Fatigue in porous PMMA: The effect of stress concentrations

This paper reports on an investigation into the effect of notches and other stress concentration features on the high-cycle fatigue strength of polymethylmethacrylate. The material was used in a porous form, as is found in bone cement and other medical applications. Sharp notches and circular holes reduced the fatigue strength significantly and this could be predicted with an approach known as the theory of critical distances (TCD) which uses two parameters: a critical distance and a critical stress. Introducing large, blunt notches or small hemispherical depressions into the porous samples had no effect on fatigue strength. Mixing under vacuum to reduce porosity gave no significant improvement, but pore-free samples had a much higher fatigue strength. These findings could be explained with regard to the role of porosity. The results provide a framework for assessing the effects of stress concentrations in bone cement and thus improving the design of implants and the procedures used in their fixation.