Separation of humidity, strain rate and temperature effect on the orientation dependent micromechanical properties of cortical ovine bone

• New indentation setup, for testing at variable relative humidity levels (5–90% RH), strain rates (10 -3 to 10 2 s −1 and temperatures (24–60 °C). • Separation of humidity, strain rate and temperature effect on the orientation dependent micromechanical properties of cortical ovine bone. • Influence of bound and free water on the plastic deformation and fracture behavior. • Indication of the rehydration process of cortical bone. Bone is a hierarchically structured composite material, the basic building blocks are type I collagen, hydroxyapatite and water. Water has a major influence on load transfer by facilitating interface sliding. This has a considerable effect on the quasi-static mechanical properties. For this reason, bone must be characterized at quasi physiological conditions, in order to understand the mechanisms allowing inelastic deformation and causing fracture. In the current work, compression tests were performed on 419 ovine cortical bone micropillars at the lamellar length scale in axial and transverse fibril orientation. Experiments were carried out at varying strain rates (0.1 s −1 - 100s −1 ), temperatures (24°C − 60°C) and tissue water contents (3.1 – 8.3 wt%), allowing to study the influence of these crucial factors. The effect of temperature and water on the mechanical properties could be separated by applying a linear modeling approach. This results in a temperature related softening of 1.2 MPa/K and 33–111 MPa/wt.% water. The results of this study highlight the significance of temperature and tissue water content on the compressive strength of bone and are of value for future multiscale simulations of patient fracture risk allowing to take into account age and disease related decrease in bone water content.