Small Fatigue Crack Growth Behavior and Fatigue Strength of Squeeze-Cast Aluminum Alloys, AC8A-T6 and AC4C-T6

Fatigue strength, crack initiation and small crack growth behavior in two kinds of squeeze-cast aluminum alloys such as AC8A-T6 and AC4C-T6 were investigated using plain specimens subjected to rotating bending fatigue at room temperature. Fatigue resistance of these alloys was almost same as that of the wrought aluminum alloys because of fine microstructure and of decrease in defect size due to squeeze-casting. Fatigue crack initiation site was at the eutectic silicon particle on the specimen surface or at microporosity in the specimen. Crack initiation life was successfully estimated from the evaluation of initiation site by the fracture mechanics and the statistics of extrema. Small fatigue crack growth of two kinds of alloys obeys the relation proposed by H. Nishitani et al. , that is d(2c)dN= C(σ_a/σ_B)^n(2c), where σ_B is the ultimate tensile strength. It was pointed out that an improvement of fatigue strength in cast aluminum alloy can be expected by refining eutectic silicon rather than by increase of static strength