TIME-DEPENDENT DEFORMATION OF POLYPROPYLENE IN RESPONSE TO DIFFERENT STRESS HISTORIES

Tensile strains have been determined as a function of time for polypro pylene during (a) two-step loading, (b) creep recovery following remov al of a load, and (c) intermittent load application. Data are presente d at 23 degrees C for specimens of different physical age, for differe nt stress levels in the non-linear range and various durations of load ing. The results are compared with predictions based on a pseudo-linea r model. They have also been analysed using a modified superposition p rocedure that allows for changes in mean retardation time due both to physical ageing and to the application and removal of loads. This anal ysis has provided useful information on the variations of molecular mo bility during the different loading histories. The functions and assoc iated parameters used in the analyses could also form the basis of a m ethod for presenting design data on plastics. Crown Copyright (C) 1997 Published by Elsevier Science Ltd.