MATRIX CRACKING WITH IRREGULAR FRACTURE FRONTS AS OBSERVED IN FIBER-REINFORCED CERAMIC COMPOSITES

As a result of matrix cracking in fiber reinforced composites, fractur e planforms assume a wide variation of profiles due to the fact that f iber bridging strongly affects the behavior of local crack fronts. Thi s observation raises the question on the legitimacy of commonly used p enny-shaped crack solutions when applied to fiber reinforced composite s. Accordingly, investigation of the effects of fracture front profile s on mechanical responses is the thrust of this paper. We start with t he solution of a penny-shaped crack in a unidirectional, fiber reinfor ced composite, which demonstrates necessarity of considering wavy frac ture fronts in fiber reinforced composites. A theoretical framework fo r fiber reinforced composites with irregular fracture fronts due to ma trix cracking is then established via a micromechanics model. The diff erence between small crack-size matrix cracking and large crack-size m atrix cracking is investigated in detail. It is shown that the bridgin g effect is insignificant when matrix crack size is small and solution of effective property are obtained using Mori-Tanaka's method by trea ting cracks and reinforcing fibers as distinct, but interacting phases . When the crack size becomes large, the bridging effects has to be ta ken into consideration. With bridging tractions obtained in consistenc y with the micromechanics solution, and corresponding crack energy bac ked out, the effective properties are obtained through a modification of standard Mori-Tanaka's treatment of multiphase composites. Analytic al solutions show that the generalization of a crack density of a penn y shaped planform is insufficient in describing the effective response s of fiber-reinforced composites with matrix cracking. Approximate sol utions that account for the effects of the irregularity of crack planf orms are given in closed forms for several ii-regular crack planforms, including cracks of cross rectangle, polygon and rhombus.