technology as input towards
selecting induction- heated adhesive bonding as their possible next- generation, out- of- autoclave curing process.
This report is split into five major parts. The first part, found in chapter two, analyses
different susceptor materials, their influence on the heat- generation and the influence of
different process parameters. Chapter three elaborates on modelling the induction heating
process. Chapter four assesses the cure characteristics of the adhesive in scope and proposes
an optimized cure cycle. The fifth chapter evaluates the mechanical performance of inductioncured adhesives. The final chapter focusses on the actual comparison between oven- cured
and induction- cured samples.
Chapter 2
Susceptor Analysis
2.1
Introduction
Induction heating is a complex process, influenced by different process-, material- and design
parameters. As a first step towards the development of an induction heating curing- process
for paste adhesives, it is important to understand the effect of those parameters on the
induction heating process.
As mentioned before, induction heating can be performed both susceptorless and susceptorassisted. This chapter will therefore assess the effect of the involved parameters on both
heating techniques. Susceptorless induction heating will be assessed through the heating of
bare CFRP substrates. The susceptor- assisted heating technique will be analysed by mixing
electromagnetic particles into the adhesive in scope of this project. A third possible set- up
is the combination of the susceptorless and susceptor- assisted technique, by using CFRP
adherends in combination with susceptor particles within the adhesive. However, in order
not to suffer from cross- over interference effects between both techniques, initial experiments
will only be performed separately.
The following objectives were established for the first experimental phase of this project:
• Assess the heat- generation characteristics of CFRP and different susceptor particles by
induction heating
• Assess the effect of different process parameters, such as coil current, coupling distance
and coil geometry on the heat- generating characteristics of the induction process
• For the susceptor- assisted set- up, determine the effect of susceptor- particle content
• Assess the feasibility/compatibility of a combined induction heating system of CFRP
adherends and susceptor- assisted adhesive
The findings from those experiments will be used to assess the suitability of both susceptorassisted and susceptorless processes for the adhesive bonding of CFRP substrates. At the end
10
Susceptor Analysis
of this phase, one set- up will be selected for further investigation and for the development of
a case- study on induction- heated adhesive bonding process.
2.2 Materials & Specimen
2.2
2.2.1
11
Materials & Specimen
Susceptor-Assisted Heating
Susceptor Particles
A total of four different susceptor particles has been selected for this phase of the project,
being Iron powder (Fe), two Nickel- Zinc- Ferrite powders (FP110 & FA100) and GrapheneOxide (GO). All of them have been chosen from different sources of literature. Table 2.1
summarizes the most important properties for each of the selected particles.
Table 2.1: Susceptor particle properties
Iron Powder
Nickel-ZincFerrite
FP110
Powder
Processing
& Technology
Nickel-ZincFerrite
FA100
Powder
Processing
& Technology
Graphene- Oxide
GO
Graphendo
60% Fe
25% Zn
10% Ni
unknown
Brand Name
Supplier
Fe
Acros Organics
Composition
Pure Iron Powder
(99%)
50%
20%
10%
10%
Particle Size
70 Mesh
(<200 µm)
50-200 µm
50-200 µm
50-200 µm
Specific
Gravity
7,8
4,7
5,2
unknown
Fe
Zn
Ni
Cu
Despite being a crucial parameter, information concerning the magnetic permeability of each
of the susceptor particles has not been found. Experimental determination of the magnetic
properties of each susceptor particle was considered to be out- of- scope for this project.
Information on the permeability of other materials found in literature can, however, be used
to establish a preliminary estimation of the magnetic properties of the materials used, as can
be seen in table 2.2.
Table 2.2: Susceptor particle magnetic permeability
Iron
Magnetic permeability [H/m]
2, 5 · 10−1
Nickel-ZincFerrite
8 · 10−4
12
Susceptor Analysis
Adhesive
The adhesive used for this project is the structural epoxy adhesive EC9323-B/A produced
by 3M. It is a two- component paste adhesive with an excellent adhesion to a wide variety
of substrate materials, including both CFRP and GFRP. The reason for using this adhesive
specifically for this project is because of the fact that the adhesive is widely used by Fokker
in their final assembly lines.
Table 2.3 shows some of the most important characteristics of the adhesive. According to
the specifications provided by the manufacturer, EC9323-B/A will gel in three hours after
mixing, build- up handling strength after four hours and fully cure at room temperature in
seven days. At 65◦ C, a full cure will have been established after two hours.
Table 2.3: Adhesive characteristics
Base
Component A
Modified Amine
Component B
Modified Epoxy
Density
1.03 g/cm3
1.04 g/cm3
Mixing Ratio
1
2
Specimen Preparation
A crucial aspect in the development of susceptor- assisted set- ups is to achieve a good
dispersion of the nanoparticles. Sample preparation has therefore been an important part of
this experimental phase. The samples were created in small- scale batches, making it easy to
tweak certain aspects of the production method in short amounts of time. Particle dispersionand concentration have been important performance indicators to assess the correctness and
reproducibility of the samples made. The sample preparation process consisted of three
phases, the mixing-, moulding- and quality- control phase.
A total of ten specimen types have been created with different weight percentages and susceptor material in order to perform all the required tests for this phase of the project. Each type
of specimen has been produced three times, as a mean of averaging- out the experimental
results. Table 2.5 shows a test matrix, including all the samples tested in this phase of the
project.
2.2 Materials & Specimen
13
Table 2.4: List of all specimen produced
Specimen Specification
3 x Fe - 1v%
Intended tests
3 x Fe - 2v%
Effect
Effect
Effect
Effect
Effect
3 x Fe - 5v%
Effect of volume- percentage
3 x Fe - 10w%
Effect of susceptor material
3 x FP110 - 2v%
Effect of susceptor material
3 x FP110 - 10w%
Effect of susceptor material
3 x FA100 - 2v%
Effect of susceptor material
3 x FA100 - 10w%
Effect of susceptor material
3 x GO - 2v%
Effect of susceptor material
3 x GO - 10w%
Effect of susceptor material
Effect of volume- percentage
of
of
of
of
of
susceptor material
volume- percentage
coil current
coupling distance
coil geometry
Mixing Process & Particle Dispersion Both components of the adhesive, as well as the
nanoparticles were mixed manually. Other mixing processes, such as ultrasonic mixing or
automated sheer mixing were considered to be infeasible due to the high viscosity of the
epoxy used. The initial mixing process consisted of the following steps:
• Prepare component A of the adhesive in the mixing cup
• Add the susceptor particles and mix until a homogeneous mixture is achieved
• Add component B and perform another manual mixing
This mixing process was considered to be the most convenient, as the lower viscosity would
enhance the mixing process. Additionally, component A of the adhesive has a white colour,
allowing an initial, visual check on the dispersion of the susceptor particles. However, mixing
the susceptor particles first into only one component of the adhesive resulted in a inhomogeneous dispersion of the particles, with in incorrect, lower particle concentration than intended.
This was primarily caused by the fact that not all the particles were taken-up by component
A of the adhesive, leaving some amount of the particles trapped at the bottom of the mixing
cup. Therefore, the mixing process was altered, and the following process was used:
14
Susceptor Analysis
Figure 2.1: SEM image from a 2v% Fe
specimen
Figure 2.2: SEM detection of Fe elements in a 2v% Fe specimen
• Prepare component A of the adhesive in the mixing cup
• Add component B of the adhesive and mix until a homogeneous mixture is achieved
• Add the susceptor particles and mix another three minutes
Because of the black colour of component B of the adhesive, a visual check on the dispersion
of the susceptor particles was not possible anymore. Scanning Electron Microscopy (SEM)
was therefore used to assess the quality of the dispersion. The dispersion of the susceptor
particles was assessed by checking the dispersion of ferrite (Fe) elements within the specimen
for two different samples. This was considered to be a correct method, as ferrite is contained
by each of the susceptor materials, while it was not detected when analysing a sample of
bare adhesive. As can be seen from Figure 2.1 and 2.2, a homogeneous dispersion has been
achieved. The only source of inhomogeneity comes from the difference in particle size, as can
be seen in the center- left part of Figure 2.2, which shows a higher concentration of ferrite
elements due to the presence of a larger Iron- powder- particle.
Sample Moulding & Quality Control The prepared adhesive was poured into plastic moulds
in order to achieve maximal reproducibility of the samples. The moulds were circular shaped,
producing samples with a 27 mm- diameter and a thickness of 2.95 mm, as can be seen in
Figure 2.3.
2.2 Materials & Specimen
15
Figure 2.3: Samples used for the susceptor-particle experiments
A final step in the specimen production was to validate the specimen’s characteristics. A
one- day cure in the plastic moulds was required for the adhesive to build- up sufficient
strength to be released from the mould. Thereafter the specimen were trimmed, measured
and weighted for the sake of quality control. The particle percentage was determined by
measuring the weight of the sample and by comparing it to the weight of a reference sample
of bare adhesive. Table 2.5 summarizes the findings, as measured for all of the thirty specimen
produced for the experiments.
Table 2.5: Specimen characteristics
2.2.2
Number of specimen
measured
30
Size
Thickness: 2.95 mm (+- 0.1 mm)
Diameter: 27 mm (+- 0.1 mm)
Accuracy of susceptor
concentration
Weight percentage : +- 0,1w%
Volume percentage : +- 0,02v%
Susceptorless Heating
CFRP Adherends
The CFRP material used for these experiments was HexPly M18/1 woven fabric. It is a high
performance, tough epoxy matrix which is often used in the aerospace industry. The CFRP
samples used for these experiments consisted of eight layers of woven fabric, with dimensions
of 75 mm by 25 mm by 2 mm, as can be seen in Figure 2.4.
16
Susceptor Analysis
Figure 2.4: The CFRP samples used for the phase I experiments
2.3 Experimental Set- Up & Methodology
2.3
17
Experimental Set- Up & Methodology
The test set- up used for Phase I of the project covers two aspects, the induction system
itself and the temperature measurements used to generate the temperature profiles. It is
of crucial importance to understand the basic working mechanisms, operating methods and
limitations of each of those systems in order to be able to draw correct conclusions from the
performed experiments. Each of the sections below will provide some in- depth information
on the equipment used.
2.3.1
The Induction System
Induction systems generate an alternating electromagnetic field through the generation of
an alternating current within the system with relative low voltage. An induction system
consists of an alternating- current (AC) generator, a workhead and a cooling system. The
generator will send an alternating current through the workhead, which then produces the
actual electromagnetic field.
The induction system used throughout the project is an Easyheat LE-10 kW unit, made by
Ambrell. The system is intended for both industrial and research applications, has a straightforward operating interface and generates an auto- tuned, stable magnetic field. It has a
maximum rated power of 10 kW, allowing it to generate coil currents of up to 600 Amperes.
It is capable of generating magnetic fields within a frequency range of 100 - 400 KHz. While
the system’s current can be tuned through the operator panel, frequency and voltage of the
system depend on machine parameters, such as circuit properties and capacitance used for
the excitation of the circuit. They can therefore not be tweaked manually and are fixed for a
certain system- coil combination.
Different coils can be mounted onto the Easyheat unit, in order to generate different electromagnetic fields. Two coil geometries were assessed during this phase of the project, a pancake
coil and a simple ’one- turn’ coil (fig. 2.5 and 2.6 respectively). Pancake coils are often used
for large- scale surface heating, as they generate a homogeneous magnetic field over a wider
heat- affected area. One- turn coils on the other hand are a more powerful system when only
a limited, rectangular area has to be heated, as it only generates a homogeneous magnetic
field within the coil- width.
Additionally, different coil types generate a different field frequency as they affect the electrical circuit’s properties. As the Easyheat system does not allow a manual tweaking of the
frequency for a certain set- up, changing the coil geometry is the only way to alter the field
frequency of the test set- up used. The pancake coil available for this project generates a 250
KHz field, while the one- turn coil generates a magnetic field of approximately 400 KHz.
The induction heating set- up is provided with a water- cooling system, used to keep the
temperature of the workhead below the maximum allowed temperature, 25 ◦ C. The cooling
required from the unit is directly linked to the current generated trough the system, as higher
levels of coil current generate more heat. As a limitation of the cooling unit, high levels of coil
current can only be withstand for short amounts of time. A coil current of 400 A could, for
example, only be sustained for about 5 minutes before the maximum operating temperature
of 25 ◦ C was reached. The test- duration for each experiment was adjusted to 200 seconds
due to this limitation.
18
Susceptor Analysis
Figure 2.5: A one- turn coil
Figure 2.6: A pancake coil
2.3 Experimental Set- Up & Methodology
2.3.2
19
Temperature Measurements
Two techniques were used for the temperature measurements in this phase of the project, being an infra- red (IR) camera set- up and a thermocouple system. Each of these measurement
techniques has some specific advantages, disadvantages and limitations. Those aspects have
to be taken into account when selecting one of both systems for the temperature measurement of a specific experimental set- up. Other techniques such as fibre- optical temperature
measurements were considered not to add any additional value to the project.
Thermocouple Measurements
Thermocouple temperature measurements are widely used, both in industrial- and in research
applications. It’s based on the principle known as the thermoelectric effect. When two
dissimilar conductors are brought in contact, a voltage will be generated, which is directly
related to the temperature difference between both conductors. This technique is applied in a
thermocouple as two dissimilar wires, of which one is attached to the measurement point and
the other to an object with a known reference temperature. The thermocouple measurement
system for this project is a Keithley 2701 multimeter- system.
Temperature measurements performed by a thermocouple system have the following advantages:
• Accurate spot- measurements
• Simple, straight- forward and inexpensive technique
• Ability to measure the temperature anywhere in the test set- up
• Ability to measure the temperature within samples (embedding of thermocouples)
However, thermocouples show disadvantages and limitations, compared to IR measurements:
• Only spot- measurements possible, making it very difficult to generate a heating profile
over a certain area
• Possible interference between the induction field and the thermocouple wires
Infra- Red Measurements
The Infra- red temperature measuring technique, also called thermography, is based on the
principle of black- body radiation. This type of radiation is emitted by any body having an
absolute temperature of more than 0◦ K, and the amount of black- body radiation emitted
depends only on the object’s temperature [25]. An object having a higher temperature will
have a higher emission of black- body radiation. IR- cameras measure black- body radiation
and convert it back to the object’s temperature.
Temperature measurements performed by IR have the following advantages:
• No interference from the induction field
20
Susceptor Analysis
• Easy measurement of the temperature profile over a specific area (identification of local
hotspots or unaffected areas)
• More powerful data- processing possibilities
However, IR measurements also have disadvantages and limitations, compared to thermocouples:
• More complex process, requiring the input of several process parameters for correct
measurements (mostly performed as auto- tweaking by the software tough)
• Measures the temperature only of surfaces in direct, visual contact
• Expensive, fragile equipment
The IR camera available at the ASML- lab is a FLIR A655sc LWIR camera. It is a powerful
infra- red camera, intended for research- and R & D purposes. FLIR also provides a dataassessment interface, called ResearchIR, which allows for a straight- forward and quick analysis
of the performed temperature measurements.
The camera can be directly mounted on the support structure of the induction system. Several
mounting positions are possible, but only the top- position will be used for the phase I
experiments, as can be seen in Figure 2.7. This allows for a good measurement of the sample’s
top surface temperature. For some of the experiments, the induction coil is placed above the
sample, which shades part of the sample’s surface from the camera. Figure 2.8 shows a crosssectional sketch of the test set- up used, including the IR camera, induction coil and test
specimen.
2.3 Experimental Set- Up & Methodology
Figure 2.7: Test set- up for the phase I experiments
Figure 2.8: Cross- sectional sketch of the test set- up
21
22
Susceptor Analysis
Infra-Red vs. Thermocouple : Comparison of Measurement Output under Exposure to
Electro-Magnetic Fields
As mentioned above, thermocouples could be considered as inappropriate in combination
with an induction heating set- up, as the external electromagnetic field could possibly interfere with the measurement signal within the wire. However, an experimental comparison of
thermocouple- and IR- temperature measurements was performed in this stage of the project.
A CFRP reference sample was placed underneath the induction coil. Figure 2.9 shows the
temperature profile, measured by both the IR camera and the thermocouple. As one can see,
only a 5% difference has been recorded between the two systems. This error can even not
be contributed purely to the thermocouple, as a small discrepancy between the local measurement of the thermocouple and the averaged area- temperature of the IR camera exists.
The thermocouple system is therefore considered to provide acceptable results in combination
with the induction heating system, and will be used further in the progress of this project.
Figure 2.9: Temperature profiles measured both by the IR camera and the thermocouple
system
As a summary, one could argue that the IR technique is more optimal when a measurement
is required over a certain area and when the test set- up allows for a mounting of the camera
such that it has a direct view on the area of interest. Infra- red temperature measurements
are therefore considered as the preferred measurement technique for phase I, as only simple
test set- ups will be used. In later stages of the project, when temperature measurements
will be required from both the adherend’s top- & bottom surface as well as the bondline’s
temperature, the thermocouple system will be used more frequently.
2.3.3
Test Plan
The objective of the phase I experiments is to analyse the effect of different process parameters
on the induction heating process. This will be done by varying one parameter at a time,
2.3 Experimental Set- Up & Methodology
23
while keeping the others fixed. Table 2.6 shows the test matrix for assessing the effect of the
susceptor material, in which the different types of susceptor particles will be tested for their
heat- generating capacity. Table 2.7 shows the test matrix for the remaining parameters to
be assessed.
Table 2.6: Test matrix for the susceptor particle analysis
Particle
Fe
FA
FP
GO
2%
Coil
current
400 A
Coil
geometry
One- turn
Coupling
distance
1 mm
v%
2%
2%
2%
w%
10%
10%
10%
10%
400 A
One- turn
1 mm
Table 2.7: Test matrix for the assessing the effect of susceptor particle- content, coil current,
coupling distance and coil geometry
Susceptor
v%
Fe
2%
CFRP
-
Coil
current
100 A, 200 A
300 A, 400 A
500 A
Coupling
distance
1 mm, 3 mm
5 mm, 7 mm
Coil
geometry
One- turn
Pancake
20 A, 40 A
60 A, 80 A
100 A
1 mm, 5 mm
10 mm, 15 mm
25 mm
One- turn
Pancake
24
2.4
Susceptor Analysis
Results & Discussion
The following sections will describe the results of each of the experiments performed. The
primary output of each experiment is a so- called temperature profile, which shows the temperature increase over time for a specific set- up. As mentioned already in section 2.2, each
experiment was reproduced by three identical specimen, and the results shown are an average from the individual tests. All of the performed tests have shown consistent data for the
three specimen with less than 1% difference is measured temperature for all of the performed
experiments.
2.4.1
Susceptor- Assisted Set- Up
Effect of Susceptor Material
Two sets of experiments have been performed in order to assess the effect of different susceptor
materials. First, samples with a fixed weight- percentage of 10% are compared for a heating
experiment with a one- turn coil at 400 Amperes and a coupling distance of 1mm. This test can
be considered as a direct measurement of the pure material heat- generating characteristics,
as it measures the amount of heat the particles can generate per unit of weight.
On the other hand, the purpose of this project is to develop an induction heating curing
process for paste adhesives. As the mechanical performance of the end- product is most
possibly affected by the amount and type of particles mixed- in the adhesive, a good susceptor
particle would ideally generate high amounts of heat at a low volume- percentage. This is
tested by comparing samples with a volume percentage of 2% of each of the selected susceptor
particles. As for the weight- percentage experiment, a one- turn coil, 400 Amperes coil current
and a coupling distance of 1 mm is used.
Figure 2.10 shows the results from the weight- percentage experiment. Iron powder generates significantly more heat, compared to the other susceptor particles. This observation is
explained by the higher magnetic permeability of Iron powder, as shown in table 2.2. Both
FP110 and FA100 generate approximately comparable amounts of heat. FA100 performs
slightly better, which can possibly be explained by the higher percentage of Iron present in
its composition (60%), compared to FP110 (50%). No temperature change was found for
the Graphene- Oxide samples, which means that the magnetic permeability of the GrapheneOxide used is not sufficient for the project in scope.
The results from the volume- percentage experiment are shown in Figure 2.11. Comparable
trends are found as with the weight- percentage experiment. The total temperature increase
differs however from the weight- percentage experiment for each material due to the difference
between weight- and volume percentages, which is related to the material’s specific weight.
Table 2.8 shows the conversion from weight- to volume- percentage and vice versa for the
tested samples. Both FP110 and FA100 show very comparable temperature increases, as
10w% percentage equals approximately 2v% for both susceptor particles. Iron powder is
considered as the susceptor particle generating the highest amounts of heat. As its specific
weight is higher than that of the other particles, the difference in generated heat is even more
significant in this case, as more material (in terms of weight) is added to the adhesive.
2.4 Results & Discussion
25
Figure 2.10: Temperature profiles for
10w% samples, 400 A and 1 mm coupling
Figure 2.11: Temperature profiles for
2v% samples, 400 A and 1 mm coupling
Table 2.8: Weight- to- volume conversion table
Weight percentage
Volume percentage
Fe
10
1,46
13,27
2
FP110
10
8,44
2,4
2
FA100
10
9,26
2,17
2
GO
10
TBD
TBD
2
The following conclusions are drawn from the susceptor- material assessment:
• Iron powder shows the best heating characteristics, generating up to twice as much heat
per unit of weight, compared to the other susceptor particles tested
• Iron powder performs even better per unit of volume, compared to the other particles
tested, because of its higher specific gravity
• Graphene- Oxide does not show any temperature increase, most probably explained by
insufficient magnetic permeability of the material
As a summary, the heating performance of Iron powder can be considered to be several times
better, compared to the other susceptor particles in scope. It can therefore be considered as
being the most promising susceptor material to be used for the development of an inductionheated adhesive bonding technique.
Effect of Volume- Percentage
Initial testing of the susceptor particles was performed at one, fixed volume- and weight
percentage. The amount of susceptor particles added to the adhesive has however a direct
impact on the amount of heat that is generated. Experiments were performed to assess
the sensitivity of the heat- generating characteristics for different volume- percentages of Iron
powder, as it was found in the previous section to be the best- performing susceptor materials.
This experiment was performed again with the one- turn coil at a coil current of 400 Amperes
and a coupling distance of 1 mm.
26
Susceptor Analysis
Figure 2.12 shows the temperature profiles for different volume percentages. Increasing the
volume percentage enhances the heat- generating characteristics significantly. For example,
doubling the volume percentage from 1v% to 2v% triples the change in temperature achieved
(15 ◦ C for 1v% while 45 ◦ C for 2v%). The increase in temperature from 1v% to 2v% is however
more significant than the increase in temperature from 2v% to 5v%, as the volume percentage
is more then doubled, while the increase in temperature is only 50%. Therefore one can say
that the sensitivity for adding susceptor material is higher at low volume percentages, and
even small differences in volume- percentage can change the heat generation significantly.
Figure 2.12: Temperature profiles for 1,2 and 5v% of Fe particles, 400 A and 1 mm coupling
Effect of Coil Current
A second process parameter to assess is the coil current of the induction system. Different
levels of coil current have been applied with the one- turn coil on a 2v% Iron powder specimen
with a coupling distance of 1mm, for which the results are shown in Figure 2.13. Applying a
higher coil current results in a stronger magnetic field around the coil, and thereby increases
the amount of heat generated within the sample. Due to the limitations of the induction
system and its related cooling systems, coil currents were only generated up to 500 Amperes.
2.4 Results & Discussion
27
Figure 2.13: Temperature profiles for 100,200,300,400 and 500 A, 1 mm coupling and 2v%
of Fe particles
For the combination of a one- millimetre coupling distance and a 2v% - Iron powder sample,
a coil current of 100 A does not result in any increase in temperature. At higher levels
of coil current, temperatures of up to 100◦ C can be achieved. Coil current can therefore
be considered as a major process parameter, to be used for developing a specific induction
heating process in later stages of this project.
Effect of Coupling Distance
Changing the coupling distance alters the strength of the electromagnetic field, and thereby
works in the same way as the coil current. As can be seen in Figure 2.14, a coupling distance
of more than 7 mm results in hardly any heat generation in combination with a 400 Amperes
coil current generated by the one- turn coil.
Figure 2.14: Temperature profiles for a 1,3,5 and 7 mm coupling distance at 400 A and
2v% of Fe particles
28
Susceptor Analysis
Effect of Coil Geometry
A last process parameter which could be altered is the coil geometry. As explained in section
2.3, two different coil types are available for this project. Changing the coil geometry has two
direct consequences. First, the coil geometry is going to change the magnetic field produced
in the test area. Secondly, the field’s frequency is going to change because of the different
impedances of different coils.
The one- turn coil produces a field frequency of 400 KHz and the pancake coil about 250
KHz. From induction theory, this should theoretically imply a higher amount of heat generated through the one- turn coil. When looking however at temperature profiles from the
experiment, the temperature rise with the pancake coil is several times higher. This effect
can be explained by the difference in magnetic field- density produced by both coil types.
As for all coil types, the one- turn- and pancake coil do only produce a strong magnetic field
within the enclosed area. As the pancake coil has a higher amount of turns, it is capable of
generating a stronger magnetic field over a wider surface of the sample, and thereby activates
a larger number of susceptor particles. This effect can also be seen when looking at thermal
images of the heating process, taken by the IR camera (fig. 2.16 and fig. 2.17).
Figure 2.15: Temperature profiles for a one- turn coil and a pancake coil, 400 A, 1 mm
coupling and 2v% of Fe particles
2.4 Results & Discussion
Figure 2.16: The heat- affected area
with a one- turn coil
29
Figure 2.17: The heat- affected area
with a pancake coil
The conclusion is made that the pancake coil will lead to a faster heating- process. One has
to keep in mind, however, that this is only true for heating larger surface area’s, which would
not be fully covered by the one- turn coil. In the case of only heating small overlap area’s of
bonded joints, one might value the one- turn coil to be more suitable.
Another point of interest in changing the coil geometry was to alter the field’s frequency,
generated by the induction system. This method of altering the field’s frequency provides
however no comparison possibilities, as changing the coil also changes the produced magnetic
field density, and therefore impacts the heat generation significantly. The conclusion was
therefore made that the frequency effect of cannot be assessed by the available test set- up.
2.4.2
Susceptorless Set- Up
Heat- Affected Area
A first step in assessing the performance of CFRP adherends in the induction heating process
is to look at the heat- affected area. In contrast to the relatively homogeneous heating of
the susceptor- assisted samples in the previous section, preliminary experiments performed
on the CFRP panels indicated some degree of inhomogeneous heating. Figure 2.18 shows
a measurement of the heat- affected zone as recorded by the IR camera. It indicates a
higher increase in temperature towards the edges of the panel, which is caused by the edgeeffects, as explained in chapter 1. As for the samples with the susceptor particles, the hightemperature-zone is, again, limited to the width of the coil itself.
30
Susceptor Analysis
Figure 2.18: The heat- affected zone for the CFRP sample exposed under a one- turn coil,
60 A and 5 mm coupling
Figure 2.18 compares the average temperature of the heat- affected zone with the maximum
temperature for a one- turn coil set- up with 60 A coil current and 5 mm coupling distance.
One can see a significant difference of up to 15% (fig.2.19). The temperature of interest when
developing an induction bonding process using CFRP is the maximum achieved temperature,
as one has to ensure it does not exceed the disintegration temperature of the CFRP. Temperatures shown in the graphs of this chapter are therefore the maximum recorded temperatures.
Later stages of the project will also use the average temperature measurements, as these will
provide a better indication of the overall heat- transfer to the adhesive during the curing
cycle.
100
Max
Temperature [°C]
Average
80
60
40
20
0
50
100
150
Time [s]
200
250
Figure 2.19: Profiles for maximum and average temperature over the heat- affected zone of
the CFRP sample, 60 A 5 mm coupling
Effect of Coil Current
CFRP samples have been tested at different coil currents with the one- turn coil and a fixed
coupling distance of 5mm. As can be seen in Figure 2.20, CFRP generates already significant
amounts of heat at coil current as low as 40 A. Increasing the current up to 100 A results
in an uncontrollable temperature profile, reaching temperatures of more than 115◦ C in less
than 30 seconds.
2.4 Results & Discussion
31
120
20A
40A
60A
80A
100A
Temperature [°C]
100
80
60
40
20
0
50
100
150
Time [s]
200
250
Figure 2.20: Temperature profiles for the CFRP under different coil currents, 5mm coupling
Effect of Coupling Distance
As for the coil current, the coupling distance is a second main parameter used to adjust the
magnetic field strength. A standard coil current of 60 A has been used for the one- turn coil
to assess the effect of different coupling distance (fig. 2.21. A coupling distance of less than
3 mm is considered not to provide sufficient amounts of temperature controllability.
120
1mm
5mm
10mm
15mm
25mm
Temperature [°C]
100
80
60
40
20
0
50
100
150
Time [s]
200
250
Figure 2.21: Temperature profiles for the CFRP at different coupling distances, 60 A
Effect of Coil Geometry
Figure 2.22 assesses the effect of different coil geometries for CFRP adherends. It compares
the heat generation for the pancake coil and the one- turn coil at a coil current of 60 A and a
coupling distance of 5 mm. As for the susceptor- assisted process, the temperature increases
much faster by using the pancake coil, compared to the one- turn coil.
32
Susceptor Analysis
140
One−turn coil
Pancake coil
Temperature [°C]
120
100
80
60
40
20
0
2
4
6
Time [s]
8
10
Figure 2.22: Temperature profiles for the CFRP with different coil geometries, 60 A and 5
mm coupling
Figures 2.23 and 2.24 show the heat-affected zone as measured by the IR camera and shows
the difference in heat-affected zone. This could possibly explain the increased heating rate
when using the pancake coil. The pancake coil is therefore considered to be inappropriate
for the intended application, as temperature controllability is poor, and only a small area of
CFRP will have to be heated for the bonding process in scope.
Figure 2.23: The heat-affected area
with a one-turn coil
Figure 2.24: The heat-affected area
with a pancake coil
Adhesive Heating Through Thermal Conduction
Until now, this chapter has focussed on assessing the heat-generating characteristics of pure
CFRP material. The final objective of this project however, is to develop an induction- heated
adhesive bonding process, consisting of CFRP adherends and paste adhesive. Despite having
superb heat- generating characteristics, one could argue that the susceptorless inductionheated process of CFRP adherends looses some performance as the heat, after being generated
by the CFRP, still has to be transfered to the adhesive through conduction.
2.4 Results & Discussion
33
In order to understand this effect, adhesive was applied at the end of the CFRP adherends,
as can be seen in Figure 2.25. This test procedure was chosen specifically as it represents
closely the set- up of a single lap-shear specimen. Different bond thickness’s were applied
and the temperature was measured at the CFRP and at the surface of the adhesive by IR
measurements.
Figure 2.25: Test specimen of CFRP material + adhesive
Temperature [°C]
100
1mm
2mm
4mm
CFRP
80
60
40
20
0
50
100
Time [s]
150
200
Figure 2.26: Temperature profiles for the adhesive + CFRP set-up, 60 A and 3 mm coupling
As can be seen in Figure 2.26, the 1 mm layer of adhesive almost reaches the same temperature profile as the bare CFRP. Increasing the thickness of the adhesive layer reduces
the temperature increase significantly, but can be considered to be irrelevant for aerospace
applications, as typical thickness’s used in structural applications range from 0,25 up to 1
mm.
34
Susceptor Analysis
2.5
Preliminary Conclusions
The objective of this first part of the master thesis is to assess the most important materialand process parameters and their effect on the heat generating characteristics of the induction
heating system. The results should give an indication of what combinations of material- and
process parameters provide a feasible and efficient heating technique for the adhesive bonding
application in scope. Based on the results obtained, the following conclusions could be drawn:
• Temperature controllability for CFRP adherends is only feasible at coil currents below
100 A for coupling distances up to 10 mm. Lower coil currents are required for closecontact induction heating (<3 mm) not to overheat the material
• After an initial heat- up stage, CFRP adherends can be kept at 100◦ C with coil currents
of 30-60 Amperes, for coupling distances of up to 5mm.
• At 2v%, Iron- powder susceptor particles do not generate any measurable temperature
increase at coil currents below 100 A, increasing the amount of susceptor particles in
the adhesive results in a better heat generation at lower coil currents, but might impact
the mechanical performance of the joint
• Graphene- Oxide does not generate any heat under induction heating. Because of the
limited time- frame available, further investigation of Graphene Oxide was considered
to be irrelevant for reaching the project’s objectives.
• Combining CFRP- and susceptor- particle heating in one set- up is not considered to
enhance the induction heating process, as either CFRP material will overheat at higher
coil currents, or the susceptor particles will not contribute at lower current set- ups
The conclusions stated above provide a first indication of the incompatibility of susceptorassisted induction heating in combination with CFRP adherends. The coil current and coupling distance required for triggering the heat- generation through susceptor particles is far
higher than the allowable threshold set for not overheating the CFRP material. Figure 2.27
combines the susceptor- assisted and CFRP temperature profiles for a one- turn coil set- up
with 100 Amperes coil current and 5mm coupling distance. As one can see, 2v% of Ironpowder does not generate any significant amount of heat yet, while the CFRP ahderends
reach a temperature of 100◦ C in less than 200 seconds.
2.5 Preliminary Conclusions
35
120
Fe−2v%
CFRP
Temperature [°C]
100
80
60
40
20
0
50
100
Time [s]
150
200
Figure 2.27: Temperature profiles the susceptor- assisted set- up and bare CFRP material,
100 A and 5 mm coupling
Several possibilities have been assessed in order to improve a possible matching between both
techniques. One solution would be to increase the field’s frequency. Hysteresis heating of
the susceptor particles is mainly depended on the applied frequency, whereas Joule heating
of CFRP is mainly driven by the electro- magnetic field strength. An ideal set- up to combine both techniques would therefore be a high- frequency and low- strength magnetic field.
However, a 400 KHz frequency is already considered as ’high- frequency induction heating’.
No equipment has been found on the market today that can provide higher frequencies. Beside that, increasing the frequency even more would not result in a normal induction heating
process anymore, as different physical aspects will show their influence, such as radiation
reflection.
Another solution would be to decrease the particle size of the susceptor. By doing so, a
larger contact surface would exist between the adhesive and the particles, increasing the
heat transfer and thereby improving the temperature- increase of the sample. One test was
performed with significantly smaller particles of Iron powder. The difference in coil current
required was, however, still considered to be too much in order to have an efficient combined
process.
A final possibility would be to design the induction system in such a way that the adhesive
and susceptor particles are mostly affected by the magnetic field. A possible solution would
be to develop a specific coil, focussing the magnetic field on the bond layer and cancellingout the magnetic field above and below, where the CFRP is attached. However, as only very
thin layers of adhesive will be used, this technique won’t allow a precise heating of only the
adhesive layer and the CFRP would still be heat- affected.
36
2.5.1
Susceptor Analysis
Impact on the Further Progress of the Project
Unfortunately, all of the proposed solutions to achieve a combined CFRP/susceptor- particle
set- up have been evaluated to be infeasible within the scope of this project. As the project’s
main objective is to assess the impact of the susceptor particles on the curing process of the
adhesive, a different adherend material has to be selected. Aluminium is also widely used
within the aerospace industry, making it an interesting adherend material for the project in
scope. Aluminium, as CFRP, has good induction heating characteristics, making it difficult
to achieve a feasible, combined set- up. Experiments performed with 2 mm thick AL6068
aluminium showed a required coil current of 150 Amperes to maintain 85◦ C with a coupling
distance of 5 mm. At this specific set- up, added susceptor particles to the adhesive would
again not generate significant amounts of heat.
The decision is therefore made to continue the analysis with non- conductive glass- fibrereinforced- plastics (GFRP). This material does not generate any heat when exposed to the
induction field and thereby does not interfere with the heat generation through the susceptor
particles. The next step will be to assess the characteristics of the adhesive itself, and to
assess its performance when increasing the curing temperature/reducing the cure time.
Chapter 3
Modelling of the Induction Heating
Process
3.1
Introduction
The previous chapter of this report has assessed experimentally the effect of different process
parameters on the induction heating process. In order to understand better the obtained
results, a model of the induction heating process was made in COMSOL Multiphysics. Additionally, the effect of certain parameters on the induction heating process can be investigated
by using the model, which could not be assessed by the experimental set- up available in the
lab.
As modelling of the induction heating process was considered not to be the main focus of this
project, only a basic model was made of the susceptor- assisted induction heating process. It
does not model the heating of individual susceptor particles within the adhesive. Instead, it
represents the adhesive and Iron particles as a uniform, single material, having homogeneous
properties. Additionally, all required parameters for the modelling process were obtained
either from literature, or estimated through own technical insights in the induction heating
process.
The objectives of this chapter could be summarized as follows:
• Develop a model of the susceptor- assisted induction heating process
• Validate the model by using the results obtained in chapter 2
• Assess the effect of the adhesive’s electrical conductivity on its heat- generation properties
• Evaluate the effect of the field frequency on the induction heating process
38
Modelling of the Induction Heating Process
3.2
Development of the Model
Modelling of the induction heating process was performed in version 5.1 of COMSOL Multiphysics. This software provides a set of build- in, predefined interfaces with associated
modelling tools, which are referred to as physics interfaces. The model can be divided into
four main blocks. Figure 3.1 shows a schematic representation of the model, including all
inputs, model blocks and outputs.
Figure 3.1: Schematic representation of the induction heating model
The model simulates a three- dimensional induction heating process of a one- turn coil and a
sample of adhesive directly underneath it. It thereby represents the set- up used in section .
The dimensions of the one- turn coil were taken from the actual coil used in the lab. Other
relevant process parameters, such as the coil current, coupling distance and adhesive thickness
could be implemented within the model in order to simulate a certain set- up. Figure 3.2
shows a graphical representation of the model.
Figure 3.2: The induction heating set- up as modelled for this project in COMSOL Multiphysics
3.2 Development of the Model
39
The geometrical lay- out of the model was configured within the model- builder of COMSOL
itself. The induction heating and heat transfer block of the model were created within the
"AC/DC" interface. As the standard induction heating interface of COMSOL does not take
hysteresis heating into account, a separate partial differential equation interface (PDE) was
added to include this second heating- mechanism. The heat transfer was modelled within
the "heat transfer" interface. Each of the sections below will elaborate in more detail on the
specific details and features of each part.
3.2.1
Geometrical Aspects
The model represents the induction heating process of a one- turn coil and an adhesive layer.
The geometrical parameters of the model are determined such that they represent the actual
set- up in the lab as close as possible. Table 3.1 summarizes the relevant parameters used to
model the geometrical aspects of the induction heating set- up.
Table 3.1: Geometrical parameters
Parameter
Coil length
Coil width
Coil outer diameter
Adhesive
layer
width/length
Coupling distance
Adhesive thickness
3.2.2
Symbol
L
W
D
W/L
Value
70 mm
20 mm
6 mm
90 x 90 mm
Source
Lab’s coil dimensions
Lab’s coil dimensions
Lab’s coil dimensions
-
CD
t
1 mm
3 mm
as used in the lab’s tests
test specimen from chapter 2
Magnetic Field
The magnetic field, generated by the alternating coil current within the one- turn coil, was
modelled by using the "Magnetic Fields" module within the "AC/DC" interface. The magnetic
field was modelled in the frequency domain, in order to be able to assess the effect of different
field frequencies on the induction heating process in a later stage of the modelling process.
The COMSOL interface allows to define a coil geometry, a coil current and a field frequency,
which COMSOL then relates to a magnetic field distribution within the model space. Table 3.2
summarizes the parameter values. Both the coil current- and frequency have been determined
from the actual set- up in the lab.
Table 3.2: Magnetic field parameters
Parameter
Coil current
Frequency
Symbol
I
ω
Value
400 A
400 KHz
Source
As used in the lab’s tests
As used in the lab’s tests
40
3.2.3
Modelling of the Induction Heating Process
Induction Heating
Heat is generated through two different mechanisms. First, Joule- heating takes place, as
Eddy- currents are induced within the material. Secondly, magnetic hysteresis losses are also
generating heat, due to the alternating electromagnetic field. The additional generation of
heat due to the curing process of the adhesive itself has not been taken into account.
Joule- heating is governed by Ampere’s law, which relates the magnetic field, generated by
the coil, to the induced current within the adhesive layer, as shown by equation 3.1.
−1
(jωσ − ω 2 0 r )A + 5 × (µ−1
0 µr B) = Je
(3.1)
In this formula, the electric current density Je is calculated as a function of the field’s frequency ω, the electrical conductivity of the adhesive σ, the material’s relative permittivity
& permeability r & µr respectively, the air’s permittivity & permeability 0 & µ0 and the
magnetic vector potential A. Finally, the magnetic flux density B is calculated according to
formula 3.2.
B =5·A
(3.2)
In order for the Joule- heating mechanism to be modelled correctly, some material properties
have to be known about the adhesive layer, such as its electrical conductivity, permeability
and permittivity. A summary of those parameters is shown in Table 3.3. If one would intend
to model the susceptor- assisted induction heating process as realistic as possible, one would
have to model individual Iron particles within the adhesive and assign each of both materials
their appropriate properties.
However, due to the added complexity of such feature, the decision was made to model
the adhesive layer as a uniform material, having homogeneous properties. Those "mixed"
properties were estimated as a "rule of mixture" between the volume- percentage of Iron
powder (2v%) and that of the adhesive (98 v%). Additionally, the adhesive + 2v% of Iron
powder was assumed to be non- conductive, thereby having an global electrical conductivity
close to zero.
Table 3.3: Joule- heating parameters
Parameter
Electrical conductivity
Relative permeability
(Adh + Fe)
Relative permittivity
(Adh + Fe)
σ
µr
Value
0.01 S/m
4000 [-]
r
1 [-]
Source
Isolating material
Iron : 200 000 [26]
Adhesive : 1 (non- magnetic)
(Final mixture : 2v% Fe + 98v% Adh.)
Iron : Adhesive : 1
(Final mixture : 2v% Fe + 98v% Adh.)
As mentioned before, the standard COMSOL interface does not take hysteresis effects into
account for modelling the induction heating processes. It makes the model more complex by
3.2 Development of the Model
41
adding non- linearity, as not only the current field intensity influences the physical properties
of the material, but also the history of the field distribution.
Adding the effect of hysteresis- heating was done through the implementation of vector hysteresis modelling, using the Jiles- Atherton theory. It explains hysteresis heating as a result
of two phenomena. First, heat will be generated through the changing magnetization of the
material, called anhysteretic magnetization Man . Additionally, the rotation of the material’s
grains, also called wall pinning, will result in frictional heating. The theory links the amount
of heat generated through hysteresis losses to a set of five material parameters. The five JilesAtherton parameters can be described as follows:
• Magnetic saturation, MS & shape factor, α
Two parameters describing the shape & size of the material’s H- B curve, thereby giving
a direct indication of the amount of magnetization energy to be released by the material
• Langevin Parameter, a
Mathematical parameter used to link the material’s H- B behaviour (MS & α) to the
amount of heat generated
• Pinning factor, k & domain rotation loss, c
Two parameters describing the effective energy release of the grains each time they
change orientation
The Jiles- Atherton theory was implemented within the induction heating model according to
a set of constitutive equations [27] [28]. The total amount of heat generated inside a material
is calculated as a function of the domain rotation, called anhysteretic magnetization Man and
the irreversible magnetization due to wall pinning Mirr . This relation is shown in equation
3.3.
dM
dMan
dMirr
=·
+ (1 − c) ·
dt
dt
dt
(3.3)
The reversible magnetization Man (Ms , a) is determined in equation 3.4, by using the Langevin
polynomial [29]. It depends on the Langevin parameter a and the effective magnetic field Hef f .
The latter is depending on the initial magnetic field H and material’s local field factor α, as
shown in equation 3.5.
Man = MS
Hef f
a
coth
−
a
|Hef f |
Hef f = H + α · M
!
Hef f
|Hef f |
(3.4)
(3.5)
The last element of equation 3.3, describing the effect of irreversible magnetization Mirr and
is calculated according to equation 3.6. This relation includes the material’s pinning factor
k, the domain rotation loss c and the reversible magnetization Mrev . The latter is calculated
according to equation 3.7.
42
Modelling of the Induction Heating Process
dHef f
dMirr
= (k −1 · c−1 · Mrev ) ·
dt
dt
k −1 · c−1 · Mrev
|k −1 · c−1 · Mrev |
Mrev = c · (1 − c)−1 · (M − Man )
(3.6)
(3.7)
The above equations only show a summarized version of the equations used by the JilesAtherton model. A more in- depth description of the required mathematical expressions can
be found in [29].
Hysteresis losses depend mainly on the Jiles- Atherton parameters. Table 3.4 shows all of
the parameters used for this block of the model, as found in literature [14]. As for the Jouleheating module, the parameters used to describe the hysteresis losses of the adhesive layer are
determined as a mixture of the properties of the non- magnetic adhesive and the ferromagnetic
Iron particles. The amount of heat generated by hysteresis losses is depending on the size of
the H- B curve. Within the Jiles- Atherton method, this material characteristic is described
by all parameters, except the reversibility coefficient c and the local field factor α. Taking a
"rule of mixture" on the remaining parameters were therefore considered to be sufficient to
provide a first- order indication on the hysteresis- heating within the adhesive layer. The JilesAtherton parameters of the adhesive were assumed to represent a non- magnetic material,
thereby all being equal to zero.
Table 3.4: Hysteresis- heating parameters [14]
Parameter
Saturation
magnetization
Symbol
MS
Value
26.8 × 103
A/m
Source
Iron : 1.3445 × 106 A/m
Adhesive : 0 (non- magnetic)
(Final mixture : 2v% Fe + 98v% Adh.)
Langevin
parameter
a
45 A/m
Iron : 2.26 × 103 A/m
Adhesive : 0 (non- magnetic)
(Final mixture : 2v% Fe + 98v% Adh.)
Pinning
factor
k
29.7 A/m
Iron : 1.4842 × 103 A/m
Adhesive : 0 (non- magnetic)
(Final mixture : 2v% Fe + 98v% Adh.)
Domain
rotation loss
c
0.7476
Iron : 0.7476
No mixture required
Shape
factor
α
-0.0044
Iron : -0.0044
No mixture required
3.2 Development of the Model
3.2.4
43
Heat Transfer
Once heat is generated through induction, it is exchanged with the adhesive’s surrounding
in order to reach thermal equilibrium. Equation 3.8 shows the equation used by COMSOL’s
heat transfer interface to calculate the thermal equilibrium.
ρCp
dT
+ 5(−k 5 T ) = Q
dt
(3.8)
In this equation, the amount of heat generated by induction Q is presented as a function of the
material’s temperature T, density ρ, its specific heat capacity Cp and its thermal conductivity
k. Each of those parameters has been taken from literature and given in table 3.5.
Table 3.5: Heat transfer parameters
Parameter
Adhesive density
Specific heat capacity
Thermal conductivity
Symbol
ρ
Cp
k
Value
1.04 g/cm3
1300 J/(kg ·K)
0.72
Source
Product’s datasheet
[30]
Product’s datasheet
44
3.2.5
Modelling of the Induction Heating Process
Validation of the Model
Validation of the model was performed by comparing the output of the model against the
experimentally obtained results from chapter 2. Three set- ups, including differences in coupling distance and coil current, have been validated against actual experimental data. Table
3.6 summarizes the relevant process parameters for the selected validation test.
Table 3.6: Description of the validation test set- up
Parameter
Coil geometry
Coupling distance
Coil current
Frequency
Susceptor material
Particle content
Adhesive thickness
Set- up 1
One- turn
1 mm
400 A
400 kHz
Iron powder
2v%
3 mm
Set- up 2
One- turn
1 mm
300 A
400 kHz
Iron powder
2v%
3 mm
Set- up 3
One- turn
3 mm
400 A
400 kHz
Iron powder
2v%
3 mm
Figure 3.3 shows the temperature profile generated for test set- up 1. It shows the model’s
output both with and without the Jiles- Atherton equations included. As one can see, the
model does not represent the actual induction heating process when hysteresis losses are not
taken into account. Adding the Jiles- Atherton relations to the model results in a accuracy
of the model of approximately 15%, which is considered to be acceptable within the context
of this experimental project. This discrepancy is most probably the result of the "ruleof- mixture" assumption made in order to model the adhesive layer and the assumed JilesAtherton parameters, as found in literature. Further validation tests on set- up 2 & 3 can
be found in appendix B. General trends for each of the varied parameters were found to be
consistent in comparison to the experimental data.
90
80
Experimental result
Model − no hysteresis
Model − hysteresis included
Temperature [°C]
70
60
50
40
30
20
0
50
100
150
200
Time [s]
Figure 3.3: Temperature profile of the validation test for set- up 1
3.3 Results
3.3
45
Results
Chapter 2 has experimentally assessed the effect of different process- and material parameters on the induction heating process. However, the lab’s set- up did not allow a complete
assessment of all relevant parameters. For example, the frequency of the magnetic field was
automatically defined by the induction equipment. Additionally, the electrical conductivity
of the adhesive could not physically be modified in the lab. The effect of those parameters
will be assessed in more detail in the sections below.
3.3.1
Effect of Field Frequency
A last process parameter, of which the effect could not be assessed by the available experimental set- up, is the field’s frequency. The EasyHeat unit determines itself the resonance
frequency of the circuit, based on the attached coil and power settings. In combination with
the one- turn coil, the lab’s set- up provided a frequency of 400 KHz.
Different field frequencies were inserted in the model, ranging from 200 KHz up to 600 KHz.
Increasing the frequency beyond this value was considered as inappropriate, as high- frequency
induction heating is normally considered not to exceed this frequency [26]. The resulting
temperature profiles for each set- up can be seen in figure 3.4.
110
100
Temperature [°C]
90
200 KHz
300 KHz
400 KHz
500 KHz
600 KHz
80
70
60
50
40
30
20
0
50
100
150
200
Time [s]
Figure 3.4: Model prediction of the effect of the field’s frequency on the induction heating
process, one- turn coil, 400 A and 1 mm coupling distance
At first sight, the model shows improved heat- generating characteristics at higher frequency
levels. The obtained results should be treated carefully, as increased field frequencies result
in different physical aspects to intervene with the actual induction heating process, such as
the absorption ratio of wave energy, which are not included in the model.
46
Modelling of the Induction Heating Process
3.3.2
Effect of the Adhesive’s Conductivity
From the experiments performed on induction heating of CFRP material, one can see the
effectiveness of Joule- heating as a primary heat- source for the induction heating process. If
therefore the electrical conductivity of the adhesive could be modified, its induction heating
behaviour could significantly improve.
For this experiment, no hysteresis losses were taken into account, in order to be able to isolate
the effect of electrical conductivity and the resulting Joule- heating mechanism. Different
levels of electrical conductivity were implemented in the model, and the resulting temperature
profile was constructed, as can be seen in figure 3.5.
160
140
10 S/m
20 S/m
30 S/m
40 S/m
50 S/m
Temperature [°C]
120
100
80
60
40
20
0
50
100
150
200
Time [s]
Figure 3.5: Model prediction of the effect of electrical conductivity on the induction heating
process, one- turn coil, 400 A and 1 mm coupling distance
For the magnetic field induced by a one- turn coil at 400 Amperes coil current, combined with
a coupling distance of 1 mm, an electrical conductivity of only 20 S/m results in comparable
heat- generating characteristics of an adhesive layer including 2v% of Iron powder.
3.4 Discussion
3.4
3.4.1
47
Discussion
Model Accuracy
This chapter has focused on the development of the susceptor- assisted induction heating
process. One has to evaluate however the performance of the model. The accuracy of the
model was assessed to be within a 15% window of the experimentally obtained results.
All temperature profiles extracted from the model were found to be below the actual temperature profile tested in the lab. This means that the model under- simulates the heat- generating
characteristics of the adhesive/Iron powder combination. The most probably cause of this
discrepancy is the fact that Iron particles were not modelled as individual entities within
the adhesive layer. Secondly, all parametric values used in the model were based on literature, rather than being experimentally obtained by own experimental research. The model is
thereby considered to simulate the induction heating process conservatively.
Global trends were however considered to be in line with the experimental results obtained in
chapter 2 for each of the assessed parameters. The model was therefore considered to provide
sufficient accuracy within the scope of this project.
3.4.2
Electrical Conductivity
The most important conclusion drawn from the model’s output is the potential of improving
the induction heating process significantly by modifying the model’s electrical conductivity.
The addition of susceptor particles is considered not to form the required closed loops within
the adhesive [8]. The combination of adhesive and Iron particles was modelled as being nonconductive. Therefore, Joule- heating is not triggered and heat can only be generated through
hysteresis losses.
Increasing the adhesive’s electrical conductivity to about 20 S/m resulted in comparable heatgenerating characteristics as those of an adhesive layer including 2v% of Iron powder. One
has to assess however if such modifications are actually feasible. As a matter of background
information, the electrical conductivity of Iron is 1 × 107 S/m, that of Carbon is 1 × 108 S/m.
An electrical conductivity of 20 S/m could be compared to that of conductive polymers, such
as PSS.
Several research projects have focussed on the development of so- called electrically conductive
adhesives (ECA). An often- used approach is to inject Silver flakes into the adhesive [31] [32].
The major disadvantage of this strategy is that it requires a very high Silver- content of about
30w% (approximately 80v%), which has a significant impact on the adhesive’s mechanical
performance.
Another strategy found in literature is to implement carbon nano tubes (CNT) into the
adhesivey layer [33]. The advantage of this technique is that it requires only very low amounts
of CNT to be added, approximately 1-2w%. An additional advantage of CNT is that they
positively influence the mechanical performance of adhesively bonded joints. The obtainable
electrical conductivity is however still very low, up to 1 S/m. The conclusion was therefore
made that induction heating of electrically conductive adhesives is not feasible yet for the
moment.
48
Modelling of the Induction Heating Process
3.5
Preliminary Conclusions
The objective of this chapter was to model the induction heating process of susceptor- assisted
adhesive. The following conclusions were made:
• Susceptor- assisted induction heating is mainly driven by hysteresis losses
• Despite not being considered feasible yet, an improvement of the electrical conductivity
of the adhesive up to 20 S/m would result in a significant increase in heat- generating
characteristics
• Increasing the field’s frequency results in an improved heat- generation of the susceptor
particles
Chapter 4
Adhesive Characterization
4.1
Introduction
One of the objectives of this project was to assess the possibility of reducing the processing
time of the adhesive by applying alternative curing methods. Induction- heating can possibly
reduce the cycle time as heat is applied more directly to the bondline, which will be analysed
in the next chapter. This can be couple to optimizing the adhesive’s curing cycle, as increasing
the curing temperature could possibly result in shorter curing cycles.
In one of his research projects, Sanchez Cebrian has established a methodology for evaluating
and optimizing the cure cycle of epoxy adhesives [8]. By assessing a wide range of experimental
techniques, sanchez concluded that the cure characteristics of an epoxy can be analysed by
a combination of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC)
and mechanical testing at different curing temperatures.
The TGA analysis is performed to determine the onset temperature of degassing for the
epoxy in scope. This results in an initial upper boundary of the allowed curing temperature,
as higher temperatures would automatically result in excessive void formation. Thereafter, a
set of DSC experiments is performed to assess the impact of the curing temperature on the
degree of cure as a function of cure time. This determines mainly how much a cure cycle can
be reduced when the curing temperature is increased. In a final phase of the characterization
process, mechanical performance is tested by single lap- shear tests on samples, which are
cured at different temperatures. Mechanical testing will however not be treated in this chapter
of the report, as it is the main focus of the next chapter.
The purpose of this part of the project is to apply Sanchez’ methodology on the epoxy in
scope of this project, EC9323. The objectives of this phase can be summarized as followed:
• Identify the onset temperature of degassing for the adhesive in scope
• Assess the effect of cure temperature on the degree of cure of the adhesive and the time
required for a full- cure
50
4.2
4.2.1
Adhesive Characterization
Experimental Set- up
Thermogravimetric Analysis (TGA)
A first characterization of the EC9323 adhesive is done by assessing the onset of gas formation.
This material property is assessed through thermogravimetric analysis, better known as TGA.
Those experiments consist of a precise measurement of the mass change of a material when
exposed to a certain temperature.
When the temperature is increased, certain compounds of the adhesive’s material will start
to degas, resulting in a certain weight loss detected by the TGA analysis. Thereby, a TGA
analysis can assess the effect of increasing the curing temperature on the stability of the
adhesive. If the adhesive is cured at a temperature for which the epoxy shows significant
degassing, one can state that this will result in an increased formation of voids and thereby
reduces the mechanical performance of the joint. By determining the onset temperature of
degassing, an initial upper limit can be determined for the curing temperature to be used for
the bonding process.
The equipment used for the TGA experiment was the Pyris Diamond TG/DTA, manufactured
by Perking Elmer. The experiment was performed on a sample of uncured, bare adhesive
material, as the hypothesis was made that the Iron powder would not have any impact on the
cure chemistry of the epoxy. Preliminary DSC experiments have confirmed this statement to
be valid, which is discussed in more detail in section 4.3.
The initial temperature used for the analysis was set at 30◦ C, a stable temperature at which
the adhesive does not show any weight loss yet. Thereafter a heating rate of 20◦ C per minute
was used until a maximum of 200◦ C was reached. A summary of the experimental set- up
can be seen in table 4.2.
Table 4.1: TGA experimental set- up
Equipment
Inert Gas
Number of Samples
Sample Weight
Test Cycle
4.2.2
Pyris Diamond TG/DTA
Nitrogen
1
2.7 mg
Initial Temperature : 30◦ C
Final Temperature : 200◦ C
Heating Rate : 20◦ C / minute
Differential Scanning Calorimetry (DSC)
Fundamentals of Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) is a thermo- analytical technique. The basic working
principle of this technique is to measure the difference in amount of energy absorbed or
released by a certain material when compared to a reference, when both are exposed to
the same temperature. The technique is often used for determining phase transitions of
4.2 Experimental Set- up
51
mixtures of materials, such as assessing crystallization and determining the glass transition
temperature. In the field of adhesive bonding, DSC experiments can be used to assess the
curing characteristics of adhesives.
The primary output of a DSC experiment is the measured heatflow. A negative heatflow
means that the sample is taking- up energy from the environment (endothermic process),
while a positive heatflow indicates a release of energy of the sample to the environment
(exothermic process). As the curing process of an epoxy adhesive is exothermic, measuring
the amount of energy which is released by the adhesive can be used as an indicator of the
cure process that has taken place.
Sample Preparation
Samples where prepared few minutes before the start of the experiment, in order to be sure
that no significant curing activity has taken place already upon starting the measurement.
Around 20 mg of adhesive is poured into standard aluminium test- cups of 5 mm diameter.
Thereafter the cups are sealed, as can be seen in Figure 4.1. The sample weight is determined
before the start of the experiment, as it has to be taken into account afterwards for the data
processing.
Figure 4.1: Empty DSC test cup (left) including adhesive (middle) sealed (right)
Cure Cycle
Several cure cycles have been tested, in order to assess the effect of the Iron powder and the
cure temperature on the curing characteristics of the adhesive in scope. Each DSC experiment
was performed twice, in order to check for consistency among the data.
Once a DSC experiment was finished, it was immediately proceeded by a second, identical
cure cycle, on the same cured sample. This is done firstly to check that a full cure has
effectively taken place during the initial DSC cycle. The second reason for performing a postcure cycle is to measure the amount of energy required by the sample during the heat- up
and isothermal stage of the experiment.
52
Adhesive Characterization
The DSC equipment used for this project is an EXSTAR 6000 from SII Seiko Instruments.
It allows to program the intended cure cycle by several program parameters:
• Starting & finishing temperature: A fixed value of 25◦ C was used for all tests, as
it approximates the lab’s room temperature.
• Program temperature & time : This determines the cure cycle of the actual experiment. Initial tests where performed at the manufacturer’s recommended cure cycle
of two hours at 65◦ C. Additional tests where performed at higher curing temperatures,
which would allow a reduction in curing time. The program time used for the experiments was taken conservatively long for each temperature, in order to be sure that most
of the curing activity would have taken place already by the end of the initial cure cycle.
Table 4.2 shows the parameters used for each of the tested cure cycles.
Table 4.2: Cure cycles tested during the DSC analysis
Program
Temperature
65 ◦ C
80 ◦ C
95 ◦ C
110 ◦ C
* Recommended
by manufacturer
Program
Time
2h*
2h
1h
50’
• Heat- up & cool- down rate: A fixed value of 25◦ C per minute was used for the
heat- up stage. As the DSC machine used for these experiments is not equiped with
any cooling unit, only a cool- down rate of 1◦ C per minute could be achieved. As for
each cure temperature a conservatively high cycle time was inserted as program time,
most of the cure activity is assumed to have taken place already in the first part of
the cure cycle. Therefore, this relatively slow cooldown is considered not to affect the
experiment’s results significantly.
4.3 Results
4.3
4.3.1
53
Results
TGA
Figure 4.2 shows the results from the TGA experiment. Both weight-percentage (blue line, left
axis) and weight loss rate (green line, right axis) are depicted, as both provide an indication
for the material’s degradation. One can conclude that the material remains fully stable
until a temperature of about 50◦ C. According to Sanchez’ theory, the onset temperature
of degassing can be estimated by the intersection of the starting- and ending asymptote of
the weight-percentage graph [8]. According to this methodology, the onset temperature for
EC9323 was estimated to be 130 degrees Celsius, as can be seen in Figure 4.2.
By looking at the curve of the weight loss rate however, one could already identify a first
weight loss peak at 110◦ C. The decision was therefore made to use 110 degrees as the onset
temperature of degassing of the adhesive for further experiments.
Figure 4.2: TGA Analysis for EC9323 adhesive
54
4.3.2
Adhesive Characterization
DSC Analysis
The previous section has established the initial boundary on the maximum cure temperature
to be 110 ◦ C. This leaves a set of possible cure temperatures, ranging from the manufacturerrecommended temperature of 65◦ C until the gas- formation onset temperature of 110◦ C. This
chapter will assess how the cure cycle is affected by altering the cure temperature, as well as
the effect of the Iron powder on the cure chemistry of the adhesive. All of those aspects have
been analysed by Differential Scanning Calorimetry (DSC).
Data Processing
Figure 4.3 shows a raw- data plot of a DSC experiment performed with EC9323 adhesive on
a cure cycle of two hours at 65◦ C. It shows the heat flow in milliwatt as a function of time on
the left y- axis and the program’s temperature on right y- axis. One can see a negative initial
heatflow up to the moment the program reaches its final temperature, which is explained by
the energy required for the heating- up the sample. Thereafter, the heat flow starts to become
positive, as the curing process releases heat.
After approximately 60 minutes, the energy balance becomes negative again, as the amount
of energy required for keeping the sample at 65 ◦ C is higher than the amount of energy still
released by the curing process. Towards the end of the experiment, the heat flow levels- out
at a constant, negative value of approximately 0.5 milliwatt. This is the amount of energy
required for keeping the sample at 65◦ C and the adhesive can be considered as been fully
cured.
Figure 4.3: A heatflow graph of a sample cured at 65◦ C for 2 hours
Figure 4.4 shows for the same sample the raw data plot of the post- cure cycle. As one
can see, the second cure cycle levels- out at a constant, negative value of approximately -0.5
4.3 Results
55
milliwatt straight after the heat- up stage. It is therefore considered that this sample was
fully cured during the initial cure cycle of the experiment. Figure 4.5 on the other hand,
shows the energy balance of a sample which was only cured for 45 minutes during its initial
cure cycle. As one can see from its post- cure cycle in Figure 4.6, heat- flow from the sample
to the environment was still taking place after the initial cure cycle at 65◦ C was finished, as
the post- cure heatflow does not balance out at a constant value straight after the heat- up
stage.
0
80
−2
60
−3
−4
40
Temperature [°C]
Heatflow [mW]
−1
−5
−6
0
20
40
60
80
100
120
20
140
Time [s]
Figure 4.4: Post-cure cycle, 2 hours at 65◦ C
3
100
0
70
−1
60
−2
50
−3
40
−4
30
2
50
−2
Temperature [°C]
−1
Heatflow [mW]
0
Temperature [°C]
Heatflow [mW]
1
−3
−4
0
10
20
30
40
0
50
Time [s]
Figure 4.5: Initial cure cycle (no fullcure), 45 minutes at 65◦ C
−5
0
10
20
30
40
50
60
70
20
80
Time [s]
Figure 4.6: Post-cure cycle, 45 minutes at 65◦ C
A first manipulation of the data is done by filtering the heatflow which is not related to the
curing process. This is done by subtracting the heatflow obtained in the post-cure cycle from
the data of the initial cure cycle. Figure 4.7 shows both the heat flow for the raw data and
the one for the isolated cure energy. As one can see, the initial negative heatflow- peak is
56
Adhesive Characterization
removed and the heatflow now approaches zero towards the end of the cure cycle. The little
irregularity present in the beginning of the cycle comes from the fact that the post- cure cycle
is started while the sample’s temperature is not perfectly balanced yet at room temperature.
The post- cure cycle therefore requires slightly less energy to heat the sample up to curing
temperature. This is considered not to impact the analysis significantly, as the assessment of
the cure cycle will mainly be done by focussing at the end of the cure cycle, rather then the
beginning.
3
Total energy
Cure energy
2
Heat Flow [mW]
1
0
−1
−2
−3
−4
−5
−6
−7
0
20
40
60
80
100
120
140
Time [Minutes]
Figure 4.7: Isolated cure- energy plot, 2 hours at 65◦ C
As mentioned before, not each DSC sample contains precisely the same amount of adhesive.
In order to compare different experiments, one has therefore to construct normalized heatflow graphs per unit of mass. An example of such normalized heat- flow graph can be seen
in the graphs below. Two samples of different weight have been cured for two hours at 65◦ C.
As sample II has a higher mass, the total heatflow coming out of this sample is larger than
that of sample I (fig. 4.8. When looking however at the normalized energy balance in Figure
4.9, one can see that both samples have exactly the same heatflow per unit of mass.
4.3 Results
57
0.15
Sample 1 (22,5 mg)
Sample 2 (28,7 mg)
3
Heat Flow [mW]
2
1
0
Normalized Heat Flow [mW/mg]
4
Sample 1 (22,5 mg)
Sample (28,7 mg)
0.1
0.05
0
−0.05
−1
−2
−0.15
−3
−4
0
−0.1
20
40
60
80
100
120
140
Time [Minutes]
Figure 4.8: Total cure- energy, 2 hours
at 65◦ C
−0.2
0
20
40
60
80
100
120
140
Time [Minutes]
Figure 4.9: Normalized cure- energy,
2 hours at 65◦ C
58
Adhesive Characterization
4.4
Discussion
4.4.1
Effect of Iron Powder on the Curing Chemistry
A first important aspect to consider for the project in scope is to assess the effect of the
susceptor particles on the curing chemistry of the adhesive. Two cure cycles were used for
this assessment, including the manufacturer’s recommended cure cycle of two hours at 65◦ C
and one hour at 95◦ C. Each cure cycle was performed twice with bare adhesive and twice
with a 2v% of Iron powder mixed in the adhesive, in order to check for consistency of the
data.
Figure 4.10 shows the normalised cure- energy plot per unit of mass at the manufacturer’s
recommended cure cycle. One can see a shift downwards in the energy balance for the sample
including Iron powder. This is firstly explained by the fact that the Iron powder absorbs a
certain part of the energy released by the adhesive, as the powder also has to maintain the
cure cycle’s temperature. Additionally, the weight of the Iron powder was taken into account
when normalizing the energy balance per unit of weight. If the weight of the Iron powder is
not taken into account, one can see that the susceptor- particle sample coincides better with
the reference sample, as can be seen in figure 4.11. When performing the same test for a cure
cycle of one hour at 95◦ C the same findings are seen from the heatflow graphs, as can be seen
in figures 4.12 and 4.13.
0.15
0.15
0.1
0.05
0
−0.05
−0.1
−0.15
−0.2
0
Bare adhesive
Adhesive + 2v% Fe
Normalized Heat Flow [mW/mg]
Normalized Heat Flow [mW/mg]
Bare adhesive
Adhesive + 2v% Fe
20
40
60
80
100
120
140
Time [Minutes]
Figure 4.10: Normalized cure- energy
for sample with 2v% Iron powder, 2
hours at 65◦ C
0.1
0.05
0
−0.05
−0.1
−0.15
−0.2
0
20
40
60
80
100
120
140
Time [Minutes]
Figure 4.11: Corrected normalized
cure- energy for sample with 2v% Iron
powder 2 hours at 65◦ C
4.4 Discussion
59
0.6
0.6
Bare adhesive
Adhesive + 2v% Fe
0.4
Normalized Heat Flow [mW/mg]
Normalized Heat Flow [mW/mg]
Bare adhesive
Adhesive + 2v% Fe
0.2
0
−0.2
−0.4
−0.6
−0.8
0
10
20
30
40
50
60
70
0.4
0.2
0
−0.2
−0.4
−0.6
−0.8
0
Time [Minutes]
Figure 4.12: Normalized cure- energy
for sample with 2v% Iron powder, 1
hour at 95◦ C
10
20
30
40
50
60
70
Time [Minutes]
Figure 4.13: Corrected normalized
cure- energy for sample with 2v% Iron
powder 1 hour at 95◦ C
As the heatflow of the ’adhesive + Iron powder’ - sample does not show any different trends
than that of the reference sample of bare adhesive , the conclusion is made that the susceptor
particles do not interact with the curing properties of the adhesive. Also at higher curing
temperatures, such as 95◦ C, the Iron powder does not interact with the adhesive’s curing
characteristics.
4.4.2
Effect of Increasing Cure Temperature
One of the main objectives of the DSC experiments is to determine required curing time to
reach a fully cured state at a certain temperature. A two hours at 65◦ C cure cycle is once
again used as reference, since its the one recommended by the manufacturer. Additional
curing cycles at 80,95 and 110◦ C are tested, as shown in table 4.2. Each cycle is performed by
two specimen, to check for consistency of the data. As the previous section has proven that
the Iron powder does not influence the adhesive’s curing characteristics, the current analysis
is performed only on samples of adhesive without susceptor particles.
Figure 4.14 shows the normalized heatflow graphs of each cure cycle performed. As one can
see, the cure cycle shifts towards the left for higher temperatures, which indicates a reduction
in required curing time.
60
Adhesive Characterization
2h/80C
0.6
0.4
0.2
0
−0.2
−0.4
0
50
100
150
Normalized Heat Flow [mW]
Normalized Heat Flow [mW]
2h/65C
0.8
0.8
0.6
0.4
0.2
0
−0.2
−0.4
0
Time [Minutes]
50
0.6
0.4
0.2
0
−0.2
50
100
Time [Minutes]
150
1h/110C
150
Normalized Heat Flow [mW]
Normalized Heat Flow [mW]
1h/95C
0.8
−0.4
0
100
Time [Minutes]
0.8
0.6
0.4
0.2
0
−0.2
−0.4
0
50
100
150
Time [Minutes]
Figure 4.14: Normalized cure- energy of adhesive without susceptor particles, for cure
temperatures of 65◦ C, 80◦ C, 95◦ C and 110◦ C
Measuring the degree of cure can be done by measuring the ratio of the amount of curing
energy that has been released already upon that specific time, and the total amount of cure
energy present in the full cure cycle. One can say that a sample has been fully cured once the
degree- of- cure- ratio- has reached a certain value. For the recommended cure cycle of two
hours at 65◦ C, an average degree- of- cure of 99% was found for the tested samples. When
using this method for comparing cure cycles at different temperatures, one has to be aware
of the sensitivity of this method for small deviations in heatflow during the heat- up stage of
the experiment.
Another method of describing a full- cure is by defining a certain value of heatflow to be
reached upon reaching the fully cured state. As one knows from the manufacturer and from
preliminary testing, a full- cure will have taken place in two hours at 65◦ C, as seen in figures
4.3 and 4.4. By measuring the remaining heatflow per unit of mass at the end of this cycle,
one knows the residual heatflow present in the material after obtaining a full-cure. A residual
heatflow of 0.0045 milliwatt per unit of mass has been determined to be present after two
hours of curing at 65◦ C.
The required curing time for different temperatures can now be determined as the time at
which the heatflow reaches 0.0045 milliwatt per unit of mass. This method is not affected by
the heat- up stage of the cure cycle and can therefore be considered to provide a more reliable
way of defining the required full- cure cycle time at different curing temperatures.
The required curing time can be reduced significantly when higher curing temperatures are
used, but might have an effect on the mechanical performance. Table 4.3 a summary of the
results from figure 4.14. The most significant time reduction is achieved between 65◦ C and
4.4 Discussion
61
80◦ C. Increasing the curing temperature beyond 95◦ C does not have a significant impact on
the required curing time anymore.
Table 4.3: Required curing time for different temperatures to reach a fully cured state
Program
Temperature
65 ◦ C
80 ◦ C
95 ◦ C
110 ◦ C
Program
Time
2h
1h11’
51’
47’
From the conclusions made above one could state that a significant cycle time reduction can be
achieved by increasing the cure temperature of the adhesive. One aspect which has not been
covered however is the impact of this increased temperature on the mechanical performance
of the joint. This will be assessed in the third section of this chapter.
62
Adhesive Characterization
4.5
Preliminary Conclusions
The second part of this project has assessed some physical properties of the adhesive in scope.
Several experiments have been performed in order to assess the curing chemistry- and speed.
The following conclusions have been made according to the cure chemistry of EC9323:
• The onset of degassing occurs at a curing temperature of 110◦ C, identified by an increased weight loss rate at this temperature
• A 2v% of Iron powder does not impact the curing chemistry and thereby does not
require a modification of the standard recommended cure cycle
• The standard recommended cure cycle of two hours at 65◦ C can be reduced by increasing the curing temperature as follows:
- 1h11’ at 80◦ C
- 51’ at 95◦ C
- 47’ at 110◦ C
One has to keep in mind however, that increased curing temperatures might affect the mechanical performance of the adhesive, which will be discussed in the next chapter.
Chapter 5
Mechanical Testing
5.1
Introduction
An important aspect of evaluating new joining methods is to understand the process’ influence
on the mechanical performance of the joint. This chapter will provide a step- wise investigation
of the impact of induction curing on the mechanical performance of standard, single lap- shear
(SLS) bonded joints.
First, several aspects of the project will be assessed individually, such as the effect of the
cure temperature and the effect of susceptor particles on the lapshear strength. Thereafter,
a comparison will be made between induction- cured and oven- cured samples. Those results
will be used in a later stage for the assessment of induction- heated adhesive bonding as a
possible future joining technique for the aerospace industry.
The objectives of this phase can be summarized as followed:
• Assess the effect of different cure temperatures on the mechanical performance of bonded
joints
• Investigate the effect of adding susceptor particles on the mechanical performance of
bonded joints
• Assess the influence of induction curing on the mechanical performance of bonded joints,
as compared to oven- curing
64
5.2
5.2.1
Mechanical Testing
Materials & Specimens
Materials
Adhesive
The adhesive used is the EC9323-B/A structural epoxy, produced by 3M, as used in the
previous parts of this project. A more detailed description can be found in section 2.2.
Susceptor Particles
Only Iron- powder is used as susceptor particle for this phase of the project, as it was selected
in chapter 2 as being the most effective heat generator. A more detailed description can be
found in section 2.2.
GFRP
The GFRP material used for the adherends in these experiments consisted of eight layers
of 0◦ / 90◦ glass fibre fabric and HEXION RIM 235 epoxy resin. It was produced through
vacuum infusion as a large plate, measuring 550 mm by 550 mm. Therafter, the plate was cut
into single lap-shear test specimen through automatic cutting/grinding. The final sample’s
dimensions were 101.6 mm by 25.4 mm by 2.1 mm, as can be seen in Figure 5.1.
Figure 5.1: GFRP specimen used for producing the single lap- shear test coupons
5.2.2
Specimens
Mechanical testing has been performed on standard single lap- shear joints, in accordance to
ASTM standard D5868. This test method has been developed in complement to standard
D1002, which is the general standard used for lap- shear testing of metal adherends. D5868
extends D1002’s applicability towards single lap- shear joints of fibre- reinforced plastic adherends.
5.2 Materials & Specimens
65
One deviation has however been made from the ASTM standard. The production method
prescribed in the ASTM D5868 is to adhesively bond larger test panels first. Only after curing
has taken place, the bonded panels are then cut into individual lap- shear specimen. This is
mainly done in order to achieve a better accuracy in the alignment of both sides of the lapshear joint.
However, due to the size limitation of the induction heating set- up available in the lab, lapshear samples for this experiment had to be cured individually. GFRP panels were therefore
cut first into lap- shear coupons, before they were bonded together. In order to have an equal
production process for all tested coupons, the decision was made also to cure the oven- cured
samples individually. Figure 5.2 shows all related dimensions of the lap- shear specimen used
for those mechanical tests.
Figure 5.2: Single lap- shear specimen used for mechanical testing [13]
The recommended bond layer thickness for lap- shear joints is 100 µm, as specified by 3M.
However, the Iron susceptor particles used for these experiments have a size of 200 µm. This
means that the susceptor- assisted lap- shear specimen would not be able to achieve a bond
layer thickness of 100 µm. The decision was therefore made to opt for a 200 µm bond layer
thickness for all tested specimen, such that all test specimen would have the same dimensions.
Lap- shear Specimens Production
The production of the single lap- shear specimens consisted out of four major phases, being
the adhesive preparation, surface pretreatment,assembling and curing phase.
Just prior to application, a 1w% of 200 µm glass beads was mixed- in manually into the
adhesive, for controlling the thickness of the bond layer. If susceptor particles had to be
added to the adhesive as well, this was done at the same time. This was done in order to
avoid an additional mixing- stage, which could possibly increase the risk of gas formation
within the adhesive.
The surface of the bonding area of both GFRP specimen was carefully pretreated before the
adhesive was applied. First, the surface was physically abraded with sandpaper (mesh 80) and
66
Mechanical Testing
thereafter degreased with PF-SR, a solvent made by PSG and typically used in the aerospace
industry as cleaner and degreaser. This process of abrading and degreasing was repeated
one more time for sake of quality of the pretreatment. Thereafter the specimen were left for
fifteen minutes at lab temperature in order to allow the remaining PF-SR to evaporate.
As mentioned before, SLS specimens were assembled individually for these experiments. In
order to guarantee a correct alignment of the two adherends, a mould was used for the
assembly, as can be seen in Figure 5.3. Once the SLS specimen was assempled, it was
transferred onto a mould- type aluminium sheet, which supported the samples during the
oven- curing process. Because of the conductive properties of aluminium, a comparable
wooden support mould was used for the induction- cured samples. After the curing process
had been finished, a final alignment check was performed for sake of quality control. In case
of non- conformities, samples were discarded and reproduced.
Figure 5.3: Mould used for assembling individual single lap- shear specimen
Oven- cured samples were cured in a Heraeus T6030 Oven. It’s a small- scale oven, perfectly
suited to cure three lap- shear specimens at the same time, making it perfectly suitable to
the batch- size used in these experiments.
Tests for assessing the effect of the cure temperature were initially performed up to the onset
temperature of degassing, 110◦ C. The cure cycles were performed such that a 99% degree of
cure would be reached, according to the DSC experiments in chapter 4, as shown in table 4.3.
The oven was pre- heated up to the curing temperature prior to the actual curing process.
Thermocouple measurements have been performed within the bond layer of a lap- shear
sample for each cure cycle. This was done first to check the accuracy of the preselected
curing temperature and to adjust it if needed. Secondly, it was used to assess the significance
of the heat- up rate of the curing process.
The heat- up rate was found to be for all cure cycles 5%- 10% of the total curing time. The
curing time was started as soon as the samples entered the pre- heated oven. Approximately
24 hours after finishing the curing process the samples were mechanically tested. In between
they were stored in the lab at room temperature.
5.2 Materials & Specimens
67
Induction Curing
As mentioned already before, the effect of an increasing curing temperature and particle
content was only assessed on oven- cured samples. This decision was made in order not
to suffer from any additional effects due to the induction curing process, and thereby to
investigate each effect in an isolated way. As a consequence, only one cure cycle has been
selected for the induction- cured samples, in order to compare their mechanical performance
against samples cured in the oven at the same cure cycle.
The induction heating equipment used for this experiment is still the Easyheat LE-10 kW
unit, as used in the previous parts of this project. The cooling unit in the lab has limited
cooling capabilities when using the induction system for continuous processes. Because of this
limitations, only coil currents of up to 175 Amperes could be used for achieving the curing
process. As a consequence, a volume- percentage of 7.5% of Iron powder is already required
to achieve a cure cycle of two hours at just 65◦ C. As increasing the volume- percentage of
particles even further was considered to be inappropriate, the decision was made not to test
at higher temperatures on the induction set- up than 65◦ C.
The cure cycle of the induction curing process was first tested by the use of a thermocouple
measurements within the bond layer and IR measurements on top of the set- up. This
experiment was used, first of all, to check which specific set- up would result in a bond layer
temperature of 65◦ C when applying a coil current of 175 A.
The advantage of induction curing is that the initial heat- up rate can be modified by applying
different levels of coil current. A second aim of the thermocouple/IR measurements was to
find the required settings for achieving a comparable heat- up stage of the induction- cured
samples to that of the oven- cured specimen.
Figure 5.4 shows the temperature profile of the oven- and induction- cured samples for two
hours at 65◦ C. As can be seen on this figure as well, a top- surface temperature of approximately 59◦ C was measured by the IR camera at the same time as the bond layer temperature
was 65◦ C. As implementing thermocouples within the actual test- specimen would possibly
affect their mechanical performance, IR measurements of 59◦ C were used as a validation
method of the cure cycle when the actual lap- shear specimen were produced.
68
Mechanical Testing
70
65
60
Temperature [°C]
55
50
45
40
35
30
Induction − IR
Induction − Thermocouple
Oven − Thermocouple
Oven − Temperature Setting
25
20
0
20
40
60
80
100
120
Time [min.]
Figure 5.4: Temperature profile for both the oven- cured and induction- cured samples for
two hours at 65◦ C
All process parameters used for the final cure cycle can be seen in Table 5.1.
Table 5.1: Process parameters used for producing the induction- heated adhesive bonding
single lap- shear specimen
Susceptor Particle
Iron powder
Particle content
7.5v%
Coupling distance
1 mm
Coil geometry
Pancake
Coil current
Heat- up stage
Rest of cure cycle
6’ @ 220 A
175 A
As for the oven- cured samples, the specimen were all mechanically tested approximately 24
hours after finishing the curing process.
5.3 Experimental Set- Up
5.3
5.3.1
69
Experimental Set- Up
Test Bench
The test equipment used for these experiments was a 10 kN tensile bench made by Zwick
International, as can be seen in Figure 5.5. Table 5.2 shows the related process parameters
used for all mechanical tests.
Figure 5.5: ZWICK 10kN bench including a test specimen
Table 5.2: Test parameters for Zwick 10 kN tensile bench
Preload
Test speed
Test- end threshold
5.3.2
10 N
5 mm/ min.
80% of Fmax
Test Plan
This part of the project consists of both samples cured in the oven and by induction heating.
The effect of increased curing temperatures and particle- content were only tested on ovencured samples. This test set- up was chosen in order to be sure that the results could be
fully dedicated to the effect in scope, and not to any possible side- effects coming from the
induction heating process. In a last phase, induction- cured samples were used for the final
comparison between oven- and induction curing.
70
Mechanical Testing
Table 5.3 shows a test matrix, including all the samples tested within this part of the project.
Table 5.3: Test matrix for the mechanical tests
Curing
method
Oven
Cure Cycle
v%
2h at 65◦ C
1h11’ at 80◦ C
51’ at 95◦ C
47’ at 110◦ C
0 - 0.5
2-5
7.5
Induction
2 hours at 65◦ C
7.5
5.4 Results
5.4
71
Results
This section contains the results of the performed lap-shear tests. Each configuration was
tested three times in order to check for consistency of the data. Reporting of the data has
been done through a combination of bar- plots and tables, showing both average values of
lap-shear strength as well as standard deviations and maximum- and minimum values.
Test data was extracted from the experiments in the form of load- displacement curves for
each specimen. Figure 5.6 shows the load- displacement curves of some of the set- ups used.
None of the tested specimens did show any recorded data beyond the maximum achieved
load, FM ax . This is most probably explained by the instantaneous character of the adhesive’s
failure. A detailed overview of all test results can be seen in appendix A.
9000
65C−2h−0v%−oven
8000
110C−0h47−0v%−oven
65C−2h−7.5v%−oven
7000
65C−2h−7.5v%−Induction
Load [N]
6000
5000
4000
3000
2000
1000
0
0
2
4
6
8
10
Discplacement [mm]
Figure 5.6: Examples of load- displacement curves of the tested specimen
Lap- shear strength for each sample was obtained by dividing the maximum load by the
bonded area of the joint. Table 5.4 shows a summary of all tested set- ups, including the
average obtained lap- shear strength and standard deviation. The results show a very high
level of consistency, with standard deviations not exceeding 2%.
Despite providing very consistent results in terms of lap- shear strength, in terms of loaddisplacement behaviour the same level of consistency was not achieved, with differences in
maximum displacement of up to 15%. This could possibly be explained by the the alignment
tabs, which have to be used when testing on the ZWICK 10kN tensile bench. Those alignment
tabs are attached to the test sample by two- sided sandpaper, which might have shifted during
the tensile testing. Still, providing consistent lap- shear strength results is considered to be
more relevant within the project’s scope.
72
Mechanical Testing
Table 5.4: Summary of the lap- shear strength of all tested set- ups
Curing
method
Oven
Oven
Oven
Oven
Oven
Oven
Oven
Oven
Induction
Cure Cycle
v%
2h at 65◦ C
1h11 at 80◦ C
51’ at 95◦ C
47’ at 110◦ C
2h at 65◦ C
2h at 65◦ C
2h at 65◦ C
2h at 65◦ C
2h at 65◦ C
0
0
0
0
0.5
2
5
7.5
7.5
τmax
[MPa]
23.48
20.14
21.43
22.32
20.11
19.68
19.28
19.15
20.38
St. Dev.
0.14 (1%)
0.39 (2%)
0.42 (2%)
0.19 (1%)
0.1 (1%)
0.13 (1%)
0.08 (0.5%)
0.39 (1%)
0.07 (2%)
At the end of each test, the fracture surface of the joint was examined in order to determine
the failure mode. Some coupons showed preliminary failure of the joint due to inter- laminar
failure of the adherend. This was considered to be caused by minor manufacturing errors
during the production of the GFRP test- coupons, and were therefore excluded from further
analysis of the effects of interest. Those samples were then reproduced and re- tested, such
that each configuration was still performed three times correctly.
All of the tested specimen showed a cohesive failure mode. Samples containing susceptor
particles all showed a pure cohesively failed fracture surface, of which an example can be
seen in Figure 5.7. Specimen which did not contain any Iron powder also showed cohesive
failure as primary failure mode, but including minor area’s of adhesive failure (less than 10%
in terms of surface area), as can be seen in Figure 5.8.
Figure 5.7: A fracture surface
of a tested specimen, including
0.5v% of Iron powder and ovencured for two hours at 65◦ C
5.4.1
Figure 5.8: A fracture surface of a tested specimen without Iron powder, oven- cured
for two hours at 65◦ C
Effect of Cure Temperature
Figure 5.9 shows the achieved lap- shear strength for samples cured in the oven at different
temperatures. The bar- plot shows the average lap- shear strength for each cure temperature.
The error- range plotted on top indicates the maximum and minimum lap- shear strength
5.4 Results
73
found for each set of samples. All samples used for the assessment showed cohesive failure
and the test’s standard deviation did not exceed 2%, as can be seen in table 5.5
25
Lap−shear Strength [MPa]
20
15
10
5
0
2h / 65°C
1h11’ / 80°C
51’ / 95°C
Cure Temperature [°C]
47’ / 110°C
Figure 5.9: Lap- shear strength of specimen, oven- cured at different temperatures
Table 5.5: Lap- shear strength of specimen, oven- cured at different temperatures
Cure
Temperature
Lap- shear
strength [MPa]
65◦ C
80◦ C
95◦ C
110◦ C
23.48
20.14
21.43
22.32
St. Dev.
1%
2%
2%
1%
Failure mode
Cohesive
Cohesive
Cohesive
Cohesive
An initial increase in cure temperature above 65◦ C (manufacturer’s recommendation) results
in the most significant drop in mechanical performance. Thereafter, the mechanical performance of the adhesive recovers up for curing temperatures up to 110◦ C.
5.4.2
Effect of Susceptor Particle- Content
The average lap- shear strength for oven- cured samples with a different content of Iron powder
are shown in Figure 5.10. As one can see, even 0.5% of added Iron particles results already in
a reduction of lap- shear strength of 14%. Increasing the particle content beyond 0.5% does
not result in a significant additional reduction in mechanical performance of the adhesive. All
tested specimen show again a cohesive failure mode. The findings of this experiment are also
summarized in table 5.6.
74
Mechanical Testing
25
Lap−shear Strength [MPa]
20
15
10
5
0
0
0.5
2
5
7.5
Iron powder volume− percentage [%]
Figure 5.10: Lap- shear strength of specimen, oven- cured for 2 hours at 65◦ C with different
volume-percentages of Iron- powder
Table 5.6: Lap- shear strength of specimen, oven- cured for 2 hours at 65◦ C with different
volume- percentages of Iron- powder
Volumepercentage
Lap- shear
strength [MPa]
0
0.5
2
5
7.5
23.48
20.11
19.68
19.28
19.15
St. Dev.
1%
1%
1%
0.5%
1%
Failure mode
Cohesive
Cohesive
Cohesive
Cohesive
Cohesive
5.4 Results
5.4.3
75
Effect of Induction Curing
The last experiment performed in this section is a direct comparison of oven- cured and
induction- cured single lap- shear joints. Figure 5.11 and table 5.7 show the average lapshear strength of specimen including a 7.5v% of Iron powder. Despite not being required
for the actual oven- curing process, the same amount of Iron powder was added to the ovencured samples in order to be able to compare the results with induction- cured samples. The
induction- cured joints show an increase in lap- shear strength of about 6%, compared to the
oven- cured samples.
25
Lap− shear Strength [MPa]
20
15
10
5
0
Oven
Induction
Curing Method [−]
Figure 5.11: Lap- shear strength of oven- and induction- cured for 2 hours at 65◦ C and
7.5v% of Iron powder
Table 5.7: Lap- shear strength of oven- and induction- cured for 2 hours at 65◦ C and 7.5v%
of Iron powder
Curing
method
Lap- shear
strength [MPa]
Oven
Induction
19.15
20.38
St. Dev.
1%
2%
Failure mode
Cohesive
Cohesive
76
Mechanical Testing
5.5
5.5.1
Discussion
Effect of Cure Temperature & Induction Heating, a Comparison with
Sanchez’ Work
At the end of these experiments it is important to reflect on the results. Some of the results
obtained in these experiments can be compared to the research performed by Sanchez. As
mentioned in the introduction of chapter 4, the assessment of a cure cycle optimization was
based on a methodology developed in his research.
Figure 5.12 shows the results of Sanchez experiment on the influence of the cure temperature.
First of all, one has to realize that Sanchez used different adhesives. Cure temperatures and
lap- shear strength can therefore not be compared directly in a quantitative way. However,
general trends can still be compared to those found in this project’s experiments. Sanchez
used the LME 6687-1 epoxy and LME 10049-3 hardener from Huntsman Advanced Materials,
having a recommended cure temperature of 80◦ C and an onset temperature of degassing of
120◦ C. The onset temperature of degassing is represented in the figure below by a solid red line.
The reduction in lap- shear strength for each curing temperature is shown as a percentage,
compared to the strength obtained at the recommended cure temperature of 80◦ C.
Figure 5.12: Experimental results of Sanchez’ work on assessing the impact of the curing
temperature on lap- shear strength of adhesively bonded joints [10]
Sanchez’ results show initially a very moderate decrease in lap- shear strength when curing
at temperatures above that recommended by the manufacturer. Exceeding the onset temperature of degassing results however in a first significant decrease in lap- shear strength,
which levels- off at approximately 70% of the initial strength. Increasing the curing temperature even further starts to result in a full deterioration of the mechanical performance of the
adhesive, which can be seen in Sanchez’ experiment as of a curing temperature of 160◦ C.
While this project has mainly focussed on temperatures up to the adhesive’s onset temperature of degassing, Sanchez performed his experiments on a wide range of temperatures. If one
5.5 Discussion
77
would like to compare general trends of Sanchez’ results with those obtained in this project,
one has to perform some additional testing beyond the adhesive’s onset temperature of degassing. Therefore, additional tests were performed up to a curing temperature of 155◦ C, as
can be seen in Figure 5.13.
Figure 5.13: This project’s extended results on assessing the impact of curing temperature
on lap- shear strength of adhesively bonded joints
As shown in Figure 5.13, EC9323’s lap- shear strength does not decrease significantly up
to a curing temperature of 125◦ C. The largest decrease in mechanical performance takes
place when curing at temperature just above the manufacturer’s recommended temperature.
Thereafter the lap- shear strength recovers up to a curing temperature of 125◦ C. Only at
155◦ C, the highest temperature tested in this project, the lap- shear strength starts to reduce
significantly.
One can now establish common trends in the results of both projects. One remark that has to
be taken into account however is the fact that a minor deviation from Sanchez methodology
was used to define the onset temperature of degassing for this project. According to Sanchez,
the onset temperature of degassing for EC9323 should have been set at 130◦ C and not 110◦ C,
as explained in section 4. Therefore, when talking about the onset temperature of degassing
in this comparison, one refers to 130 ◦ C for the EC9323 adhesive.
Both projects show initially a fairly stable mechanical performance when increasing the curing
temperature beyond the manufacturer’s recommended one. Exceeding the onset temperature
of degassing results in both cases in a first significant drop in lap- shear strength. This can be
seen as the 77% of the initial lap- shear strength still present in this project’s results for the
155◦ C- cured samples, and the 68% of the initial lap- shear strength present in Sanchez’ experiments at 120◦ C, both being the first curing temperature tested beyond the onset temperature
of degassing.
The initial drop beyond the recommended temperature and the recovery until the onset
temperature, found in this project’s results, can however not be compared to the results
from Sanchez project. The reason for this is that Sanchez only started testing at 80◦ C
78
Mechanical Testing
(recommended cure temperature for that adhesive) and tested thereafter at a 20◦ C- interval.
He therefore has only tested at one intermediate temperature (100◦ C), before reaching his
onset temperature of degassing (120◦ C).
Additionally, Sanchez also performed some preliminary tests on the effect on induction- heated
adhesive bonding on the mechanical performance of single lap- shear joints [11]. However,
almost all of his tested samples have failed by adherend fracture. It is therefore considered
not to be relevant to compare the outcome of this project with this obtained results. Unfortunately, no other research has been found on susceptor- assisted, induction- heated adhesive
bonding.
5.5.2
Effect of Susceptor Particle- Content
Some research has focussed already on the effect of susceptor particles on the lap- shear
strength of bonded joints, in the context of nanoparticle reinforcement of structural adhesives
[34]. Modified epoxy adhesives, including up to 5w% of alumina or silica particles have
shown a significant increase in lap- shear strength of 15% up to 40%. The influence of such
particles on the mechanical performance of adhesively bonded joints can either be positive or
negative, and depends mainly on the type of susceptor material, particle size- & shape and
the adherence between the susceptor particles and the adhesive.
One possible explanation of the significant reduction in lap- shear strength found in this
project could be that the susceptor particles did not achieve good adherence to the adhesive.
Figures 5.14 and 5.15 show a SEM- analysis performed on the fracture surface of failed lapshear specimen. The Iron particles, identified as white particles in Figure 5.14 or by the green
dots in Figure 5.15 are well surrounded by a layer of adhesive for most of its surface. This
can be considered as an indication of good adhesion between the suscptor particles and the
adhesive itself.
Figure 5.14: SEM image from the
fracture surface of a single lap- shear
specimen, containing 7.5v% of Iron
powder
Figure 5.15: SEM image from the
fracture surface of a single lap- shear
specimen, containing 7.5v% of Iron
powder
5.5 Discussion
79
Iron powder was selected as susceptor particle for this project purely because of its better
heat- generating characteristics, compared to the other tested susceptor particles. Iron powder
shows however a highly irregular particle- shape, as can be seen in Figure 5.16. Additionally,
former research mentioned in the above paragraph made use of nanoparticles with sizes up
to 100 µm, while the Iron powder used in this project has a particle size of 200 µm. Both of
those aspects might result in local distortions within the adhesive, another possible cause of
the reduction in lap- shear strength. Additional concerns for using Iron powder come from
durability aspects, such as corrosion.
One possible recommendation towards future research would be to develop susceptor particles
having both good heat- generating characteristics under induction, as well as a positive effect
on the mechanical performance of the adhesive.
Figure 5.16: SEM- picture of pure Iron powder (x200), used as susceptor particle for this
project
80
Mechanical Testing
5.6
Preliminary Conclusions
This chapter has assessed the effect of induction- heated curing on the mechanical performance
of single lap- shear joints. The preliminary conclusions from this chapter could be summarized
as followed:
• The additional of small amounts Iron particles to the adhesive, such as 0.5v%, result
in a significant reduction of about 15% in lap- shear strength of the tested specimen,
compared to bare adhesive
• Further increasing the particle content does not significantly add to the reduction in
mechanical performance of the joint, with only 18% reduction in lap- shear strength for
samples including 7.5v% of Iron powder
• The cooling equipment available in the lab has been a major limitation for assessing a
full cure cycle of the adhesive for high temperatures, as only coil current up to 175 A
can be effectively cooled for long operating cycles
• Curing from inside the adhesive, as done in susceptor- assisted induction heating, results in a slight increase in lap- shear strength (6%), compared to oven- cured samples
including the same amount of susceptor particles
With the mechanical tests finished, all experimental work has been performed in order to
complete the assessment on induction- heated adhesive bonding. All knowledge gained from
the experimental phase will now be used in the next chapter to construct a clear comparison
between oven- curing and induction curing.
Chapter 6
Comparison of Oven- vs. Induction
Curing
6.1
Introduction
In this chapter of the report, a comparison study is discussed to benchmark susceptor- assisted,
induction curing for adhesively bonded joints against conventional oven- curing. Literature
often refers to induction heating as being more energy efficient and cost- effective. The
objective of this chapter is to check if those statements are also valid in the case of susceptorassisted induction- cured adhesive bonding. The analysis will be mainly based on the casestudy of this project, single lap- shear joints.
The following objectives were established for this phase of the project:
• Compare oven- curing and induction curing of adhesively bonded single lap- shear joints
in terms of energy consumption, process time and investment cost
• Identify advantages, disadvantages, limitations and opportunities of susceptor- assisted
induction curing adhesive bonding
With the results of this chapter, one should be able to understand better the opportunities and
limitations of susceptor assistend, induction curing adhesive bonding. It will thereby provide
a first indication for assessing this technique’s potential to become an energy- efficient, nextgeneration adhesive bonding technique for the aerospace industry.
82
6.2
Comparison of Oven- vs. Induction Curing
Energy Consumption Monitoring
The energy consumption for the oven set- up is measured using a Brennenstuhl PM231 meter
device. When equipment is connected to the electrical grid through the PM231, as shown in
Figure 6.1, the meter measures the total amount of energy consumed by the equipment.
As the measurement device has no data logging function, the energy consumption had to
be monitored manually. Therefore, values shown in the results section of this chapter shows
averaged values of the energy consumption. Within the scope of this project this measuring
technique was considered to be acceptable.
For comparing the energy consumption of the curing techniques, instantaneous energy consumption at a certain time will be shown in Watt [W]. The total energy consumption of the
curing process will be expressed in kilowatt- hour [kWh].
Figure 6.1: The measurement equipment used for the energy consumption monitoring
For the oven- cured samples, energy consumption was measured from beginning of the heatup stage when the oven is switched on. Besides this, the energy consumption of the cooling
unit of the induction equipment was measured by using the same meter in a separate run.
The PM231 meter could not be used for measuring the energy consumption of the induction
equipment itself, as the maximum rated power of the induction equipment (10 kW) exceeds
the capabilities of the meter (<3.6 kW). However, the EasyHeat induction equipment has a
build- in energy consumption measuring function, which monitors the energy consumption
directly at any time.
6.3 Results
6.3
83
Results
6.3.1
Energy Consumption
Curing
Figure 6.2 shows the energy consumption profile for the T6030 oven, when used for two
hours at 65◦ C. Roughly said, one can assume the oven to work on its maximum rated power,
800W, for the first seven minutes of the heat- up phase. Thereafter, one additional minute
at approximately 120 W is required for the oven to trim itself precisely at 65◦ C. The actual
cure cycle consumed on average 60 W during the cure cycle. No difference was noted between
the energy consumption of an empty oven and an oven containing three single lap- shear
specimen.
As mentioned before, the values shown in this graph are averaged values, manually noted
from the power meter. The actual energy consumption fluctuated at any time around the
average value, with deviations of up to 5%. As only the total energy consumption of the
production process is of interest, this limitation was considered not to effect the outcome of
the experiment.
1000
900
Energy consumption [W]
800
700
600
500
400
300
200
100
0
0
20
40
60
80
100
120
Time
Figure 6.2: Energy consumption of the T6030 oven for heating- up and maintaining 65◦ C
From Figure 6.2, the conclusion can be made that a significant part (+- 45%) of the total
energy is consumed during the initial heat- up stage, in which the air inside the oven has to
be heated- up to the required temperature. Table 6.1 shows an estimation of the total energy
consumption in kilowatt- hour for the curing process of the single lap- shear specimens.
84
Comparison of Oven- vs. Induction Curing
Table 6.1: Total energy consumption of the oven- curing process at 65◦ C
Task
Heat- up
Continuous Heating - 65◦ C
Total
Time
[min.]
7’
1’
120’
Energy
[W]
800
120
60
Total energy
[kWh]
0.09
0.002
0.12
0.212
Induction Curing
Table 6.3.1 shows the energy consumption of the the EasyHeat induction equipment at different coil currents in combination with the pancake coil.
The energy consumption of the cooling unit, as shown in table 6.3 can be split up in the
initial cooling phase and the continuous cooling. The initial cooling stage is used to cool the
cooling water to 18◦ C prior to starting the heating- process. The continous cooling used for
cooling the equipment during the production process depends on the coil current used, as can
be seen in table 6.3.
Table 6.2: Energy consumption of the EasyHeat unit at different levels of coil current
Coil current [A]
50
100
150
175
200
Energy [W]
5
145
396
541
750
Coil current [A]
250
300
350
400
Energy [W]
1251
1820
2546
3380
Table 6.3: Energy consumption of the EasyHeat’s cooling unit
Stage
Initial cooling
Continuous cooling - 175A
Continuous cooling - 400A
Energy [W]
1350
750
1350
As one can see from table 6.3.1, the energy consumption of the induction heating equipment
increases exponentially when increasing the coil current. Figure 6.3 shows an approximation
of the induction-, cooling- and total energy consumption as a function of coil current.
6.3 Results
85
4000
3500
Energy Consumption [W]
3000
Induction Equipment − Regression
Induction Equipment − Data Points
Cooling Equipment − Regression
Cooling Equipment − Data Points
Total Energy Consumption
2500
2000
1500
1000
500
0
−500
0
50
100
150
200
250
300
350
400
Coil Current [A]
Figure 6.3: Graphical representation of the EasyHeat’s energy consumption at different
levels of coil current
By using the data from table 6.3.1 and 6.3, one can estimate the total energy consumed during
a production process of two hours at 175 A coil current, as used for curing the lap- shear
joints in chapter 5. Table 6.4 shows a summary of the estimation. All energy consumptions
are now expressed in kilowatt- hour.
Table 6.4: Total energy consumption of the induction curing process at 175A coil current
Task
Initial cooling
Continuous cooling - 175A
Induction heating - 175A
Total
Time
[min.]
15’
120’
120’
Energy
[W]
1350
750
541
Total energy
[kWh]
0.3375
1.5
1.08
2.92
86
6.3.2
Comparison of Oven- vs. Induction Curing
Process Time
The required curing time for adhesively bonded joints depends mainly on the curing characteristics of the adhesive itself. The adhesive will therefore, at a certain curing temperature,
require the same amount of time to obtain a full cure, independent of the curing method used.
No significant changes in curing process were therefore found between oven- and induction
curing.
One possible source of process time reduction is to modify the heat- up rate of inductioncured joints. When cured at a certain temperature, oven- cured samples will always achieve
the same heat-up rate, which depends only on the heat- conducting properties of the adherend
material and the specimen’s dimensions.
The heat- up rate of induction- cured samples can however be modified by applying a momentary higher coil current within the system. The bond layer of the joint can therefore reach
its final curing temperature in a shorter time window, allowing a moderate reduction in cycle
time. In the case of the production set- up used in the lab, a coil current of 400 Amperes
resulted in a heat- up time of approximately two minutes, while the cooling of the system
would not be impacted significantly.
A further decrease in processing time is considered to be only feasible by using higher curing
temperatures, as discussed in chapter 4. In that case, the heat- up stage in the oven will take
more time, while induction curing could still achieve the same heat- up time by increasing the
coil current. The benefit of induction curing will therefore become more significant at higher
curing temperatures.
6.3.3
Investment Cost
Table 6.5 shows a summary of the investment cost for both the oven- and induction heating setup, as provided by the manufacturers of both systems. The induction system is significantly
more expensive than the T6030 oven.
Table 6.5: Investment cost for the oven- and induction- set- up used in this project
Oven
Induction
Item
T6030
Cost [A
C]
1 884
Easyheat LE-10 kW
Workhead
Cooler
Grand Total
16 380
4 050
4 080
24 510
6.4 Discussion
6.4
87
Discussion
As mentioned already in the introduction of this project, the promising features of inductionheating, in general, are its low investment cost, combined with a lower energy consumption.
When looking at the results of this project however, none of those statements can actually
be proven for the case study of this project: susceptor- assisted, induction- cured single lapshear joints.
When looking back at the results from chapter 2, the conclusion was made that susceptorless
induction- heating of CFRP adherends is much more effective than any susceptor-assisted
set- up. This was already a first indication of a possible inefficiency of susceptor-assisted
induction- heating. It is therefore also of interest to estimate the energy consumption of a
susceptorless CFRP set- up.
Table 6.6 estimates the energy consumption of a susceptorless set- up using CFRP adherends
to cure the same single lap- shear specimens for two hours at 65◦ C. From the analysis performed in chapter 2, one could estimate the required coil current to be approximately 50 A
in combination with a coupling distance of 5 mm, in order to reach a temperature of about
65◦ C.
One can see that the actual heating process is extremely energy- efficient, using only about
10% of the energy consumed by the T6030 oven at 65◦ C. Still, the additional energy consumption taken by the cooling unit results in an overall higher energy consumption than the ovencuring set- up. The consideration has to be taken, however, that the EasyHeat equipment
does not show any significant heat- generation at those low coil currents, and a smaller, less
power- consuming cooling unit could probably be used. Susceptorless induction- heating of
CFRP material can therefore be considered to be very energy- efficient, and possibly even
more efficient than conventional oven- curing.
Table 6.6: Estimation of the total energy consumption of a susceptorless curing process
with CFRP adherends and 60A coil current
Task
Initial cooling
Continuous cooling - 50A
Induction- heating - 50A
Total
Total oven- curing
Time
[min.]
15’
120’
120’
Energy
[W]
1350
100
5
Total energy
[kWh]
0.3375
0.2
0.01
0.55
0.212
Despite the less promising results of susceptor- assisted induction heating, one could think
about specific applications for which this method would still be beneficial. This will be the
main focus of the next section.
6.4.1
Up- Scaling of the Bonding Process
So far, this chapter has focussed on comparing the performance indicators of oven- curing and
induction- curing for the production of single lap- shear joints. One could argue however that
88
Comparison of Oven- vs. Induction Curing
the outcome of this comparison is affected by the type and dimensions of the application.
The single lap- shear joints used in this project are small- scale samples with a relatively large
bonded area, covering approximately 15% of the total sample’s surface area. If the size of the
adherends would be increased while the bonded area would stay the same, this percentage
would be reduced and different results would be obtained from the comparison. Joints could
now still be induction- cured by the same set- up, while oven- curing would require larger
equipment to accommodate the new, larger dimensions of the adherends.
In order to get a first estimate of the relation between an oven’s volume and its energy
consumption, experiments have been performed on different sizes of ovens in the lab. The
energy consumption for a cure cycle of two hours at 65◦ C was measured, as can be seen in
table 6.7. As the VTU 60/60 has a higher rated power than the operating window of the
PM231 meter device, no precise energy consumption measurements were performed for this
oven.
Table 6.7: Results of the energy consumption monitoring experiment performed in the lab
Energy consumption
Oven
Volume
[m3 ]
Heraeus
T6030
0.03
Rated
power
[kW]
0.8
Heraeus
UT6200
0.18
2.8
Heat- up : 7’ at 2.8 kW + 1’ at 600 W
Continuous - 65◦ C : 300 W
Votsch
VTU 60-60
0.58
7.2
Heat- up : 65◦ C reached in +- 8’
Heat- up : 7’ at 0.8 kW + 1’ at 120 W
Continuous - 65◦ C : 60 W
From the above table, one could draw some general assumptions on the energy consumption
for larger oven set- ups. As one can see, the total time required for pre- heating the oven
up to 65◦ C is approximately 8 minutes for each oven, independent of its size. Additionally,
the energy consumption profile for the heat- up stage looks very common for each oven, with
an initial heating of seven minutes at the oven’s maximum rated power, followed by a final
temperature trimming stage of about one minute. Thereafter, about 7.5 - 10 % of the oven’s
maximum rated power is required for keeping the oven at 65◦ C. The effect of the oven’s
content on its energy consumption was not taken into account when establishing the above
trends.
Based on the above findings, the energy consumption of an oven can be roughly estimated
from its maximum rated power. Ovens of different dimensions were used to estimate the
relation between the oven’s volume and its maximum rated power, as can be seen in Figure
6.4. For small- scale ovens, up to a volume of 5 m3 , this relation is almost linear. Only at
larger volumes the relation starts to flatten- off.
6.4 Discussion
89
70
Statistical Regression
Data Points
60
Rated Power [kW]
50
40
30
20
10
0
0
2
4
6
8
10
Volume [m3]
Figure 6.4: Statistical relation between the oven’s volume and energy consumption
The assumptions made above can now be used to estimate the total energy consumption of
an oven-curing set- up, as a function of the oven’s volume, as shown in Figure 6.4. In section
6.3, a total energy consumption of 2.92 kWh was obtained for the induction- curing process.
An equivalent energy consumption is obtained at an oven- volume of approximately 0.8 m3 ,
shown in Figure 6.6.
25
4.5
Statistical Regression
Measured Data Points
4
Energy Consumption [kwh]
Energy Consumption [kwh]
20
15
10
5
Statistical Regression
Data Points
3.5
3
2.5
2
1.5
1
0.5
0
0
2
4
6
8
10
Volume [m3]
Figure 6.5: Energy consumption estimations for oven- curing in function of
the oven’s volume
0
0
0.2
0.4
0.6
0.8
1
1.2
Volume [m3]
Figure 6.6: Energy consumption estimation : Break- even estimation for
induction- and oven- curing
Further analysis of the energy consumption of induction- curing was considered to be rather
difficult. The area which can be heated by an induction set- up is mainly depending on the
coil’s geometry and dimensions. For example, the EasyHeat unit used for this project can
accommodate coils covering up to approximately 50 cm2 .
90
Comparison of Oven- vs. Induction Curing
When trying to generalize the relation between bonded area, the equipment’s rated power and
its energy consumption, one has to realise however that the link between those parameters is
much more complex than for oven- curing, as it depends on many additional parameters such
as the coil geometry, frequency and coil current. Furthermore, only one set- up was available
in the lab for actual energy consumption measurements.
When looking at manufacturer’s data sheets, increasing the equipment’s rated power is commonly used for generating larger coil currents, and thereby stronger magnetic fields, rather
then increasing the coil size. The decision was therefore made not to extrapolate any further
the energy consumption for larger set- ups.
Summarizing the findings from above, one could now argue that applications having a bonded
area of less than 50 cm2 , but requiring an oven- volume of more than 0.8 m3 for the curing
process could be processed more efficiently by susceptor- assisted induction- curing.
When looking at the investment cost, the oven- curing process is considered still to be cheaper.
For example, a 0.75 m3 oven from Hereaus would cost 6110 Euro, only 25% of the cost of an
EasyHeat set- up.
6.5 Preliminary Conclusions
6.5
91
Preliminary Conclusions
The objective of this chapter was to establish a comparison between oven- curing and induction
curing in the concept of adhesively bonded joints. Key performance indicators used in this
analysis are energy consumption, investment cost and process time. From this analysis, the
following conclusions could be drawn:
• The production of standard, single lap- shear joints through induction curing is both
more expensive and more energy consuming than oven- curing
• Joints having a bonded area of less than 50 cm2 , but requiring an oven- volume of more
than 0.8 m3 can be cured more energy- efficiently by induction heating, resulting in a
lower energy consumption
• The processing time can not be reduced significantly by using induction curing for any
type of application, if a full cure has to be achieved
• The investment cost for an induction set- up is significantly higher than that of smalland medium scaled ovens
• Susceptorless induction heating of CFRP is more energy- consuming than oven- curing
due to the additional energy consumption of the cooling equipment.
92
Comparison of Oven- vs. Induction Curing
Chapter 7
Conclusion & Recommendations
This report has been written in the context of an experimental project on induction- heated
adhesive bonding. The project’s main objective was to investigate the effect of susceptorassisted induction heating on the curing behaviour, mechanical performance and energy consumption of adhesively bonded lap- shear joints.
As an energy- transfer mechanism, induction heating is often referred to as an energy- efficient,
quick and clean process. The hypothesis was therefore made that curing paste adhesives by
induction would lead to a greener & more cost- effective process, compared to conventional
oven- curing.
The project has been split in four major parts, each assessing different aspects of inductionheated adhesively bonded joints. The following sections will summarize the major findings of
each of those experimental phases.
Susceptor Analysis
An assessment was made on the heat- generating characteristics of different susceptor materials, including several types of susceptor particles (Iron-, Nickel- and Graphene- based) and
CFRP adherends. Among all tested susceptor particles, Iron powder has been found to be
the most effective heat- generator. CFRP in turn, showed heat- generating characteristics
several times better than those of Iron powder. The conclusion was therefore made that a
combined set- up of susceptor particles and CFRP adherends would not be feasible.
As only limited research has been performed yet on susceptor- assisted, induction- heated
adhesive bonding, the decision was made to focus on this technique for the further process
of the report by using Iron powder susceptor particles in combination with non conductive
GFRP adherends.
94
Conclusion & Recommendations
Modelling of the Induction Heating Process
In addition to the experimental research, performed in the previous part of this report, a
model was made of the induction heating process in COMSOL multiphysics. A simplification
was implemented by modelling the samples including adhesive and Iron particles as a uniform,
homogeneous material. The properties of the uniform material were estimated as a "rule- ofmixture", based on the volume- percentage of the adhesive and the Iron particles.
The model was used to assess the effect of different process parameters on the induction
heating process, which could not experimentally be tested on the lab’s set- up. Despite not
being considered feasible yet, an improvement of the electrical conductivity of the adhesive up
to 20 S/m would result in a significant increase in heat- generating characteristics. Increasing
the field’s frequency also results in an improved heat- generation of the susceptor particles.
Adhesive Characterization
A second phase of the project has focussed on characterizing the adhesive and its cure cycle,
through a set of TGA and DSC experiments. First, the conclusion was made that adding
Iron powder to the adhesive does not affect its curing chemistry. Secondly, by increasing
the curing temperature from 65◦ C (manufacturer recommendation) to 110◦ C, the required
curing time of the EC9323 adhesive could be reduced from two hours to 47 minutes. At this
elevated curing temperature, the lap- shear strength would only reduce by 5%, compared to
the strength obtained at 65◦ C.
Impact of Induction Curing on the Lap- Shear Strength
Single lap- shear specimens have been tested to assess the impact of induction curing on the
mechanical performance of the adhesive. The effect of particle content was assessed first on
oven- cured samples. Even at small volume percentages of 0.5v% of Iron powder, the lapshear strength is reduced by approximately 15%. Increasing the volume percentage further
does not result in any significant additional decrease in performance. The inside- out curing
taking place during the induction heating process shows a slight beneficial effect on the lapshear strength of adhesively bonded joints (6%).
Induction vs. Oven benchmark
As a final part of this project, a benchmark has been developed in order to compare energy
consumption, investment cost and processing time of the induction- and oven curing process.
When assessing the energy consumption used for producing the single lap- shear samples
as used in the previous phase of this project (two hours curing at 65◦ C), induction curing
consumes about fourteen times more energy. Also the investment cost for the induction
equipment is approximately thirteen times more than that for the oven- curing set- up. The
required curing time can also not be modified significantly by changing the curing process.
However, increasing the size of the adherends, while keeping the area to be bonded relatively
small, can result in the induction process to become more energy efficient. An induction set-
95
up able to cure area’s of up to 50 cm2 consumes, for a curing process of two hours at 65◦ C,
the same amount of energy as an oven of approximately 0.8 m3 .
The main advantages of induction- heated adhesive bonding are the fact that no constraint
would exist anymore on the external dimensions of the joined adherends and its better controllability of the bond layer’s temperature. Its main disadvantages however, are the reduced
mechanical performance due to the addition of Iron powder and a difficult in- depth curing
due to the susceptor particle’s sensitivity for the coupling distance.
Induction- heated adhesive bonding’ s main opportunities lie in its application on large- scale
structures, requiring small area’s to be bonded. If the surrounding area of such joints can
be made free of conductive material (aluminium, CFRP) and allow a close coupling distance
between the workhead and the joint, susceptor- assisted induction- heated adhesive bonding
can effectively become a greener, next- generation manufacturing process for the aerospace
industry.
Advantages, Disadvantages, Opportunities & Limitations of Susceptor- Assisted, InductionCured adhesive bonding
With all information gained in the previous parts of this project, some conclusions could be
drawn on the potential of induction- heated adhesive bonding on becoming a next- generation
manufacturing process for the aerospace industry. Those findings will be split in advantages,
disadvantages, limitations and opportunities. The statements made below do not only apply
on single lap- shear joints, as tested during this project, but can be considered as being
applicable on all adhesively bonded joints in general.
The following are considered to be the principal advantages of susceptor- assisted, inductionheated adhesive bonding:
• Susceptor- assisted induction curing results in small improvements of lap- shear strength
• Compared to oven- curing, no size- limitations on the application’s external dimensions
exist anymore. This is specifically interesting for large- scale, irregular- sized applications with only small bonded area’s.
The technique possesses, however, also some specific disadvantages:
• Adding Iron powder to the adhesive results in a reduction of mechanical performance
of the adhesive of approximately 15%, even at a low particle content
• In- depth curing of thicker bondlayers is difficult to achieve, due to the sensitivity of
the susceptor particles for increasing coupling distance
• The initial investment cost of an induction equipment is high, compared to that of
small- and mid- scaled ovens of up to 15 m3
The following list contains all major opportunities of susceptor- assisted induction- heated
adhesive bonding:
96
Conclusion & Recommendations
• Adhesively bonded joints, requiring an oven set- up of at least 0.8 m3 , but having
a bonded area not larger than 50 cm2 could be processed more energy- efficient by
induction curing than by oven- curing
• By adding susceptor particles, non- conductive materials can also be adhesively bonded
by induction curing
• The induction curing set- up allows a much better temperature control of the bondlayer
compared to oven- curing. It also allows an easy implementation of specific temperature
profiles during the heat- up-, curing- and cool down phase of the process.
• Today, nanoparticles exist that improve the mechanical performance of paste adhesives.
If those particles could be modified into susceptor particles, maximum utility could be
taken from those particles, as they would result in an improved mechanical performance
of the joint and would allow the adhesive to be cured by induction at the same time.
Lastly, the following statements could be considered as the technique’s principal limitations
towards future applications:
• Small- scale applications can be cured more efficiently, cheaper and energy- efficient by
using conventional oven- curing techniques
• Because of the relatively slow curing behaviour of epoxy adhesives, and the limited area
of an induction set- up to be heated at the same time, it is not possible to use an
induction set- up for curing large bond areas simultaneously, exceeding a bonded area
of 50 cm2
• The surroundings of the joint (up to +- 30 cm as proposed by EasyHeat’s manufacturer,
Ambrell) have to be free of conductive materials (Aluminium, CFRP), as the required
magnetic field strength for the susceptor particles will trigger significant heat- generation
within those materials. Examples such as clamps, tools, rivets or other parts of the
structure have therefore to be carefully separated from the induction set- up.
• Small coupling distances are required to achieve maximal energy efficiency of the setup, which requires good accessibility of the joint and reduces operating flexibility
• The need for continuous cooling limits the portability of the induction heating set- up
97
Recommendations
Based on the results of this project, the following set of recommendations has been constructed:
• A first point of interest would be an assessment on the effect of different material
properties on the induction heating process. Examples of such material properties are its
micro- structure (grain size- & shape) and magnetic properties (remanence, coercivity,
permeability). This knowledge could thereafter be used to select a new, possibly more
efficient susceptor particle than those used in literature and in this project.
• MagSilica, a superparamagnetic susceptor particle tested by Sanchez, showed superior
heat- generating characteristics, compared to both Nickel- and Iron- based particles.
Unfortunately, this material has not been found within the time window of this project.
A further investigation in this material, or other superparamagnetic materials could lead
to the identification of more suitable susceptor particles with enhanced heat- generating
characteristics.
• The mechanical performance of induction- cured joints has only been assessed by static,
single lap- shear tests. It is however also important to understand the effect of susceptor
particles on the long- term performance of adhesively bonded joints, especially in the
aerospace industry. An assessment could therefore be made on the durability of such
induction- cured joints, such as crack growth properties and corrosion issues. Additionally, testing of the peel- strength of susceptor- assisted adhesives is also considered to
be of major interest.
• Curing the adhesive at an increased temperature might modify the adhesive’s physical
properties, such as its glass transition temperature. Additional testing at elevated
curing- and testing temperatures might therefore be performed to understand those
influences better.
• As learned from the 1st chapter’s results, Joule heating of conductive materials is more
effective than hysteresis heating of ferromagnetic susceptor particles. Developing an
adhesive having sufficient conductive properties for Joule heating to be activated could
therefore possibly result in improved heat- generating characteristics.
• Despite not having been the main focus of this project, susceptorless induction heating
of CFRP adherends has shown to be extremely energy- efficient. Moreover, CFRP has
a wide range of applications within today’s aerospace industry. A final recommendation
would therefore be towards industrial organizations to assess the opportunity of shifting
towards this alternative materials for adhesively bonded joints when cured by induction.
98
Conclusion & Recommendations
References
[1] S.
Niks.
Structural
adhesive
bonding
in
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References
Appendix A
Appendix A : Detailed Test-Data of
Single Lap- Shear Tests
This appendix shows the detailed results of all single lap- shear tests performed in the context
of chapter 5 of this report. Each of the graphs below contains the load- displacement curves
of all three samples tested at a certain set-up.
A.1
Effect of Cure Temperature
9000
8000
7000
7000
6000
6000
Load [N]
Load [N]
8000
9000
65−2h−1
65−2h−2
65−2h−3
5000
4000
5000
4000
3000
3000
2000
2000
1000
1000
0
0
2
4
6
8
10
Discplacement [mm]
Figure A.1: Load- displacement curves
of single lap- shear tests, oven- cured, 2h
@ 65◦ C
80C−1h11−1
80C−1h11−2
80C−1h11−3
0
0
2
4
6
8
10
Discplacement [mm]
Figure A.2: Load- displacement curves
of single lap- shear tests, oven- cured,
1h11’ @ 80◦ C
104
Appendix A : Detailed Test-Data of Single Lap- Shear Tests
9000
9000
8000
95C−0h51−1
95C−0h51−2
95C−0h51−3
8000
110C−0h47−1
110C−0h47−2
110C−0h47−3
7000
7000
6000
Load [N]
Load [N]
6000
5000
4000
4000
3000
3000
2000
2000
1000
1000
0
0
5000
1
2
3
4
5
6
7
8
9
0
0
10
2
Figure A.3: Load- displacement curves
of single lap- shear tests, oven- cured, 51’
@ 95◦ C
A.2
6
8
10
Figure A.4: Load- displacement curves
of single lap- shear tests, oven- cured, 47’
@ 110◦ C
Effect of Particle Content
9000
8000
9000
65C−2h−2v%−1
65C−2h−0.5v%−1
65C−2h−0.5v%−2
65C−2h−0.5v%−3
8000
7000
6000
6000
5000
4000
5000
4000
3000
3000
2000
2000
1000
1000
0
0
2
65C−2h−2v%−2
65C−2h−2v%−3
7000
Load [N]
Load [N]
4
Discplacement [mm]
Discplacement [mm]
4
6
8
10
Discplacement [mm]
Figure A.5: Load- displacement curves
of single lap- shear tests, oven- cured, 2h
@ 65◦ C, 0.5v% Fe
0
0
2
4
6
8
10
Discplacement [mm]
Figure A.6: Load- displacement curves
of single lap- shear tests, oven- cured, 2h
@ 65◦ C, 2v% Fe
A.3 Effect of Induction Curing
105
9000
8000
7000
7000
6000
6000
Load [N]
Load [N]
8000
9000
65−2h−5v%−1
65−2h−5v%−2
65−2h−5v%−3
5000
4000
5000
4000
3000
3000
2000
2000
1000
1000
0
0
2
4
6
8
0
0
10
65−2h−7.5v%−1
65−2h−7.5v%−2
65−2h−7.5v%−3
2
4
Discplacement [mm]
Figure A.7: Load- displacement curves
of single lap- shear tests, oven- cured, 2h
@ 65◦ C, 5v% Fe
A.3
6
8
10
Discplacement [mm]
Figure A.8: Load- displacement curves
of single lap- shear tests, oven- cured, 2h
@ 65◦ C, 7.5v% Fe
Effect of Induction Curing
9000
8000
65−2h−IND−1
65−2h−IND−2
65−2h−IND−3
7000
Load [N]
6000
5000
4000
3000
2000
1000
0
0
2
4
6
8
10
Discplacement [mm]
Figure A.9: Load- displacement curves of single lap- shear tests, induction- cured, 2h @
65◦ C, 7.5v% Fe
106
Appendix A : Detailed Test-Data of Single Lap- Shear Tests
Appendix B
Appendix B
This appendix shows the validation results of test set- up 2 & 3, used for validating the
induction- heating model.
60
55
Experimental result
Model − hysteresis included
Temperature [°C]
50
45
40
35
30
25
20
0
50
100
150
200
Time [s]
Figure B.1: Temperature profile of the validation test for set- up 2
65
60
Experimental result
Model − hysteresis included
Temperature [°C]
55
50
45
40
35
30
25
20
0
50
100
150
200
Time [s]
Figure B.2: Temperature profile of the validation test for set- up 3