A controlled-notch specimen to study fatigue crack initiation in bone cement

Despite the extensive literature on the mechanical characteristics and failure properties of poly(methyl methacrylate) bone cement, little is known of its fatigue crack initiation process. The most likely in vivo bone cement fatigue crack initiation sites are internal flaws and irregularities on the bone cement surface. The stress concentration created by a flaw, and subsequently the stress state at that flaw, depends on the flaw geometry. To model the fatigue crack initiation process of a flaw, it is necessary to reproduce the stress state at that flaw. In this study, a special mold was designed to introduce notches with specific tip radii into fatigue specimens. The notch was molded into the specimen to simulate the in vivo flaw formation process. The molding method allows control of the stress concentration by specifying the notch tip radius. We created notched specimens where the tip radii of the notches ranged from "sharp" (< 3 microm) to 400 microm. The results demonstrated that notched specimens created by the special mold satisfied two necessary requirements for fatigue crack initiation studies: (1) the material microstructure at the notch tip must not be disrupted by the notching process, and (2) the notch tip stress field, determined by the notch tip geometry, must be reproducible.