Damage accumulation of bovine bone under variable amplitude loads.
Stress fractures, a painful injury, are caused by excessive fatigue in bone. This study into damage accumulation in bone sought to determine if the Palmgren-Miner rule (PMR), a well-known linear damage accumulation hypothesis, is predictive of fatigue failure in bone. An electromagnetic shaker apparatus was constructed to conduct cyclic and variable amplitude tests on bovine bone specimens. Three distinct damage regimes were observed following fracture. Fractures appeared ductile from low cyclic amplitude (< 4,000 με), brittle, from high cyclic amplitude (> 9,000 με), and a combination of ductile and brittle from mid-range cyclic amplitudes (6,500 – 6,750 με). Brittle and ductile fracture mechanisms were isolated into variable amplitude loading tests. PMR predictions of cycles to failure consistently over-predicted fatigue life when mixing isolated fracture mechanisms. However, PMR was not proven ineffective when used with a single damage mechanism.