A physically-based method for predicting high temperature fatigue crack initiation in P91 welded steel

A methodology is presented for physically-based prediction of high temperature fatigue crack initiation in 9Cr steels, with a focus on material inhomogeneity due to welding-induced metallurgical transformations. A modified form of the Tanaka-Mura model for slip band formation under cyclically-softening conditions is implemented in conjunction with a physically-based unified cyclic viscoplasticity constitutive model. The physically-based constitutive model accounts for the key strengthening mechanisms, including precipitate hardening and hierarchical grain boundary strengthening, successfully predicting cyclic softening in 9Cr steels. A five-material, finite element model of a P91 cross-weld test specimen, calibrated using the physically-based yield strength and constitutive models, successfully predicts the measured detrimental effect of welding on high temperature low-cycle fatigue crack initiation for P91 cross weld tests, via the modified Tanaka-Mura model. A key finding of the current work is the requirement to adopt an energy-based Tanaka-Mura method to account for cyclic softening in 9Cr steels, with packet size as the critical length-scale for slip band formation. The work demonstrates the significant effect of highly flexible operation on the service life of welded connections.