Microstructural Effects on Short Fatigue Crack Propagation and Their Modelling

Cyclically loaded components in structural applications often undergo a stress amplitude which is close to the fatigue limit of the material used. Under such conditions, crack initiation and short crack propagation is considered to play an important role. It is well established that in high cycle fatigue (HCF) cracks propagate as stage I cracks during a rather large fraction of fatigue life (up to 90% of the number of cycles until failure, Nf). The propagation behaviour of these so-called microstructurally short fatigue cracks is strongly affected by microstructural properties, such as the grain size and the presence of phase boundaries, and cannot be described using linear-elastic fracture mechanics. Because of the significance of crack initiation and early crack propagation for cyclic life, new concepts and experimental methods have to be applied in order to provide a robust and reliable fatigue life assessment. Results from two interdisciplinary research projects are reported in this presentation, which were carried out by the authors during the past seven years, in order to shed light on the relevant mechanisms and to illustrate the implementation of these findings in a life prediction model.