Characterization of ductile damage in polyethylene based on effective stress concept

Abstract

In this paper we introduce a new approach to the characterization of ductile damage in polyethylene (PE) material using the damage variable defined as D=1−σ/σeff. Quasi-static stress-strain relationship has been established using a spring-damper-plastic model. The viscous stress was then removed from the experimentally measured total stress to determine the quasi-static stress. The results were used to investigate the influence of crosshead speed and specimen geometry, through varying the ligament width of the PE specimen, on the quasi-static stress-strain curves. Results show that reduction of the crosshead speed leads to larger quasi-static stress due to less damage generated at a lower crosshead speed, while variation of the ligament width has little effect on the quasi-static stress. Using a curve that is fitted to the experimentally determined quasi-static stress as a function of crosshead speed and extrapolated to zero crosshead speed, the effective stress in the undamaged configuration was successfully estimated and used to determine the stress-based damage variable. The damage variable so determined is compared with those measured through the degradation of elastic modulus either based on the hypothesis of strain or energy equivalence. The stress-based damage variable shows good agreement with the damage variable based on the energy-equivalence approach.