Relaxation of residual thermal stresses and strains: Effects on tensile stress relaxation and creep

Abstract

The tensile stress relaxation and creep behavior of thermally quenched and aged polymer glasses have been investigated. Theoretical and experimental results for the effect of residual thermal stresses and strains on the creep and stress relaxation behavior of polymer glasses are presented. Novel optical and mechanical techniques were used to determine if either the residual strains and/or the residual stresses were changing at room temperature in quenched samples of bisphenol‐A polycarbonate. Relaxation of the residual internal stresses at room temperature was observed and this relaxation is proposed to occur by a conversion of the residual elastic strains into anelastic and persistent strains. The effects of these residual strains and stresses on the tensile stress relaxation and creep behavior have been modeled by dividing up the sample into n contiguous slices. The dimensions of these slices are assumed to be dependent upon the rate of quenching of the slice and upon a mechanical equilibration of the slices. Incorporation of these effects into constitutive equations for tensile stress relaxation and creep yields equations which predict how residual volume, strains, and stresses can alter the mechanical response. Simulations with the model indicate that relaxation of the internal stresses will reduce the long term compliance, while recovery of the internal strains will increase the long term modulus of the glass. It is suggested that similar effects may be observed in other types of mechanical testing and in solvent cast samples.