In order to understand understanding of the fracture/damage process and risk of fracture with decreased human cortical bone properties, there has been great number of studies for predicting the stiffness and strength of bone, such human cortical bone. However, the effects of damage are usually studied in terms of the mechanical property degradation of modulus and strength in the loading mode in which damage was accumulated. In-vivo loading and understanding of damage is complex. Therefore, in this dissertation, the effect of damage on human cortical bone in one loading mode on other mode’s mechanical properties was investigated.

To investigate the effect of test conditions and recovery time, mechanical tests were conducted with zero-stress hold and zero-strain hold, and with from 1 minute to 100 minutes recovery time. Damage effects were measured from pre- and post-damage diagnostic cycles. Results showed that zero-strain hold condition and longer recovery time yield less degradation in modulus measures.

To investigate cross-modal effect of damage, 7-cycle damage test was conducted. Cross-modal effect of damage were measures from three pre-damage (tension, compression and torsion) and three-post-damage diagnostic cycles about the one of three damage loading mode with three different damage strain magnitudes. Results showed that axial damage affected modulus degradation in axial mode and torsional damage affected torsional modulus degradation. Viscoelastic parameter and strength degradation showed similar results. This may caused by different damage morphology of tension, compression and torsion damage (i.e. microcracks and diffuse damage).

Also bi-modal damage test was conducted to investigate cross-modal effect of damage on human cortical bone. Damage was induced by tension+torsion or compression+torsion mode with two different sets of magnitudes. The results also showed that degradation of axial modulus was affected by both damage mode and damage magnitudes while torsional degradation was only affected by damage magnitude. This results support un-coupled effects between axial and torsional mode, which investigated in previous single mode damage test.

Finally, the contribution of linear microcracks and diffuse damage to mechanical properties degradation was studied by micromechanical damage model. The model compared to tensile damage experimental data and showed the contribution of linear microcracks to the total degradation was very small;​ suggesting that diffuse damage could have primarily responsibility to property degradation by tensile damage in human cortical bone.