Modeling of double yield points in polyethylene: Temperature and strain‐rate dependence

Abstract

The work described in this article is a more quantitative development of previous work on three grades of polyethylene which identified two yield points under deformation over a range of conditions. This behavior was modeled as two processes acting in parallel such that the total stress is the sum of the two individual stresses, termed the recovery stress and the effective stress. The individual stresses on each arm were found by use of a strain‐dip experiment, which enabled the yield stress for each arm to be found as a function of the temperature and strain rate. The effective stress measurements on one of these polymers (medium density polyethylene) have been extended over a wider range of temperatures and strain rates. The results obtained were then modeled in two different ways: firstly assuming that the stresses on the two arms were each modeled by a single activated rate process; and secondly assuming that the recovery stress is strain‐rate independent and the effective stress was modeled by a cooperative jump process. The results show that the second approach is more successful, but further analysis has to be carried out to confirm this proposition. Activation volumes found using the two methods are in agreement with previous results. However the activation energies found are smaller than previously measured due to either differences in data analysis or in definitions of yield points. A tentative assignment of the effective stress to an interlamellar shear process has been made.