Significance of Fracture Toughness of Laser Welds for Structural Steels and Fracture Performance Assessment by Weibull Stress Criterion

—Improvement of Steel Structural Performance by Laser Beam Welding for Mid-Thick Section Steel Plate—

Abstract

The significance of fracture toughness test results of laser welds of structural steels has been discussed. In the laser welds of structural steels, the weld metal corresponds to a locally hardened zone and generally has a reduced fracture toughness. Nevertheless, the weld metal toughness measured in the toughness test, where a notch is located in the center of laser welds, is essentially being overestimated. This is due to a development of plastic zone in the base metal region adjacent to the hardened welds. However, such "reduced fracture toughness" of the laser weld metal does not necessarily mean a poor fracture performance of the welded joints. The welded joints subjected to tension exhibit a significant amount of constraint loss near the near crack-tip, whereas the fracture toughness specimen holds a constrained stress state due to bending loading. Besides, the plastic deformation of the base metal region adjacent to the welds provides a shielding effect on the CTOD of the crack in the weld metal. The equivalent CTOD concept, that links the fracture toughness specimen and the structural component at the same level of the Weibull stress, is applied for the evaluation of the constraint loss in the laser welded joints.