Heterogeneous nucleation of crazes below notches in glassy polymers

Abstract

A slip-line field study of craze initiation at plastic zone tips below notches in polycarbonate has been carried out over a wide temperature range. Combination of the critical values of hydrostatic tension with Yee's temperature data for bulk modulus leads to a calculated critical elastic dilation in the region beyond the zone tip that is nearly temperature-independent. Microscopic investigation, however, shows strong evidence that crazes in polycarbonate and other glassy resins initiate at micrometre-size foreign particles. Assuming that the foreign particle is a rigid adherent sphere, a stress analysis for the particle situated on the elastic side of the elastoplastic boundary has been effected; the particle is found to raise the hydrostatic tension over the value in the homogeneous field by 104%. In view of this analysis the polycarbonate experimental results suggest a temperature-independent, critical local elastic dilation of greater than 2% as a crazing criterion. Based on these results it may be suspected that all crazes — surface or internal — in glassy polymers, except at crack tips, are heterogeneously nucleated at impurity sites.