Crystal plasticity modeling of microstructure influence on fatigue crack initiation in extruded Al6082-T6 with surface irregularities

The influence of surface roughness and microstructure heterogeneities in the vicinity of macroscopic stress concentrations are investigated by crystal plasticity simulations. It is shown that in the extruded Al6082-T6 alloy under consideration, twin grain boundaries and other highly misoriented grain interfaces, which constitute significant barriers to plastic slip, provide internal domains in the material where initiation of fatigue damage is more likely to occur than at the stress concentrations which are due to the surface roughness. In addition, the microstructure influence on the appearance of stress gradients is found to be significant. Further, the present study indicates that frequently used fatigue initiation parameters (FIP) such as the locally accumulated plastic strain or stored energy may be insufficient to identify fatigue crack initiation. Such parameters should be complemented or replaced by other FIP which take additional characteristics of the microstructure into account. This is exemplified in the present study by employing two additional FIPs, based on a modified Fatemi-Socie critical-plane parameter and on the Dang Van criterion, respectively.