|Ph.D Student||Cooper Israel Zvi|
|Subject||A Constitutive Model for Dynamic Failure of Ceramics|
Including Micro-Cracking and Porous Dilation
below the Hugoniot Elastic Limit
|Department||Department of Mechanical Engineering||Supervisor||Professor Emeritus Miles Rubin|
|Full Thesis text|
Silicon carbide is one of a few ceramics used in various armor applications due to its high hardness and low density. Spall tests on different grades of silicon carbide have been conducted over a wide range of impact levels. The Hugoniot Elastic Limit (HEL) conventionally represents the impact level at which inelastic deformation is initiated. It is typically determined by measuring the axial stress in plate impact experiments at the point when a significant change in slope occurs in the plots of particle velocity versus time. For many types of polycrystalline silicone carbide the spall stress can decrease with increasing impact stress even for impact stress levels significantly below the HEL. Consequently, the observed HEL does not truly separate elastic and inelastic response in these materials. A constitutive model has been formulated within a thermomechanically consistent framework in which relationships between damage, stiffness, inelastic distortion and changes in porosity are proposed. In the model, the reduction in the spall stress below the HEL is caused by rate-dependent damage evolution that is coupled to effective rates of inelastic distortion and porous dilation. Simulations of shock-wave propagation using the model show good agreement with test results. In particular, the model captures the variation of spall strength with impact loading, which has not been addressed by previous models. It has been shown that while damage and inelastic distortion may reduce the strength of the ceramic, bulking (i.e. porous dilation at positive pressure) coupled to rate-dependent inelasticity may stiffen the stress loading wave such that the HEL of the material appears to be higher.