Fatigue crack nucleation at interfaces

Fatigue crack nucleation at grain boundaries near the endurance limit was examined combining the results of fatigue tests with local orientation measurements and with finite element calculations. Coarse-grained thin sheets were examined in which the orientation and inclination of each boundary could be measured directly. These results were used as input for FEM calculation in which the stress concentrations near the grain boundaries were calculated. In the fatigue tests, the boundaries, at which the first cracks nucleate, were measured by a replica technique. A comparison between the experimental results and the FE calculation showed that the cracks nucleated and propagated along the boundaries with the largest stress concentration. When a crack hits a boundary with a lower stress concentration, it either stops or propagates transgranularly depending on the local stress concentration given by the microstructure. From these results, it can be concluded that elastic strain incompatibility combined with the microstructure (grain size and orientation) can be used to predict the crack nucleation sites and the scatter in life time for fatigue near the endurance limit.