THE EFFECT OF TEMPERATURE AND PRESSURE ON NECKING OF POLYCARBONATE

The effect of temperature and pressure on necking behaviour of polycar bonate was studied in tension. Following the concepts elucidated by Co ates and Ward, neck geometry and neck propagation were related to the materials parameters: strain rate sensitivity and strain hardening par ameter. The relationship between neck profile and the true stress-true strain curve was established by combining an analytical expression fo r the macroscale mechanics of the flow process with a constitutive equ ation describing the true stress, true strain and true strain rate dep endence of the material. Having assumed a constitutive equation, the m aterials parameters were obtained from the stable neck geometry. The s train hardening parameter was found to be insensitive to changes in pr essure and temperature in the range studied and had a value of approxi mately 3.3. Since the strain hardening parameter affected primarily th e draw ratio, the latter was essentially independent of temperature an d pressure. The increase in strain rate sensitivity parameter with pre ssure and decreasing temperature from 0.028 to 0.035 was manifest as a gradual broadening of the neck. The molecular significance of the mat erials parameters is discussed in terms of the contributions of two se parate processes to plastic deformation: thermally activated yielding and the temperature and pressure independent stretching of a molecular network.