A 3-D view on the mechanisms of short fatigue cracks interacting with grain boundaries

Short fatigue cracks propagate with a fluctuating crack growth rate while interacting with microstructural barriers. Although some models of the interaction exist, it is not clear what determines the strength of the resistance of a grain boundary to crack propagation. Therefore we developed a method for artificial crack initiation which uses a focused ion beam (FIB) to nucleate cracks crystallographically on single slip planes identical to natural stage I cracks. The crack parameters as well as the grain boundary parameters can be varied independently for systematic experiments. For the first time the crack path through a grain boundary is shown in 3-D by FIB tomography. This enables the interaction between microcracks and grain boundaries to be observed in 3-D with a high spatial resolution. It is 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.