COMPRESSIVE FATIGUE BEHAVIOR OF BOVINE TRABECULAR BONE

We studied the fatigue behavior of bovine trabecular bone specimens un der stress control using a sinusoidal uniaxial compressive load profil e with a frequency of 2 Hz. The stress range was determined from the c orresponding initial global platen-to-platen strain range, where the m aximum initial strain was between 0.8 and 2.1% and the minimum strain was 0.6%. The local strain distribution was measured on the same type of specimen by affixing glass spheres and photographing them in the un loaded and loaded positions using multiple exposures. The number of cy cles to failure (defined as a 5% decrease in secant modulus) was stron gly correlated with the initial global maximum strain (r2 = 0.78) and ranged from 20 cycles at 2.1% strain to 400,000 cycles at 0.8% strain. All of the fatigue specimens showed a region of transverse failure ap proximately 1 mm from the end of the specimen Microscopic examination of the failure zones revealed two failure modes: a straight transverse brittle-like fracture through the trabeculae, most often found in tra beculae transverse to the loading direction, and buckling-like failure , common in oblique trabeculae, involving bending and splitting. The l ocal strain increased towards the ends of the specimens to a value 2-4 times that in the middle. Modulus degradation with the number of cycl es was distinctively different for high-cycle and low-cycle fatigue, s uggesting the possibility that both creep and damage accumulation cont ribute to fatigue failure of trabecular bone.