Characterising the environmental stress cracking behaviour of thermoplastics: a fracture mechanics approach

Abstract

Environmental stress cracking (ESC) is a slow crack growth process which affects thermoplastics. It occurs under the combined effect of applied stress and an ingressing environment, without large-scale diffusion or chemical reaction. This thesis focusses on the development of a fracture mechanics test method, which is applied to characterise the ESC behaviour of several polymer-environment combinations of industrial interest.

The materials investigated were PE in Igepal solution, HIPS in sunflower oil, PMMA in methanol, and PEEK in acetone. A rig was built for testing using the single edge notched bend (SENB) configuration, with crack growth measured via specimen compliance. Tests were conducted in both air and the liquid environment, with the effects of temperature and initial applied G (energy release rate) also investigated.

Results were obtained in the form of crack initiation (G versus time) and propagation (G versus crack speed) plots. Standard material tests were also performed, as well as scanning electron microscopy (SEM) of the fracture surfaces to describe the different failure processes observed.

For most of the materials tested, the models of relaxation- and flow-controlled crack growth were considered to hold, yielding values of critical crack opening displacement (COD) and characteristic crack length (L0) respectively. The ESC results were also used to perform component life predictions (tf) for the different materials, using either known operating conditions or `critical' values, the latter leading to a proposed parameter λ. Overall, PEEK was judged as having the highest ESC resistance, and PMMA the lowest.

The developed fracture mechanics method was found to successfully describe the ESC behaviour of several different polymer-environment combinations. It produced geometry-independent, inherent material information compared to standard industry tests, which typically output failure times under conditions that may not represent the situation in service.