Environmental crack initiation and propagation in Polyethylene

Environmental crack initiation and propagation in Polyethylene under different loading conditions L. Castellani, A. Castiglioni
L. Andena, A. Mendogni, M. Rink, F. Sacchetti
A. Adib
Polimeri Europa SpA, Mantova, Italy
Politecnico di Milano, Italy
Rey Juan Carlos University, Madrid, Spain
Introduction
ESC in a material exposed to a specific environment results in initiation and propagation of fracture, driven by mechanical force. The Bell Telephone Test
(BTT) is usually adopted to evaluate ESC resistance (ESCR) of PE in presence of detergents. We adopted a linear elastic fracture mechanics (LEFM)
approach to study this phenomenon. Williams [1] showed that for a given polymer, depending on the test temperature, unique relations can be obtained
between the applied stress intensity factor K and the fracture initiation time ti and, during crack propagation, between K and the crack speed da/dt.
The application of LEFM to PE is not straightforward due to its ductile behavior and inherent toughness. Results obtained with different experimental
configurations, characterized by different loading histories [2] (constant load, constant strain rate [3] and constant strain [4]), are hereby presented.
Materials
Methods
Tests were conducted under three different loading histories and configurations:
constant load on SE(B) samples (a), constant strain rate on DCB samples (b)
and constant strain on BP samples (c,d).
Crack advancement, a (t), was detected by a compliance method (SE(B)) or optically (DCB, BP) Relaxation data (at 50°C)
Results
Crack initiation
Crack propagation
Conclusions
LEFM is applicable to characterize crack initiation and propagation in air and detergent, results are in good agreement with those of the BTT test.
Each loading condition is best suitable for experimentation in a specific range of applied K.
The active environment seems to equally affect the crack initiation and propagation phases for both materials.
A critical interaction time is needed to observe an effect of the detergent; its value is higher for the material with better ESCR. References
[1] Williams J. G., In: various editors. Advanced Polymer Science 27, Berlin Heidelberg: Springer‐Verlag; 1978, p 67‐120.
[2] Andena L., Castellani L., Castiglioni A., Mendogni A., Rink M., Sacchetti F., Engineering Fracture Mechanics, in press
[3] Andena L., Rink M, Frassine R, Corrieri R., Engineering Fracture Mechanics 2009, 76, 2666‐2677.
[4] Chang P., Donovan J. A., Journal of Material Science 1989, 24, 816‐820.