The microstructure as crack initiation point and barrier against fatigue damaging

From the emission of dislocations till short crack propagation fatigue is a local process determined by the microstructure. We present experiments based on electron channelling contrast imaging (ECCI) as refined application of the scanning electron microscope (SEM) and new focused ion beam (FIB) technique like FIB crack initiation and FIB tomography which give detailed information about crack initiation and the interaction of short fatigue cracks with precipitates and grain boundaries as microstructural barriers. As main result the characteristic fluctuation in the propagation rate of short fatigue cracks in front of grain boundaries that has so far defied calculation can now be calculated analytically from the BCS-model and Tanaka model by using three constants measured in a single crystal.