Biaxial stress relaxation of high impact polystyrene (HIPS) above the glass transition temperature

A procedure was developed and experiments were performed to measure the biaxial stress relaxation of High Impact Polystyrene (HIPS) sheets above the glass transition temperature. Tests were performed at various temperatures and strain magnitudes. The data were normalized to unity at time zero to isolate the time dependence of the stress from the strain dependence. The results show that the time dependent biaxial stress relaxation properties were independent of the magnitude of the stress or strain for small strains. The stress relaxation curves were fitted to a Prony series and i1 was found that time‐temperature superposition could be applied to interpolate the normalized relaxation. Uniaxial stress relaxation tests were also performed and the results were compared to that of the biaxial testing program.