Blunt and Ballistic Impacts On Human Head Models: An Analytical And Numerical Study

Head injuries, as a leading cause of death, have become a major health care issue for civilians and soldiers. There has been an urgent need to understand mechanisms of such injuries. The objective of this dissertation research is to study some head injuries and related mechanisms using analytical and computational models. In Chapter 2, a new analytical (non-linear) model for the impact of a solid sphere on a fluid-filled spherical shell is developed by including the stress wave propagation effect in addition to the Hertzian contact deformations and the shell membrane and bending actions. A simplified (linearized) model incorporating the elastic energy loss due to the stress wave propagation is then formulated by using a linear force-deflection relation, which leads to a closed-form expression for the impact duration. By directly applying the newly obtained non-linear and linearized models, three representative problems simulating blunt head impacts are analyzed. In Chapter 3, head injuries induced by golf ball impacts are studied through computational modeling. A full human body model and a three-piece golf ball model are integrated to construct a new finite element model, and LS-DYNA is employed to perform simulations. To assess head injury risks, the impact force, von Mises stress, pressure, and first principal strain are computed in the current model and compared with existing experimental and simulation data. In Chapter 4, a finite element model is developed for an Advanced Combat Helmet (ACH) and validated against the experimental data obtained at the Army Research Laboratory. Ballistic head impact simulations are then performed for an ACH placed on a ballistic dummy head form embedded with clay as specified in the current ACH testing standard by using the validated helmet model. In Chapter 5, new constitutive models for hyperelastic materials are proposed using the upper triangular decomposition of the deformation gradient tensor, which are simpler than those based on the invariants of the right and left Cauchy-Green deformation tensors. Two examples are provided to illustrate applications of the new constitutive models, which can be adopted and further modified to simulate brain tissues