In situinvestigation of small fatigue crack growth in poly‐crystal and single‐crystal aluminium alloys

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Abstract
In situscanning electron microscope observations of short crack growth in both a poly‐crystal and a single‐crystal alloy revealed that fatigue cracks may grow in a shear decohesion mode over a length that is several times the grain size, far beyond the conventional stage I regime. In the poly‐crystal aluminium alloy 2024‐T351, fatigue cracks were found to continue to grow along one shear band even after two mutually perpendicular shear bands had formed at the crack tip. For the single‐crystal alloy specimen with the loading axis being nearly perpendicular to its main shear plane, mode I fatigue cracks were found to grow along the shear band. These two types of fatigue crack growth pose a significant challenge to the existing fatigue crack growth correlating parameters that are based on crack‐tip opening displacement. In particular, it has been found that the cyclic crack‐tip opening displacement, which accounts for both large‐scale yielding and the lack of plasticity‐induced crack closure, is unable to unify the growth rates of short and long cracks in aluminium 2024‐T351, suggesting a possible dependence of crack growth threshold on crack length.

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