Influences of stress ratio, tensile and compressive residual stress on fatigue crack growth rate in butt welded joint of 600MPa grade steel.

A fatigue crack initiated in welded structure propagates through the welded residual stress field. It is a very important problem to clarify the effect of welded residual stress on fatigue crack growth rate. For the butt welded joint of 600MPa grade steel, the effects of tensile and compressive residual stresses were investigated in association with the crack closure phenomenon. The results obtained are summarized as follows.(1) For the welded joint in which the fatigue crack propagated through the tensile residual stress field, the fatigue crack growth rate was accelerated by the tensile residual stress. But, the acceleration of fatigue crack growth rate was independent of the stress ratio and the intensity of residual stress.(2) For the welded joint in which the fatigue crack propagated through the compressive residual stress field, the fatigue crack growth rate decreased with increasing stress intensity range, and the crack arrested as the stress intensity range approached the threshold for crack growth, ΔK0. At the stress intensity range above ΔK0, the fatigue crack growth rate increased with increasing stress intensity range. The increase of stress ratio has given rise to increase in fatigue crack growth rate and decrease in ΔK0 value. Such a stress ratio effect was the same as the results of base metal.(3) It is considered that the welded residual stress has an effect on the fatigue crack growth rate to change the mean stress at the crack tip. This effect may be explained by the equivalent stress ratio, Req, defined by the following equation.Req=R-2Kres/Kmax