The influence of weld imperfections on fatigue strength and the current relevant design guidelines

Weld quality could have a significant influence on the fatigue strength of welded components. The correlation between weld imperfections and fatigue strength is defined in different guidelines as IIW-recommendations or the German technical bulletin DVS 0705 in a conservative manner. In order to assess the influence of weld porosity on fatigue strength in a more accurate way, experimental and numerical investigations on welded specimens with internal pores were performed. Experimentally, the specimens were tested under cyclic loading until failure and finally, the fracture surfaces were evaluated with respect to the size and location of the defects and the fracture behaviour. Numerically the specimens were simulated including the inner pores and evaluated regarding the number of cycles to c rack initiation and crack propagation, respectively. Besides the conventional approach to fatigue assessment of welded components based on their fatigue classes, the applicability of the fatigue damage parameter according to Fatemi and Socie was examined in order to estimate the fatigue life until the formation of a crack from the initial pore. The remaining life until failure was estimated using fracture mechanics methods in combination with the fatigue crack growth curve da/dN. The results demonstrate that in general the number of cycles until crack formation is larger than the cycles of crack propagation until failure. Therefore, the assumption of the pore as a sharp crack would underestimate the fatigue life significantly. With the combination of both methods, the estimation of crack f ormation using an appropriate damage parameter and crack extension based on fracture mechanics the fatigue life of welded plates with internal pores can be described in accordance with the experiments.