In situ SEM study of fatigue crack initiation in Kaowool(TM)/Saffil(TM) discontinuous fiber-reinforced aluminum-silicon alloy matrix composite

Specimens of A1-Si alloys reinforced with discontinuous Kaowool$sp{TM}$ (50% $rm Alsb2Osb3$-50% SiO$sb2$) and Saffil$sp{TM}$ (97% $rm Alsb2Osb3$-3% SiO$sb2$) fibers were fatigued under strain-controlled repeated bending loading conditions, using an in-situ SEM fatigue stage to identify the fatigue crack initiation site(s). From the series of pictures recorded from particular locations on the surface of the specimen, at various stages of the cyclic process, the crack initiation site(s) was identified. The results from the study of 15 vol.% Kaowool$sp{TM}$ composites showed that crack initiation occurred at processing defects called "shot" particles, very early in the cyclic process and at other microstructural features. Cracks from multiple sites were observed to grow and coalesce into a single major crack that led to specimen fracture after about 250,000 cycles. The premature cracking of these shot particles motivated further study of the behavior of these shot particles under tensile loading using an in-situ SEM tensile testing apparatus. In the case of 15 vol.% Saffil$sp{TM}$ composites, cracks initiated at fiber/matrix interfaces of transversely oriented fibers as early as 5 cycles of loading. Fracture occurred after only about 2500 cycles due to link up of several such cracks along the interfaces. The difference in behavior of the fiber/matrix bond for the Kaowool$sp{TM}$ and Saffil$sp{TM}$ composites may be attributed to the differences in chemical reaction products at the interface due to different amounts of silica in the two fibers.