A model for the formation of fatigue striations and its relationship with small fatigue crack growth in an aluminum alloy

The fatigue crack growth process involves damage accumulation and crack extension. The two sub-processes that lead to fatigue crack extension were quantified separately in a recent model for small fatigue crack growth applicable to engineering alloys. Here, we report the results of an experimental investigation to assess the assumptions of that model. The fatigue striation formation in an aluminum alloy is modeled, and it is verified that the number of cycles required for striation formation is related to the cyclic crack-tip opening displacement. The striation spacing is related to the value of the monotonic crack-tip displacement. It is concluded that extensive crack-tip geometry changes due to plasticity in the aluminum alloy causes a reduction in the slope of the fatigue crack propagation curves. The implications of these results on the fatigue crack propagation lifetime calculations are identified.