SUMMARY

The human skull is a remarkable structure that exhibits a host of complexities in its structural dynamics. Motivations to research the human skull dynamics in the existing literature have spanned from studying bone-conducted sound to head injuries. Most recently, its potential leveraging in the context of medical ultrasound for the brain via guided waves has emerged as another research domain. Each of the cranial bones comprising the human skull is in general a three-layered composite structure, consisting of the porous diploë sandwiched by cortical tables, although geometric irregularities including regions where the diploë is absent are common. Aside from geometric complexity, cranial bone may also exhibit spatially varying porosity and material properties. Furthermore, sutures, which are joints between different cranial bones, may affect the overall skull stiffness and damping. This work is a fundamental investigation into the human skull dynamics for (1) high-fidelity modeling of composite bone vibration in non-sutured (monolithic) and sutured, dry and degassed bone segments, (2) elastic parameter and damping identification to use in vibration/vibroacoustic simulations and in guided wave simulations, and (3) ultimately to create a complete cranial vault model capable of producing accurate skull natural frequencies and mode shapes in the absence and presence of cavity fluid loading representing the brain. To this end, here we create a framework for developing composite finite element models of cranial bone segments. The numerical models are used in an optimization scheme with experimental vibration results to extract the elastic moduli from different cranial bone regions. The effects of sutures are to be explored in an analogous fashion to the work on monolithic bone segments presented here. Additionally, the effects of fluid-loading due to degassing and submersion of skull bone will be considered. Finally, these studies will be synthesized to develop an experimentally validated finite element model of the human skull (cranial vault), including the cavity fluid representing the brain, to accurately capture the overall vibroacoustic behavior. The rigorous experiments and modeling framework, as well as their reconciliation, and the identified elastic and dissipative parameters presented in this work are expected to be of use for a broad spectrum of research from blast-induced response to guided waves and mode conversion in medical ultrasound.