Effect of Undercut and Nonpropagating Crack Depths on Fatigue Strength of Welded Joint

Abstract

The relationship between a depth of undercut and a depth of nonpropagating crack has been investigated experimentally and systematically under the condition of bending load.
Though the fatigue strength depends on a length of nonpropagating crack in the direction of plate thickness, many fatigue tests were usually carried out by using rotatory bending fatigue test and surface cracks were observed.
Welded joints are more often loaded with axial stress or bending stress than rotatory bending stress. Furthermore the fatigue failure of welded joint may occur at the toe of weld at which undercut exists and grain size of weld is larger than that of base metal.
Grain size of material used (SM400) was increased by using heat treatments 1273 K-7200 s and 1073 K-14400 s. The depth of artificial undercut was varied from 0 to 250 μm. The stress ratio R was zero. After fatigue testing, nonpropagating cracks lying on the section of a specimen were observed by using an optical microscope.
All nonpropagating crack existed within two grains from the bottom of the undercut of which the depth was varied from 0 to 250 μm. A crack within the first grain was the first stage crack and a crack within the second grain was the second stage crack.
The depth of nonpropagating crack depended on the size of two grains at the tip of the undercut. The shorter the total depth of undercut and nonpropagating crack was, the higher the fatigue strength was. In the case that the difference between undercut depths of two spacimens was smaller than one grain size, the fatigue strength was similar.