Crack-microstructure interactions in ceramic matrix composites

A theory has been proposed to predict the combined influence of residual stresses from thermal and elastic mismatch on crack path in ceramic matrix composites. The theory predicts changes in crack-reinforcement interactions with crack tip stress intensity for nine possible combinations of thermal and elastic mismatch in which all two phase ceramic composites can be classified. For two of the nine combinations, radical transitions in crack propagation behavior, such as between attraction/avoidance and debond/fracture of reinforcements, were expected, while, for the rest of the combinations, different degrees of crack-reinforcement interaction are predicted but with gradual or no transitions in crack propagation behavior.; The theoretical predictions were verified by quantification of crack path in model ceramic composites representing five of the nine thermal and elastic mismatch combinations. Cracks at different mean velocities were obtained by a novel technique involving loading and unloading the indentor at different constant crosshead rates. A stereological parameter, the degree of crack-particle contact, R, that measures the amount of crack-reinforcement contact beyond random was defined and used to characterize the crack path at different crack velocities or stress intensities. Other parameters, such as the ratio of number of particles debonded to those fractured, the number of particles intersected per unit crack length and average debond length per particle, have also been used to quantify the crack path tendency. This work is the first systematic quantification of radical transitions in crack path in ceramic matrix composites.; The effect of indentation peak load on crack propagation behavior was also investigated. Quantification of crack path at different loads showed differences in crack propagation behavior which were interpreted in terms of change in indentation crack velocity with indentation peak load. Approximate crack growth rates were determined by using the values of indentation crack length produced in the time taken to unload the indentor. The approximate crack growth rates and the quantitative measurements of crack path indicated that, with increase in indentation load, the crack velocity increased in glass composite materials, but it decreased in crystalline composite materials. Differences in the nature of damage at the indentation site were also correlated with the residual stress in the ceramic composites.