Subcycle fatigue crack growth mechanism investigation for aluminum alloys and steels

In this paper, the existence of crack closure and its sufficiency for crack growth prediction is investigated by multi-resolution in-situ optical microscopy experiment and SEM experiment. In in-situ optical microscopy testing, the digital image correlation analysis is used to measure the plastic zone size in front of the crack tip. In in-situ SEM testing, the crack tip opening displacement and the crack growth kinetics are investigated. Besides, crack closure behavior under constant loading with a single overload is studied in in-situ SEM test. This 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 first, there exists crack closure phenomena for aluminum alloys, but not for steels in our current researches; second, the crack closure will significantly change the crack tip plasticity behavior; third, the crack closure concept is able to account for crack growth kinetics uniquely for constant loading, but it is insufficient for constant loading with a single overload. Finally, the necessarily and the insufficiency of crack closure for crack growth prediction is discussed.