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Rotating bending fatigue tests were conducted using A6061 and Al-Si eutectic alloys to investigate fatigue strengths and small fatigue crack growth behavior. Both alloys were fabricated by continuous casting and extrusion, and the effect of fabrication processes on the fatigue properties was investigated. The fatigue strengths of continuously-casted and extruded A6061 alloys were nearly comparable. The specimens, which were sampled parallel to the extrusion direction, exhibited higher fatigue crack initiation resistance than the specimens sampled perpendicular to the extrusion direction. However, the fatigue crack growth resistance was higher in the specimens sampled perpendicular to the extrusion direction. The differences in the crack initiation and growth resistances were attributed to the different microstructures depending on the extrusion direction. The fatigue strength of the extruded Al-Si eutectic alloy was higher than that of continuously-casted alloy, which could be attributed to the higher fatigue crack initiation resistance of the extruded alloy. The continuously-casted Al-Si eutectic alloy had typical dendritic microstructure, in which eutectic Si dispersed heterogeneously. The extrusion process homogenized the dispersion of eutectic Si in the microstructure, which resulted in the higher fatigue crack initiation resistance.

Fatigue crack propagation (FCP) tests were performed using CT specimens of extruded magnesium (Mg) alloy, AZ61, at a frequency of 10 Hz and stress ratios, R, of 0.05 and 0.7. The FCP tests were conducted in the environmental chamber, in which the humidity was controlled from 20%R.H. up to 80%R.H. The FCP tests were also performed in dry air and pure water for comparison. The effect of humidity on FCP behavior of Mg alloy was investigated. The FCP rates and crack closure behavior were strongly affected by humidity. When the FCP rates, da/dN were evaluated by stress intensity factor, ΔK , the FCP rates in dry air were the lowest irrelevant to the stress ratio. The humidity in air accelerated FCP rates in the low ΔK region even at the lowest humidity of 20%R.H. The FCP rates under the humid conditions were similar to those in pure water. After allowing for crack closure, the effect of environment still existed, where the FCP rates were the fastest at 30%R.H. under R=0.05, then in the higher humid conditions and in dry air, in decreasing order. The acceleration could be attributed to both anodic dissolution and hydrogen embrittlement at the fatigue crack tip.