A model for non-linear creep in polypropylene

Abstract

Measurements of the creep behaviour of a polypropylene polymer under uniaxial tension have been modelled using a stretched exponential function with four parameters. Non-linear behaviour arises because one of the parameters, related to a mean retardation time for the relaxation process responsible for creep, is dependent on stress. Creep curves measured under a uniaxial tensile stress and a uniaxial compressive stress of the same magnitude are different. The differences can be described by relating the retardation time parameter to an effective stress that is determined by the magnitude of both the shear component of the stress and the hydrostatic component. This analysis has then been generalised to enable expressions to be formulated for creep behaviour under an arbitrary multiaxial stress state. This requires an assumption that either the Poisson's ratio or the bulk modulus is independent of time. The validity of this assumption has been evaluated through comparisons of predictions of creep under a pure shear stress with measurements, which show that a time-independent Poisson's ratio is the better approximation. Although not the main theme of the paper, examples are given illustrating the dependence of model parameters on the structure of the crystalline and amorphous regions of the polymer. This is particularly relevant to the application of the model to the analysis of the creep behaviour of welded polypropylene where properties will, in general, be influenced by the heat treatment.