FATIGUE PROPERTIES AND FATIGUE FRACTURE MECHANISMS OF SIC WHISKERS OR SIC PARTICULATE-REINFORCED ALUMINUM COMPOSITES

Fatigue properties and fracture mechanisms were examined for three commercially fabricated aluminium matrix composites containing SiC whiskers (SiC(w)) and SiC particles (SiC(p)) using a rotating bending test. The fatigue strengths were over 60% higher for SiC(w)/A2024 composites than that for the unreinforced rolled material, while for the SiC(p)/A357 composites, fatigue strengths were also higher than that for the unreinforced reference material. For the SiC(p)/A356 composites at a volume fraction of 20%, the fatigue strength was slightly higher than that of the unreinforced material. Fractography revealed that the Mode I fatigue crack was initiated by the Stage I mechanism for the SiC(w)/A2024 and SiC(p)/A357 composites, while for the SiC(p)/A356 composite, the fatigue crack initiated at the voids situated beneath the specimen surfaces. On the other hand, the fatigue crack propagated to the whisker/matrix interface following the formation of dimple patterns or the formation of striation patterns for SiC(w)/A2024 composites, while for the SiC(p)/A356 and SiC(p)/A357 composites the fatigue crack propagated in the matrix near the crack origin and striation patterns were found. Near final failure, dimple patterns, initiated at silicon carbide particles, were frequently observed. Mode I fatigue crack initiation and propagation models were proposed for discontinuous fibre-reinforced aluminium composites. It is suggested that the silicon carbide whiskers or particles would have a very significant effect on the fatigue crack initiation and crack propagation near the fatigue limit.