Application of local approach to inhomogeneous welds. Influence of crack position and strength mismatch

In steel welds there is often a large variation in fracture toughness and m echanical properties between the weld metal, base material and the various heat affected zone (HAZ) microstructures. The stress field in front of a cr ack in a weldment can be noticeably affected by the strength mismatch betwe en the weld metal, HAZ and the base material. The crack position relative t o the various microstructures will clearly influence the strength mismatch effect. In this paper the influence of crack tip positioning on the fractur e performance of strength mismatched steel welds has been studied both expe rimentally and by FEM analysis. For a mismatched weld with local brittle zo nes small changes in crack tip location can give considerable changes in th e fracture performance of a CTOD specimen. A high degree of strength mismat ch increases the effect of crack positioning. Weld metal overmatch increase s the stress level in the heat affected zone due to material constraint and thereby reduces the cleavage fracture resistance of the weldment when the coarse grained HAZ (CGHAZ) controls the fracture. The detrimental effect of high overmatch is most pronounced for specimens with notch position at fus ion line and a short distance into the brittle CGHAZ. The Weibull stress ha s been shown to be a suitable fracture parameter in the case where one micr ostructure clearly controls the cleavage fracture and the calculation of th e Weibull stress therefore can be limited to this zone. (C) 1998 Elsevier S cience Ltd. All rights reserved.