Successive stress relaxation testing was used to investigate the strain hardening of polypropylene and polystyrene in the stage of deformation before yielding. By combining this information with that of a stress relaxation test it was possible to measure the change in flow stress with plastic strain or “workhardening” parameter K. K has been associated with the nucleation of “defects” of some sort which slow down the kinetics of the deformation process.
Both polymers were found to strain harden in this deformation region. In polystyrene, the amount of time need to relax through a fixed stress increment reached a plateau at a point corresponding with visible crazing in the gage section of the sample. The workhardening parameter K was determined and found to decrease with stress. By plotting the rate of change of flow stress with total strain plots were obtained which avoided the use of strain, an ill-defined parameter in materials which change state during deformation. From these plots it was seen that polystyrene exhibits a well-defined linear region at small strain whereas polypropylene deviates from linearity immediately. Hardening of polystyrene was observed even in the linear response regime.