Short fatigue cracks propagate with a fluctuating crack growth rate while interacting with microstructural barriers. Although some models of the interaction exist, it was not clear what determines the strength of the resistance of a grain boundary to crack propagation. Therefore a method for artificial crack initiation was developed which used a focused ion beam to nucleate cracks crystallographically on single slip planes identical to natural stage-I cracks. The crack parameters as well as the grain boundary parameters could be varied independently for systematic experiments. For the first time the crack path through a grain boundary was shown in three dimensions by focused ion beam tomography. This enables the interaction between microcracks and grain boundaries to be observed in three dimensions with a high spatial resolution. It was not only the inclination angle between the active slip systems, but also the inclination angle of the grain boundary, which determines the strength of these microstructural barriers.

A 3-D View on the Mechanisms of Short Fatigue Cracks Interacting with Grain Boundaries. W.Schaef, M.Marx, H.Vehoff, A.Heckl, P.Randelzhofer: Acta Materialia, 2011, 59[5], 1849-61