The macroscopic yield behaviour of polymers

A yield criterion, not previously compared with the actual macroscopic behaviour of polymers, is herein compared with the pressure-modified octahedral shear stress criterion earlier suggested by others.This new relation, which is a version of the von Mises criterion, accommodates differences in tensile and compressive yield strengths and accounts for any dependence of yielding on the hydrostatic component of the applied stress state. With the use of thin-wall tubes accounting for the majority of experimental points, the yield behaviour of polycarbonate and polyvinylchloride was investigated. Besides these findings, results previously reported by others have also been utilized in this paper. Since these various studies employed quite different polymers, the excellent overall correlation of experiment with prediction should merit the serious attention of persons interested in the macroscopic yield behaviour of polymers. Comparisons between this new criterion and the modified octahedral shear stress are also made in regard to the effect of pressure on subsequent yield behaviour. Although not fully verified, it is suggested that the predictions which result using this new criterion, seem a little more reasonable.