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

摘要

The present paper describes a novel experimental technique recently presented that allows one tostudy 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 byapplying a series of mechanical and heat treatments. Once the thermo-mechanical treatment is completed andthe desired microstructure obtained, a circular notch is machined on each specimen, and the samples aresubjected to push-pull fatigue loading. Several combinations of notch and microstructural sizes have beentested. This method provides an easy way to record and analyse the effect of the microstructure upon crackgrowth rate. It was observed that the space between successive crack-tip arrests correlates well with the materialgrain size. Another interesting observation is that in the majority of the cases studied the cracks did not initiateat the point of maximum stress concentration. This is surprising since the classical methods of notched fatiguelimit analysis clearly indicate the horizontal symmetry axis as the initiation and propagation direction for pushpullloading.