A study of the crack wake closure/opening behaviour of short fatigue cracks and its influence on crack growth

Crack closure behaviour has long been found to have a significant influence on fatigue crack growth and a large amount of research has been done in the past 3 decades. Whereas the majority of these research efforts focus on long cracks, relatively little is known for short fatigue cracks, in particular for microstructurally short cracks. This paper presents a quantitative characterisation of the wake closure/ opening behaviour of microstructurally short fatigue cracks under cyclic loading for a structural aluminium alloy 2024-T351 using an in situ SEM fatigue testing system. The results clearly indicate that crack closure level was at a peak value at crack lengths of ∼100-150 μm. Beyond the regime of short crack growth, the crack closure stress decreases gradually with crack length, approaching asymptotically to a constant value at crack lengths of about 1.5 mm. In addition, it was found that the crack closure/opening stress intensity factor, K_cl (K_op), is nearly constant while the maximum stress intensity factor increases with crack size. Therefore, the effect of crack closure is most significant in the early stage of short fatigue crack propagation. The experimental results have also been compared with those predicted by Newman's model and a model previously developed by the authors.