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Fatigue crack growth tests were carried out using CT specimens of ultra-fine grained P/M (Powder Metallurgy) aluminum alloy of which grain size was from 200 to 500nm. Fatigue crack growth behavior was investigated under constant amplitude and repeated two-step variable amplitude load sequences. It was found that roughness induced crack closure played an important role in closure behavior of this material. Although the roughness of fracture surface was smaller than the other conventional aluminum alloys, the roughness induced crack closure was dominant because of the small crack tip opening displacement (CTOD) with respect to the roughness level. Under repeated two-step loading, crack opening load was lower than that under constant amplitude loading with the same K value as the high level load, and the crack growth retardation caused by load history was not prominent comparing with other aluminum alloys. It is considered that the crack opening load under variable amplitude loading was dominated by the roughness induced crack closure. The small contribution of the plasticity induced crack closure was due to the constraint of plastic strain by the grain boundaries of ultra-fine grains.

Fatigue Crack Growth Behavior in Ultra-Fine Grained P/M Aluminum Alloy under Repeated Two-Step Variable Amplitude Load Sequences