Shear-Punch Testing of Human Cranial Bone and Surrogate Materials

In recent decades, traumatic brain injury has been at the forefront of public health and military injury research. Improved understanding of skull deformation and fracture are required to advance the development of numerical simulations and surrogate materials aimed at providing critical feedback for future protective equipment design. Numerical simulations of skull fracture require material properties and identification of fracture mechanisms for compressive, tensile, and shear loading; skull–bone failure mechanisms are complex and stress-state dependent. A knowledge gap exists on the mechanical response of human cranial bone subjected to pure shear loading. A shear-punch experimental method was designed to understand the shear response of human skull as a function of through-thickness for cadaveric calvaria specimens. The compliance of the shear-punch system was first tested on a polyurethane foam trabecular bone simulant and a stereolithographic additively manufactured cortical bone surrogate, and their shear strength values were compared to known values of human bone. The sandwich-like structure of human cranial bone consists of dense outer and inner tables separated by a highly porous central diploe layer. Force data obtained from the shear-punch experiments was analyzed as a function of normalized penetration depth to assess the shear strength compared to the microstructure. Preliminary results show 1% offset yield and ultimate shear strengths of human calvaria to be 17.7 ± 4.9 MPa and 25.5 ± 5.1 MPa, respectively.