Comparative assessment of slow strain rate, 4-pt bend and constant load test methods for measuring environment stress cracking of polymers

The environment stress cracking (ESC) susceptibility of polycarbonate and acrylonitrilebutadiene-styrene (ABS) in a number of environments has been evaluated using slow strain rate, four-point bend and constant load tests. Testing was conducted for the materials in the as-processed and annealed state. For the as processed materials, the longitudinal direction of the specimen was generally perpendicular to the melt flow direction. Nevertheless, a few tests were carried out with the specimen orientation parallel to the flow direction. The ESC techniques gave consistent ranking of aggressivity of environments although the extent of relaxation of the ABS when exposed to particular environments limited the significance of results from the bend tests. Otherwise, the results validate further the use of the slow strain rate technique as an accelerated test method for assessing the ESC of plastics, albeit primarily as a ranking technique. The threshold stress for crazing was highest for the slow strain rate method which can be attributed partly to the particular strain rate adopted. There was no effect of molecular orientation on ESC for the limited range of test conditions evaluated. The difference in threshold stress measured for the annealed and as-processed specimens indicated the existence of a net tensile residual stress in the very near-surface region, not detected by the layer removal method of residual stress measurement. Crown copyright (C) 1999 Published by Elsevier Science Ltd. All rights reserved.