The effect of temperature and pressure on necking of polycarbonate

Abstract

The effect of temperature and pressure on necking behaviour of polycarbonate was studied in tension. Following the concepts elucidated by Coates and Ward, neck geometry and neck propagation were related to the materials parameters: strain rate sensitivity and strain hardening parameter. The relationship between neck profile and the true stress-true strain curve was established by combining an analytical expression for the macroscale mechanics of the flow process with a constitutive equation describing the true stress, true strain and true strain rate dependence of the material. Having assumed a constitutive equation, the materials parameters were obtained from the stable neck geometry. The strain hardening parameter was found to be insensitive to changes in pressure and temperature in the range studied and had a value of approximately 3.3. Since the strain hardening parameter affected primarily the draw ratio, the latter was essentially independent of temperature and pressure. The increase in strain rate sensitivity parameter with pressure and decreasing temperature from 0.028 to 0.035 was manifest as a gradual broadening of the neck. The molecular significance of the materials parameters is discussed in terms of the contributions of two separate processes to plastic deformation: thermally activated yielding and the temperature and pressure independent stretching of a molecular network.