Existence and insufficiency of the crack closure for fatigue crack growth analysis

In this paper, the multi-resolution in situ experiment, optical microscopy experiment and SEM experiment are used to investigate the existence of crack closure and its sufficiency for crack growth prediction. In situ optical microscopy testing and the digital image correlation analysis are used to measure the plastic zone size in front of the crack tip. In situ scanning electron microscopy testing is used to measure the crack tip opening displacement and crack growth kinetics. Crack closure behavior under constant loading with a single overload is studied under SEM. The experimental methodology is applied to two different metallic materials (aluminum alloys and steels). Detailed imaging analysis and experimental results are presented and compared. It is found that the crack closure phenomena exist for aluminum alloys, but not for steels in the current investigation. If the crack closure happens, it will significantly alter the crack tip plasticity behavior. Under constant amplitude loading, the crack closure concept is able to uniquely correlate the crack growth kinetics. However, under single overload loading, the crack closure is not able to uniquely correlate with the crack growth kinetics and statistical crack growth experiment also shows the inefficiency of crack closure. Finally, a discussion about the necessarily and the insufficiency of crack closure for crack growth prediction is given.