Initiation and growth behavior of very-long microstructurally short fatigue cracks

The present paper describes a novel experimental technique recently presented that allows one to study interactions between the crack and microstructural barriers with an unprecedented level of ease and detail. The method consists in increasing the grain size of Al1050 Aluminium alloy until the centimetre scale by applying a series of mechanical and heat treatments. Once the thermo-mechanical treatment is completed and the desired microstructure obtained, a circular notch is machined on each specimen, and the samples are subjected to push-pull fatigue loading. Several combinations of notch and microstructural sizes have been tested. This method provides an easy way to record and analyse the effect of the microstructure upon crack growth rate. It was observed that the space between successive crack-tip arrests correlates well with the material grain size. Another interesting observation is that in the majority of the cases studied the cracks did not initiate at the point of maximum stress concentration. This is surprising since the classical methods of notched fatigue limit analysis clearly indicate the horizontal symmetry axis as the initiation and propagation direction for push-pull loading