Evaluation of fracture toughness and proposal of microstructurally-controlled composites by fracture-mechanics simulation

Recently we have constructed a simulation program based on the fracture mechanics for crack initiation and growth in discontinuously-reinforced MMCs. Parametric studies are carried out to evaluate the effects of reinforcement properties, such as volume fraction, tensile strength, interfacial bonding stress and spatial pattern of distribution, on crack initiation and growth characteristics through SiC whisker-reinforced 6061Al alloy composites. The crack initiation toughness decreases and simultaneously the crack growth resistance increases with increasing volume fraction. Degradation of both interfacial bonding stress and fiber strength affect mostly the crack initiation toughness. On the other hand, spatial patterns of whisker distribution have a certain influence on crack growth resistance, and there exists an optimum extent of whisker clustering for the crack growth resistance. Based on the results of the simulation, MMC with artificial and periodic clustering of whiskers has been proposed. It has been fabricated actually by high pressure infiltration casting using preforms made from granulated whiskers, and its mechanical properties and fracture characteristics are evaluated. It is certain that the crack growth resistance of the MMC is improved without spoiling other mechanical properties by introducing clustering.