STATISTICAL-ANALYSIS OF CRACK ADVANCE IN CERAMIC COMPOSITES

Crack propagation in multi-phase composites is affected not only by th e externally applied loads, but also the internal residual stresses ar ising from thermal expansion mismatch and the strength of individual c omponents. The influence of internal stresses is pronounced in brittle materials due to their sensitivity to local stress concentrations. Th e material being modelled in the current investigation is a SiC-reinfo rced Si3N4 composite with a random microstructure and a range of volum e fractions, particle sizes and interfacial toughnesses. These three v ariables all influence crack advance, but to very different degrees. F or example, the particle size is calculated to have a greater effect o n the overall toughness in a specimen in the initial stages of crack a dvance than the volume fraction, with larger particles leading to high er macroscopic toughness. The strongest influence on the apparent crac k resistance is interfacial toughness. Variable interfacial toughness is also shown to have a strong effect on the amount of expected interf acial failure and the resulting fracture surface roughness. The most i nteresting result is that the effective composite toughness is calcula ted not to approach a maximum equal to the critical values of the comp onents, rather it can be up to 30% higher due to the residual thermal stresses. This result highlights the importance of understanding and i ncluding residual stress effects into composite failure analyses.