Systematic Error in the Measure of Microdamage by Modulus Degradation During Four-Point Bending Fatigue

The accumulation of fatigue damage in bovine and human cortical bone is conventionally measured by modulus or stiffness degradation. The initial modulus or stiffness of each specimen is typically measured in order to normalize tissue heterogeneity to a prescribed strain [1,2]. Cyclic preloading at 100 N for 20 cycles has been used for this purpose in both uniaxial tension and four-point bending tests [1–3]. In four-point bending, the specimen modulus is often calculated using linear elastic beam theory as,   where F is the applied load, l is the outer support span, b is the specimen width, h is the specimen height, and ε is the maximum strain based on the beam deflection [2]. The maximum load and displacement data from preloading is used to determine the initial specimen modulus. The initial modulus and a prescribed maximum initial strain are then used to determine an appropriate load for fatigue testing under load control.