Multiscale response of the human skull to quasi-static compression

Full thickness skull specimens were extracted from the human crania, with both the inner and outer surfaces intact. The BVF-morphology (bone volume fraction) of these specimens had been previously characterized in detail and reported, with high-resolution micro-computed tomography at ~5 μm resolution. A subset of these specimens was loaded in the direction normal to the outer surface in quasi-static compression. In contrast to many previous mechanical characterization studies of skulls, following two additional procedures were used in this study. (1) Fresh skull specimens were used, which were stored refrigerated before mechanical loading, instead of using embalmed or dried specimens. (2) Furthermore, using digital image correlation, non-contact full-field inhomogeneous strain measurements were made using the speckled specimen surfaces and the compression platens, also avoiding possible errors in strain measurements from machine compliance and due to irregularities in the loading surfaces of the specimen. The averaged far-field compressive mechanical response was obtained from these local full-field measurements on the composite bone specimens. Assuming a layered structure for the skull bone, using the local averaged full-field strain measurements of each layer, a power law was used to represent the relationship between initial mechanical response and the averaged BVF of the layers. Using the measured porosity maps of the rest of the non-compressed specimens, this relationship was used to predict the modulus-depth dependency of the skull bone and the variabilities associated with the structure. The mechanical properties and density as a function of the normalized thickness of the skull are presented for use in finite element simulations to model the skull with the desired degrees of complexities, also based on the region of action, depending on the goals of the computer simulation of the impact: either as a single homogenous layer, three-layer sandwich, multilayer heterogeneous or continuous elemental structure. In addition, a power law was derived relating the compressive failure strength and bone volume fraction (BVF) for the skull bone.