Influence of Boron Additions and Heat Treatments on the Fatigue Resistance of CoCrMo Alloys.

Cobalt-based alloys are widely used in the manufacture of joint prostheses. In this study, the effect of boron additions and heat treatment on the ASTM F75 was evaluated by rotating bending fatigue. The boron ranged from 0.06⁻1 wt %. The alloys were tested in as-cast and heat-treated conditions. In the as-cast condition, the infinite life was observed at 380 MPa, improving to 433⁻615 MPa according to the amount of boron added. In the heat treatment condition, the fatigue resistance was improved only in the base alloy. The addition of 0.06 wt % boron and heat treatment led to the same resistance as in the as-cast condition. Adding large amounts of boron combined with heat treatment diminished the fatigue limit. The fracture analysis revealed primarily brittle behaviour with some ductile features even on the same sample; only the heat-treated alloy with 0.06 wt % boron was clearly ductile. This alloy also exhibited notably better toughness to crack propagation.

Microdamage distribution in fatigue fractures of bone allografts following gamma-ray exposure.

PURPOSE: Although significant differences in bone properties have been extensively studied, results vary when bones are exported to gamma radiation of a range usually used for sterilization purposes (25-35 kGy). Hence, the aim of this work was the study of the mechanical properties and microdamage development of human bones used as allografts following gamma-ray exposure, followed by an extensive statistical analysis of microdamage effects in fatigue behaviour. METHODS: Specimens of the cortical region of human femurs were exposed to 15-25 kGy and 26-30 kGy radiation levels, then they were subjected to compression fatigue tests until fracture. The fatigue life was determined in relation to the radiation level, and the evolution of microdamage was assessed using fluorescence microscopy in order to calculate characteristic lengths of microcracks. RESULTS: Significant differences in fatigue life were detected (p < 0.05) between non-radiated (control) and radiated specimens, resulting in a drastic 89.2% fatigue life reduction of the 15-25 kGy group, and 95.3% in the 26-30 kGy group, compared to the reference. Microdamage analysis showed a considerable increase in microcrack lengths when bone was exposed to gamma radiation, which may indicate that bones used as allografts could fracture at some point when subjected to loading conditions in vivo. CONCLUSIONS: The results of our research indicate that even if a range of 15-25 kGy is suggested to sterilize bone allografts, such practice needs to be reconsidered. In addition, using Weibull distribution, this work describes the conditions in which microcracks grow towards the fracture of bones in relation to the decrease in their mechanical properties.