Stress intensity factor (K1)-crack speed (c) plots for the high-density polyethylene-10% detergent (Igepal) system have been obtained to investigate the effect of fracture mechanics test variables such as load level, specimen geometry, and dimension on the crack growth behavior. Fracture mechanics appeared to be valid at low K1 levels. However, at high Kj values, the data showed that linear elastic fracture mechanics failed to describe the crack growth behavior. Williams’flow model was not successful in explaining a number of regions which were obtained in a K -6 plot. Craze forma-tion at the crack tip enhances the crack speed at high K1levels. © 1989, Taylor & Francis Group, LLC. All rights reserved.