NONLINEAR MECHANICAL RESPONSE OF HIGH-DENSITY POLYETHYLENE .1. EXPERIMENTAL INVESTIGATION AND MODEL EVALUATION

The nonlinear behavior of high density polyethylene (HDPE) is investig ated for samples cut from thick-walled HDPE pipe. Extensive experiment al work has been performed to characterize Vie non-linear time-depende nt response of the material tested under uniaxial compression, Tests w ere conducted under conditions of constant strain rate, creep, stress relaxation, constant loading rate, abrupt change of strain rate, creep -recovery, cyclic strain rate, and various combinations of these loadi ng conditions. Creep and stress relaxation response after strain rever sal and the effect of the transient response on the following stress-s train behavior is examined, Permanent strains for the test specimens a nd their dependence on loading histories are investigated, Specimens c ut at various orientations from the pipe are used to quantify the smal l amounts of local anisotropy in the pipe specimen. The experimental w ork has been used to develop both nonlinear viscoelastic (NVE) and vis coplastic (VP) constitutive models in a companion paper. Both the test results and the corresponding model predictions are reported in this paper. It is found that the VP model reproduces the nonlinear viscoela stic-viscoplastic behavior of HDPE very well provided that the current strain is not below tile maximum strain imposed (there is no strain r eversal). The NVE model predicts the material behavior reasonably well for some loading conditions, but inadequately for others.