Experimental study on fatigue strength of small-diameter socket-welded pipe joints

The authors conducted fully reversed four-point bending fatigue tests on socket-welded joints 20 to 50 mm in nominal diameter, and rotating bending fatigue rests on socket-welded joints 20 mm in nominal diameter. S-N curves for 33 series of different types of specimens were obtained. Examination was made of the effects of various parameters listed in the forthcoming on fatigue strength such as steel types (carbon and stainless steels), diameter, pipe thickness (Sch), fillet shape, slip-on gap, and root defects. Bending fatigue test results indicated fatigue strength for socket-welded joint to be less for longer life regions than reported in the literature by Markl and George (1950). Fatigue strength for socket joints of 50 mm nominal diameter at 10(7) cycles of fatigue life was 46 MPa for carbon steel and 60 MPa for stainless steel with nominal bending stress on the pipe surface. Cracks generally originated from the toe when stress amplitude was high with shorter fatigue life and from the root when amplitude was small with longer life. Fatigue strength was greater for smaller diameter, larger Sch (thicker pipe wall), final welding pass on the toe of pipe side, and in the absence of a slip-on gap. From fatigue test results of socket joints with weld defects at the roots, an empirical equation for the relation of defect size with decrease in fatigue strength was established. Fatigue strength was found to decrease to 60 percent the original level for defect size 25 percent of leg length.