Rib formation in the fracture of polymethyl methacrylate

Abstract

Polymethyl methacrylate was fractured under a variety of loading conditions. Various linear features were observed on fracture surfaces but attention was centered on ribs of torn-out material. Such ribs were formed only in cases where loading conditions resulted in a tensile stress field ahead of the crack tip. Moreover the width of the ribs, and also their spacing, increased as the crack traversed the specimen into zones of increasing tensile stress. These observations, along with previously published data on the dependence of rib spacing on molecular weight and temperature, led to the conclusion that rib formation was due to secondary tensile fractures. These were identified to be showers of microcracks, from side views of the ribs. Fractographic evidence was obtained that the crack accelerates after formation of each rib (i.e. shower) and a “stick-slip” mechanism adopted.