Fatigue crack behavior in power plant residual heat removal system piping including weld residual stress effects

The piping in some residual heat removal (RHR) systems in nuclear power plants, made of 304L stainless steel, is subjected to a multiaxial cyclic stress state in the cold/hot water mixing zone due to thermal fluctuations. In addition, residual stresses produced by welds and/or surface treatments can be large in magnitude and be tensile or compressive, depending on the proximity to the weld location. Crack networks have been observed to nucleate and grow under these stress states. Prominent cracks originating from micro-cracks on the order of grain size continue to grow to a length of about 2.5 mm. The thermal fluctuation is such that crack growth is in the high cycle fatigue regime. In this paper, the plane orientation and growth direction of these cracks with respect to the cyclic and mean stress states and their gradients are analyzed. The roles and interactions of shear, tensile, and compressive stresses with regards to the direction and orientation of the cracks are also discussed. Weld residual stresses (WRS) are decomposed into a local effect in tension near the weld and a structure effect in compression farther away, which also help explain some crack configurations in RHR systems. Moreover, a dependence of WRS field on spatial second derivative of weld temperature field is shown, which can have an important impact for comparison of measured and simulated WRS. Crack growth was also simulated with XFEM in the presence of WRS, with the principal hypothesis that mean stress acts only on crack opening.