Conservative data for fatigue crack propagation analysis

Abstract

Conservative data relating to the fatigue crack propagation properties and fatigue threshold were obtained by using center-cracked-type transverse-butt-welded joints. The properties were unique in spite of the difference of the materials (four ferritic steels and an austenitic stainless steel), the welding methods (three types of are welding) and heat inputs (three levels), even under variable amplitude loading. The unique properties came from the tensile residual stresses which were always induced around the crack tip due to re-distribution on crack extension, and were the main controlling factor of fatigue crack propagation. The tensile residual stresses make the fatigue crack always open because of the increase in the stress ratio around the crack tip. The role of residual stress is revealed by examining the stress ratio effect observed in base metal and the effect of post-weld heat treatment. The advantage of inducing compressive residual stresses on the inner surface of the welded pipe to improve the fatigue crack propagation properties is pointed out.