THE DEPARTMENT OF CIVIL AND SYSTEMS ENGINEERING
ADVISOR LORI GRAHAM-BRADY, PROFESSOR
ANNOUNCE THE THESIS DEFENSE OF
Friday, April 23, 2021
Contact Elena Shichkova for access to this presentation.
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. A simple phenomenological model has also been proposed that suggests a transition criterion resembling the one obtained from the numerical model.
A micromechanical multi-physics model for ceramics, that integrates key physical mechanisms, has been recalibrated using the new granular transition criterion and used to simulate impact experiments with boron carbide in ABAQUS. 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.
Finally, the dependence of microstructure on fragment morphology has been examined via a modified version of the fragmentation and crack coalescence model. 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.