Fracture of Surface-Cracked Undermatched Weld Joint in High-Strength Steel

Abstract

The objective of this paper is to study the combined effects of weld metal undermatching and crack location on the fracture behavior of transversely loaded butt-welded joints in high-strength steel. Surface-cracked tensile panels were instrumented with stain gages and tested with notch locations at the heat-affected zone (HAZ), base and 30 percent undermatched weld metals. It can be expected that for a given applied strain, the level of the strain concentration developed in undermatched weld region will be significantly lower in the high strain-hardening welds compared to the low work-hardening weld joint in high-strength steels. Therefore, in higher strength steel undermatched welds, substantial increase of the strain accumulation should be expected. Detailed strain measurements and tensile panel tests of this study reveal that if there is a defect in the undermatched weld, a combination of the low weld metal toughness and resulting strain accumulation in the weld metal lead to poor fracture performance. The HAZ notched tensile panel results indicate that weld metal undermatching causes an unsymmetrical strain distribution at the crack vicinity. Despite the occurrence of the high applied strain concentration at the weld metal side of HAZ notched panel, the gross section yielding (GSY) occurred similar to the base metal notched one. On the other hand, weld metal notched panels showed net section yielding (NSY) with a considerably reduced ductility. An important difference in 30 percent undermatched weld joint fracture performance, however, emerges when results are analyzed in terms of crack growth resistance curves. The J R -curves of the HAZ and weld metal notched panels have shown lower resistance behavior compared to the base metal panel. Therefore, it is concluded that the fracture performance of the undermatched welds cannot be fully understood if only strength or total strain capacity (applied side) of the entire panel is considered. The fracture toughness (material resistance side) of the undermatched weld joint should also be an integral part of the fracture performance assessment procedure.