Effect of Weld Induced Residual Stresses on Pipe Crack Opening Areas and Implications on Leak-Before-Break Considerations

Abstract

The USNRC is anticipating updating their leak-before-break (LBB) procedures. One of the technical areas of concern in the existing procedures is the prediction of the crack-opening-displacements (COD) needed for estimating the postulated leakage crack size for a prescribed leakage detection capability. If cracks develop in the welded area of a pipe, as is often the case, residual stresses in the weld may cause the crack to be forced closed. Earlier studies have shown that pipe welding produces high residual stresses with a sharp stress gradient ranging from tension to compression through the thickness of the welded area of the pipe. The current guidelines are inadequate to predict crack size based on leak rates for cracks in welded areas of pipes. The current guidelines rely on the calculation of the crack-opening-displacement as related to pipe loading. Values from the current guidelines are used to predict a crack’s cross sectional area and, in turn, to determine the severity of an existing crack by monitoring in-service leakage rates. The equations currently in use are applicable to service loaded pipe material only. Residual stresses caused by cold work, welding, etc. are neglected. This study uses two and three dimensional finite element models and weld residual stress calculation software created at Battelle Memorial Institute to develop correction factors to be used with the traditional design equations. The correction factors will compensate for the effects of welding induced residual stresses on cracks in pipe welds. This study concentrates on type 316 stainless steel material properties, but the COD corrections should be equally applicable to all stainless steels, and also can be used for ferritic steels. A test matrix of pipe radius, thickness, and crack size was used to develop the equation correction factors. Pipe wall thicknesses (t) of 7.5 mm (0.295 in.), 15 mm (0.590 in.), 22.5 mm (0.886 in.), and 30 mm (1.181 in.) were studied in pipes with mean radius to thickness ratios of 5, 10, and 20. Cracks with half-lengths in radians of π/16, π/8, π/4, and π/2 were introduced in these virtual pipes. The matrix of results was used to produce correction factors for crack opening displacement equations applicable to a broad range of pipe sizes.