Experimental and computational analyses on fatigue fracture and microstructure in dissimilar metal weldments with circular sharp stress raiser

Abstract

Fatigue strengths of dissimilar metal weldments with a circular sharp stress raiser, which were the inner peripheral side welded joints of flange pipes with tungsten inert gas (TIG) or metal inert gas (MIG) welding process, were defined through bending fatigue experiments. Fatigue strengths decreased according to stress concentration factors in some cases but not in samples with a different welding process, and stress intensity factors at the sharp weld notch were applied for analyses using the energy release rate method. The crack states corresponded to the fact that the fatigue cracks propagated at nearly right angles to the maximum stress direction, and it is possible to predict their propagation direction. The local fatigue resistance of the martensite-ferrite structure with the TIG welding was higher than that of the austenite-ferrite structure with the MIG welding which has a largely different penetration rate. In the samples which have similar weld metal microstructure, the nominal fatigue strength declines according to the increase in the stress concentration factor. The TIG samples and the MIG samples reach their fatigue failure in the same level of stress intensity factors, except for a MIG sample which has a large excess metal shape and a different fracture mode. The stress intensity factors can be used as the fatigue fracture threshold criterion in the weldments with a circular sharp stress raiser, where have the complex stress effects.