J-curve behaviour of human cranial bone

It has been shown that for bone, neither the strain energy release rate (Gc) nor the stress intensity factor (Kc) exist, but instead, they change as the crack grows. This behaviour is represented by the use of J-curves, which show the change of the material’s fracture resistance as a function of crack growth [1]. In this work, J-curves for Mode I are developed for human cortical cranial bone applying the Elastic-Plastic Fracture Mechanics (EPFM) principles. Samples of 5 unembalmed skullcaps (3 male and 2 female, N=72, 58 to 81 yo.) were tested in single edge notch beam (SENB) fashion. The samples were extracted from the external cortical layer of the frontal, left parietal and right parietal bones, and the notches were oriented in the radial, anterior (longitudinal) and lateral (transversal) directions. Videos of the tests were recorded at 1000 fps with an additional 10x macro-lens, with a white LED lamp (3000K) oriented at 30° above the samples. The crack propagation was measured by using videography [2] which consists of tracking the whitening zone, a phenomenon that occurs in front of the crack-tip due to microcracking. The resulting J-curve is shown in Fig. 1. The shape of the curves similar to the shape found for tibial and humeral cortical bone [3], and shows the instability of crack growth for human cortical cranial bone. No specific trend can be seen in terms of direction (radial, longitudinal or transversal). Moreover, curves for the different bones tested (frontal, left and right parietal bone) are distributed equally all over the range of crack displacement reached during the tests, and no remarkable differences can be found in terms of gender or age.