Effect of collagen damage induced by heat treatment on the mixed-mode fracture behavior of bovine cortical bone under elevated loading rates

The fracture resistance of bone has been attributed to a competition of sub-micron lengthscale intrinsic mechanisms, including plasticity conferred by collagen stretching and intermolecular sliding and much larger lengthscale extrinsic mechanisms such as crack deflection and bridging, In this study, the contribution of intrinsic toughening mechanisms on the dynamic fracture behavior of bovine cortical bone is investigated. Single edge notched cortical bone specimens were extracted from the mid-diaphysis of a bovine femur with dimensions in accordance with ASTM E399. Four specimen groups are studied, a control group, and groups subjected to two-hour heat treatments of 130 C, 160 C and 190 C, respectively. Using a trypsin-hydroxyproline assay to determine the percent of denatured collagen achieved by each heat treatment, it is shown that the 160 C and 190 C groups have accumulated substantial collagen network damage compared to the 130 C and control groups. Threepoint bend drop tower experiments with impact velocities of 1.6m/s. The selected impact velocity results in a nominal stress intensity factor rate of K = 1.5 x 10 MPa m1/2/s.Specimen's speckled surfaces were imaged at 500.000 fps during deformation and post processed using digital image correlation to determine the in-plane displacement fields, Using an orthotropic material linear elastic fracture mechanics formulation a.nd over-deterministic least-squares analysis, the critical mode-I and mode-II stress intensity factors (i.e., fracture initiation toughness) were determined immediately proceeding fracture. As the heat treatment temperature increases (and the damaged collagen content increases), a weak but decreasing trend in fracture toughness was observed. Of particular note, for the 160 C and 190 C heat treatments, it was observed that the mode-II fracture initiation toughness is larger than the mode-I fracture initiation toughness. Regardless of the heat treatment condition, the mode-II fracture initiation toughness was comparatively less affected. For the specific case of Haversi an bovine cortical bone whose collagen network has been denatured using heat treatment, a trend is observed pointing to collagen primarily conferring mode-I fracture initiation toughness, opposed to mode-II fracture initiation toughness, for the transverse fracture orientation.

Automated summary

The contribution of intrinsic toughening mechanisms on the dynamic fracture behavior of bovine cortical bone is investigated. It is shown that as the heat treatment temperature increases, there is a weak but decreasing trend in fracture toughness, and the mode-II fracture initiation toughness is larger than the mode-I fracture initiation toughness for the 160℃ and 190℃ treated specimens.