Direct high resolution in situ SEM observations of small fatigue crack opening profiles in the ultra-fine grain aluminium alloy

A study of the fatigue crack opening displacements for a small contained fatigue crack in the ultra-fine grain size aluminium alloy (IN 9052) has been carried out using in situ SEM observation technique. It is found that the left hand crack tip opening displacement is significantly different from the right hand crack tip opening displacement. At peak applied stress the measured fatigue crack tip opening displacements are also significantly smaller than the calculated crack tip opening displacement based on Dugdale model. Significant micro-damage can also be observed around the two surface fatigue crack tips. When unloading to zero applied stress, fatigue crack tip has fully closed. These observations indicate that the fatigue crack tip opening displacement is strongly affected by the damage around the crack tip and are associated with the different stages of the fatigue crack propagation process.