The mechanics of indentation fracture in poly(methyl methacrylate)

Abstract

The formation of stable radial cracks at indentations in PMMA is considered. The distribution of tensile stress in the surface adjacent to an impression made with a spherical indenter is taken to be that round an expanded hole in an infinite plate of plastic-elastic material, and the strain energy release rate G at a crack propagating in this stress field is calculated. As in the case of Hertzian fracture there is a size effect: G is proportional to the product of the radius of the plastic-elastic boundary (and hence to the radius of the impression) and the energy density of the stress field, so that according to this or any qualitatively similar model the radius (or depth) of the impression which first initiates fracture is a measure of the fracture properties. Indentation may therefore be used to assess ductile-brittle transitions in polymers in a variety of circumstances, such as increasing molecular orientation, decreasing temperature, or decreasing molecular weight. An experiment with irradiated PMMA which confirms the existence of the effect is described to illustrate the latter situation.