A novel method for fracture toughness assessment of inhomogeneous ferritic steel weldments using bimodal master curve analysis

The basic Master Curve (MC) method for analysis of brittle fracture test results is intended only for macroscopically homogeneous ferritic steel. In reality, steels and welds often contain inhomogeneities that distort the standard MC analysis. The structural integrity assessment procedure SINTAP contains a lower tail modification of the MC analysis, enabling conservative lower bound fracture toughness estimates to be determined also for inhomogeneous material. Such estimates, however, only describe the fracture toughness of the more brittle constituent. The deficiency of SINTAP, in this respect, lies in its inability to provide any information from the more ductile material. Therefore, a probabilistic description of the complete material is not possible. This paper introduces a new extension of the MC analysis for inhomogeneous material: a bimodal MC analysis method that describes the fracture toughness distribution as the combination of two separate MC distributions. This bimodal distribution model is shown to describe successfully the weld heataffected zone (HAZ) fracture toughness data sets that generally exhibit substantial microstructural inhomogeneity. This is especially the case with multipass weldments containing local brittle zones (LBZ).