Assessment of large-scale heterogeneity due to toughness variations in a multipass weld: brittle failure mechanisms and modeling

The fracture surfaces of 49 SE(B) toughness tests performed on five different geometries, were carefully investigated by SEM imaging and cross-section analysis. The specimens were extracted from a large multi-pass weld in T-S orientation. The failure characteristics were associated with three distinctly different zones of the weld. Transgranular fracture occurred primarily in the reheated zone and in the as-welded zone with a dendritic microstructure inclined relative to the crack plane. With a dendritic microstructure aligned with the crack plane intergranular fracture occurred. The toughness of the as-welded zone with a low inclination angle, was significantly lower than that obtained in the other two weld zones. Due to the relatively large size of the zones compared to the fracture process zones of the tests, it is appropriate to characterize the failure behavior as large-scale heterogeneity. Weakest-link modeling may be applied locally in each weld zone, giving rise to three different sets of model parameters. A new calibration technique is introduced and used to fit a local weakest-link model to the toughness distribution curves of the individual zones.