Comparison of Stress Intensity Factors of Part Surface Flaws at Stress Concentration in Plate and Pipe Between FE Analysis and Weight Function Approach

Abstract

Stress intensity factor (SIF) is one of the key parameters in structural integrity assessment. Weight function method has been used in flaw acceptance assessment codes and standards, such as R6 and BS7910, to calculate SIF of a semi-elliptical (part) surface flaw. In this method, stress distribution across the section thickness is described by a polynomial equation, and SIF is estimated using geometry functions fi and stress components σi. The SIF solutions are available for both the deepest and the surface points of part surface flaw in R6 and BS7910. However, a case study from this work shows that the SIF estimation using the current methods are not always conservative when a flaw is at stress concentration, such as weld toe. This results in an optimistic limiting defect sizes and jeopardizes the safety. To account for the effect of stress concentration on SIF, one solution is to use SIF magnification factor and stress concentration factor, but this approach could be overly conservative. Although the original research used power law stress distribution in calculation of SIF, it is not clear whether the developed geometry function factors are suitable for a flaw at steep gradient stress concentration zone. The same question is for the similar SIF solutions of French RCC-MR code, as the model used to derive the SIF does not include stress concentration. This paper briefly reviews the weight function SIF solutions and compares them with the 3D FEA results of surface flaws in plate and pipe with various dimensions and flaw sizes. The guidance is provided on how to use weight function SIF solutions for surface flaws at stress concentration region for structural integrity analysis.