Fatigue damage in a unidirectional SiC/Al composite due to push-pull and zero-pull loading

Axial fatigue behaviour due to fully reversible and zero-tension cyclic loads on specimens cut from a 5 mm thick panel of a unidirectional SiC/A1 composite has been investigated at room temperature. The panel contained 40 vol.% SiC fibres (SCS-2), sandwiched between 32 layers of A1 6061 foils, which were bonded together by hot-pressing. The loading was always parallel to the fibres. Steady hysteresis loops were observed in the stress-strain plot after about 3 cycles of loading. A plot of S/N curves showed that at load ratio R = 0 the fatigue strength of the composite was about 3 times higher than that of the monolithic matrix metal. At R = −1, however, the fatigue strength of the composite was even lower than that of the matrix metal. At both R = 0 and R = −1, the composite suffered large modulus losses (about 15%) at cycles well before the final failure. At R = 0 the modulus loss involved fibre breakage and matrix cracks, which were transverse as well as parallel to the loading direction, while at R = −1 it involved delamination cracks and barrelling of outer layers. Fractography after the final failure at R = 0 showed secondary cracks and fibre pull-out.