The effect of stress concentrations on the fracture strength of polymethylmethacrylate

Polymethylmethacrylate (PMMA) is a brittle material whose strength is greatly affected by stress concentrations such as notches and defects. This paper describes a method known as the Theory of Critical Distances (TCD) which uses a critical stress and a critical material distance. The method was tested against experimental data on the fracture strength of PMMA in two forms: commercial Perspex and surgical bone cement. Samples were made containing stress concentrations of various shapes and sizes: notches, holes and hemispherical depressions. Initial predictions were poor, but improved when a modified form of the theory was used, taking the critical stress to be higher than the material’s UTS. One interesting observation, which is well predicted by the theory, is that features with a stress concentration factor less than some critical value (approx. 2) or smaller than a critical size (approx. 0.5mm) cause no reduction in strength.