An axial crack growth prediction procedure initiated at butt welded joint (Growth prediction for surface crack deeper than semi-circular and that retarded at weld fusion line)

Abstract

Structural integrity of cracked pipes is assessed by predicting crack growth. In the fitness-for-service code of the Japan Society of Mechanical Engineers (JSME), the crack growth is predicted using stress intensity factor at the deepest and surface points. A semi-elliptical crack is assumed not to become deeper than a semi-circular crack. However, in reality, the stress corrosion cracking initiated at nickel alloy welds stops growing at the fusion line and becomes deeper than a semi-circular crack. Furthermore, crack shape is close to a rectangular shape rather than a semi-elliptical shape. In this study, validity of the JSME code procedure was discussed for predicting the growth of stress corrosion cracking at nickel alloy welds. Crack growth was simulated by finite element analysis together with an auto meshing technique. Various residual stress distributions and retardation of the crack growth at the fusion line were considered in the simulation. It was demonstrated that the growth prediction procedure prescribed in the JSME code brought about a conservative prediction even if the crack became deeper than the depth of a semi-circular shape crack. It was revealed that, when the growth to the surface direction was retarded at the fusion line, the change in crack size in the depth direction could be predicted conservatively by the current JSME procedure. It was suggested that, when the retardation at the fusion line is assumed in the growth prediction, the crack shape should be modelled by a rectangular shape.