Viscoelastic-plastic behaviour with mean strain changes in polypropylene

Abstract

The stress-strain curves and stress-relaxation curves of polypropylene are obtained by using a closed-loop, electrohydraulic, servo-controlled testing machine. Effects of mean strain changes on deformation behaviour are examined in a tension-compression mode under strain control at room temperature (18–23 °C). The hysteresis loops of three mean strains show a steady-state response from the stress-strain curves at a strain rate of 1 × 10−3 s−1 at a strain width of 5%, at a number of cycles of N=50 and at three mean strains (ɛm=0, + 1.0 and + 2.0%). The drop of stress at the mean strain of ɛm= -1.0% is larger in magnitude than that at ɛm=+1.0%; this is caused by the higher stress level at ɛm=- 1.0% as compared with the stress level at ɛm=+1.0%. From the results of stress amplitude and the stress drop behaviour, the magnitude of stress drop is hardly affected by the mean strain.