| The influence of crack path on fracture toughness of particle-reinforced ceramic-matrix composites was examined. The composite systems studied, namely, c-ZrO2/Al2O3, glass/SiC, and glass/Al 2O3, were fabricated using powder processing, with controlled microstructure. Vickers indentation, indentation strength in bending, and Chevron-notched bending tests were used to grow cracks stably and obtain fracture toughness or crack resistance of the composites. The crack path was characterized using a stereological parameter, degree of contact, R, representing the amount of crack-reinforced particle interactions. It was found that the thermal and elastic misfits between the matrix and the reinforcement, the loading and/or unloading rate, and the test environment all significantly influenced the crack path. The crack propagation velocity was directly related to the variation in loading rate. The amount of particle-crack interaction significantly influenced the fracture toughness of ceramic composites. For a composite in which the matrix had higher thermal expansion coefficient (α) and lower Young's modulus (E) than the particles, increasing interaction between particles and cracks decreased fracture toughness. For a composite in which the matrix had lower α and lower E than the particles, decreasing interaction between particles and cracks decreased the fracture toughness. When excluding the environmental effect, the study demonstrated that crack-particle interactions played a key role in influencing the fracture behavior of the composites. An analytical model was developed to explain the relationship between the quantified crack path and the fracture toughness of the composites. The model well describes the experimental observations and the modeling results matched the experimental results well. Microstructural based finite element analysis, OOF, was employed to simulate the fracture behavior of the composites. The OOF results have shown the same details of crack path attraction and avoidance of the particles as a function of material variables that were shown in the experiments. This study demonstrated that application of degree of contact parameter provides a simple and useful tool for analysis of the influence of variables on particle-crack interaction and influence of such interaction on fracture behavior of ceramic composites. |
