Effect of fatigue-induced microdamage on the compressive properties of bovine trabecular bone
Understanding the effect of microdamage accumulation on the failure mechanisms of bone is important for treatment and prevention in degenerative diseases such as osteoporosis. The common sites of fracture in osteoporotic patients – arm, vertebra, hip – have large proportions of trabecular bone, which make them of particular interest. Degenerative bone diseases disrupt bone’s natural remodeling ability, meaning the microdamage which is normally repaired begins to accumulate, increasing a patient’s risk of fracture. The current study aims to investigate the effect of microdamage accumulation on subsequent monotonic tests-to-failure using cored bovine trabecular bone samples. Various levels of microdamage were induced via pre-determined quantities of compressive fatigue loading on cored samples, which were subsequently tested in a uniaxial, compressive, test-to-failure. A parabolic relationship was found in the yield strain (from the test-to-failure) plotted against the reduction in modulus from fatigue loading, as well as in the normalized yield stress (from the test-to-failure) plotted against the reduction in modulus from fatigue loading. These results support the hypothesis that increases in yield strain at low quantities of fatigue induced damage could be attributed to a microdamage stress relieving mechanism in which small microdamage sites nucleate rather than growing larger sites. The results are indicative that a critical amount of this proposed mechanism exists, after which point further microdamage accumulation becomes detrimental to the mechanical properties obtained in subsequent compressive testing-to-failure. Once this critical amount of fatigue-induced damage is induced, a significant decrease in the subsequent yield strain is noted representing the growth and coalescence of few, large microdamage sites that become responsible for yielding in the test-to-failure. X-ray micro-computed tomography was used in an attempt to characterize damage propagation during post-yield loading of select trabecular bone samples during monotonic failure testing. Qualitative three dimensional imaging suggests that two distinct damage propagation types may exist: the first appearing to originate from the centre of trabeculae, while the second appears to originate from trabecular surfaces.