Development of Fracture Toughness Requirements for Weld Metals in Seismic Applications

Fracture of unreinforced welded moment frame connections subjected to simulated seismic loads was caused by the initiation of fatigue cracks and their propagation to critical size. The fatigue cracks initiated at the web-to-flange intersection at the weld access hole, the valleys on the flame-cut weld access hole surface, the weld toe, and weld imperfections. Final fracture occurred when the fatigue crack extended unstably either in the base metal or in the weld metal. Final fracture is determined by the size of a crack, the stresses and strains acting on the crack, and the fracture toughness of the material. This paper presents the methodology used to establish the necessary and sufficient fracture toughness requirement for weld metal used in seismic applications. The methodology was based on fracture mechanics principals and on empirical correlations. The proposed Charpy V-notch (CVN) toughness is 40 ft-lb at 70°F and 20 ft-lb at 0°F for components subjected to +50°F and higher. This CVN requirement should preclude weld metal toughness from being a contributing factor to the fracture of unreinforced moment frame connections. Further improvements in the fracture performance of the connections must be accomplished by changes in design, detailing, fabrication, and inspection.