FRAGMENTATION, GRANULAR TRANSITION & IMPACT PERFORMANCE OF CERAMICS

Brittle materials under impact loading exhibit a transition from a cracked solid to a granular medium. Appropriate representation of this transition to granular mechanics and the resulting initial fragment size and shape distribution in computational models is not well understood. The current work provides a numerical model to analyze competitive crack coalescence in the transition regime and provides insight into the onset of comminution and the initial conditions for subsequent granular flow. Crack statistics obtained from initial flaws using a wing crack growth based damage model have been used to discretely model elliptical cracks in three dimensions, with and without a minimal intersection constraint. These cracks are then allowed to coalesce with nearby cracks along favourable directions and the output fragment statistics are predicted. The evolving fragmentation offers insight into the onset of comminution as well as the final transition to granular mechanics and the resulting initial fragment statistics. A simple phenomenological model has been proposed that suggests a transition criterion resembling the one obtained from the numerical model. A micromechanical multi-physics model for ceramics has been recalibrated using the new granular transition criterion and new experimental data, and used to simulate impact experiments with boron carbide in ABAQUS. The dominant physical mechanisms in boron carbide have been identified and simulated in the framework of an integrated constitutive model that combines crack growth, amorphization and granular flow. The integrative model is able to accurately reproduce some of the key cracking patterns of Sphere Indentation experiments and Edge On Impact experiments. Based on this integrative model, linear regression has been used to study the sensitivity of sphere indentation model predictions to the input parameters. The sensitivities are connected to physical mechanisms, and trends in model outputs have been intuitively explored. These results help suggest material modifications that might improve material performance, prioritize calibration experiments for materials-by-design iterations, and identify model parameters that require more in-depth understanding. The dynamic performance of armor ceramics is related to their microstructure through various mechanisms that are either intrinsic to the material or the specific loading conditions. Although this relationship has previously been explored in the context of some mechanisms like crack growth and interaction, the microstructural dependence of the parameters of some other mechanisms such as fragmentation and subsequent granular flow is not well understood. This thesis studies the influence of microstructural defects on the fragment size and shape distribution in comminuted armor ceramics, which has a significant effect on the associated granular mechanics of the material. The crack coalescence and fragmentation model has been modified to account for anisotropic crack growth and to predict fragment shape more accurately. Fragment statistics predicted via the model are used to infer fragment morphology for different initial microstructures. Existing geomechanics literature then helps to link the trends to bulk granular friction. The observations suggest that an optimal defect size and spacing in the initial intact ceramic will enhance impact performance as compared to fine dispersed defects in the ceramic matrix. In some impact experiments, structural fragmentation of ceramics could precede microstructure driven granular transition and limit the strength of ceramics. In the last part of this thesis, the link between structural fragmentation, fragment sizes and analytical structural fragmentation criteria are explored and suggested as future directions