Time-dependent response of polypropylene after strain reversal

Observations are reported in tensile relaxation tests on specimens subjected to stretching up to various maximum strains e, followed by retraction to various minimum strains E. Experimental data show that shapes of relaxation curves on pre-loaded samples are strongly affected by strain increment Delta epsilon = epsilon(max) - epsilon: (i) when Delta E is small, stress monotonically decays with time, (ii) with an increase in Delta epsilon, the relaxation process becomes non-monotonic, and (iii) at relatively large increments Delta epsilon, stress grows with time. A two-phase constitutive model is derived for the viscoelastic and viscoplastic responses of semicrystalline polymers under arbitrary deformations with small strains. Adjustable parameters in the stress strain relations are found by fitting the observations. Numerical simulation demonstrates that the model correctly describes the time-dependent behavior of polypropylene subjected to cyclic pre-loading in creep and relaxation tests. (C) 2010 Elsevier Ltd. All rights reserved.