Influence of crack propagation orientation on R -curve fracture mechanics of equine cortical bone

Objectives for this research were to experimentally determine if bone exhibited R-curve behavior in multiple crack propagation orientations for specimens from the lateral and dorsal regions of equine third metacarpal cortical bone, and to investigate potential relationships of R-curve behavior with fractography and histologic findings. In the first set of experiments compliance calibration equations were developed relating the compliance of the specimen to crack length at any point in time during the R-curve test for each crack propagation orientation: transverse, longitudinal, and radial. In the second set of experiments R-curves were examined for cracks propagated across specimens in three different orientations. In the final set of experiments fractured specimens were examined using SEM imaging, histologic techniques, and surface topography mapping. Significant differences were found for the compliance calibration equations between each orientation, thus justifying using the calibration equation specific to an R-curve. All three orientations exhibited R-curve behavior, justifying a departure from traditional linear elastic fracture mechanics in order to fully characterize the fracture behavior of bone. The transverse specimens were found to have significantly higher maximum fracture toughness values, Kmax, than either of the other two orientations. For the transverse specimens the lateral region was found to have a significantly higher Kmax than the dorsal region; this was not the case for the other two orientations. The crack length associated with the maximum toughness value, amax was found to be significantly longer in the longitudinal and radial orientations when compared with the transverse orientation. The dorsal region had more alternating and bright osteons than the lateral region. But the hypothesis that these, and hooped osteons, would resist pullout, thus increasing fracture toughness, was not supported. The dorsal region had more osteons with larger diameters than the lateral region. These larger osteons provided more cement line length that could serve to initiate crack propagation and correspond to lower Kmax values for the dorsal region. For both the radial and longitudinal orientations the crack was significantly more likely to penetrate osteons with predominantly longitudinally oriented collagen fibers than other osteons with other collagen fiber orientations.