Fracture Toughness Testing of Glassy Plastics

Despite widespread use of linear elastic fracture mechanics to describe brittle fracture in glassy plastics, prior work has not established a valid fracture toughness criterion based on the unique aspects of polymer mechanical behavior. In this study, fracture toughness tests on poly(methyl methacrylate) (PMMA) were correlated with crack-tip crazing and fracture surface morphology in order to assess the influence of dissipative processes and crack growth modes on standard toughness test variables. Various K_c criteria were examined for their relevance to PMMA, a material with a velocity dependence of fracture typical of glassy plastics. Since crack-tip crazing is restricted in this material, crack extension is essentially elastic for all normal specimen dimensions and precrack conditions. The load instability at the maximum of the load-displacement record is distinguished from the point of intrinsic instability, which is indicated by the K versus ˙a relationship. The load instability leads to a geometry-dependent K_c criterion which can be described analytically, and procedures for measuring a K_c based on intrinsic instability are defined for a standard fracture test. Measured values are in reasonable agreement with estimates derived from K versus ˙a in the literature. The implications of the geometric effects on instability are discussed for other materials and specimen geometries.