Influence of weld mis-matching on fatigue crack growth behaviors of electron beam welded joints

The influences of weld mis-match effects on fatigue crack growth (FCG) rate and crack path deviation (CPD) were studied by virtue of fatigue crack growth experiment of Ni-base superalloy electron beam welded (EBW) joints. The results show that the fatigue crack growth of electron beam welded joints is instable due to the influence of mechanical heterogeneities. The fatigue crack located in the weld metal of overmatched joints deviated from its original growth direction, and then spanned the welded seam and base metal in a mixed mode while propagating. Owing to the crack deviation at the weld metal and heat-affected-zone (HAZ), the effective growth driving force at the tip of fatigue crack was reduced with the reduction of the effective stress intensity factor (SIF) which finally causes fatigue crack rate decrease. As a result, the overmatched joints have the capacity of protecting fatigue crack joints from fast growth. Fatigue crack was strongly affected by size and the symmetrical characteristics of the plastic zone at the crack tip, thus the integrity of the welded structure containing the fatigue crack mainly depended on the toughness of the low strength zone.