Effects of Local Wall Thinning With Crack on Limit Loads for Pipes Subject to Combined Pressure and Bending

Abstract

As power plants age, local thinning often occurs in pipework due to erosion/corrosion or other reasons. Local thinning reduces the strength of a pipe and increases the local stress. When the pipework is subject to cyclic loading, a crack could initiate from corrosion pit or welding defect at the local thinning. It is often required to carry out engineering critical assessment for a crack at the local thinned area to support safety case. Limit load or reference stress solution is needed to carry out flaw acceptance assessment to R6 or BS7910. These components are normally subject to combined internal pressure and global bending. Current practice for assessing these kinds of components is based on the minimum measured thickness or a factored minimum thickness. The former is overly conservative on limit load, leading to early sentencing of a component, whilst the latter is potentially non-conservative depending on the proposed factor, thinning profile and dominant load. For pipe with a crack or local thinning subject to internal pressure and global bending many closed form limit load solutions are available. However, limited information and solutions are available on how to determine the limit load of a cylinder with both crack and local wall thinning under combined pressure and moment loading. In this paper, the limit load solutions for pipe with a crack at the local thinned area are investigated using 3D finite element analysis for a range of pipe geometries with different internal excavations (local wall thinning) and circumferential cracks subject to combined pressure and global bending moment. The study also considers the differences between the global limit load of the thinned section (net-section collapse), the local limit load at the thinned area (local collapse) and the ligament limit load at the crack tip (ligament yield). Comparison is made between the elastic-plastic FE analyses and the handbook solutions.