FRACTURE ASSESSMENT OF BIMETALLIC WELD

Abstract

The available Leak-Before-Break (LBB) methodology is based on evaluating the integrity of cracked nuclear piping, having mismatch in material properties, by considering the strength properties of the base material and fracture properties of weld material. This method generally yields safe result but it is highly conservative. In order to carry out a precise assessment of integrity of a cracked welded structure, knowledge of measurement of the fracture toughness of the welded joint, determination of limit load and estimation of crack driving force are needed. At present, FEM is considered as the best method to evaluate limit load and crack driving force for these mismatched structures; however it is always useful to have some simplified estimation methods for engineering calculations. With this view in mind an estimation method has been developed for evaluating limit-load for mismatched specimen like center crack plate (CCP) specimen, three-point bend (TPB) specimen and compact tension (CT) specimen. Good agreement has been obtained between the analytical and Finite Element results. Also a J-integral estimation method is proposed for a crack located in the middle of a weld with a mismatch in mechanical properties from the surrounding base materials. The method covers both yield stress over/under matching and differences in hardening behaviour between weld and base materials. The method involves the definition of an `equivalent stress-strain relationship' based on the mechanical properties of both the weld and the base materials. An approximate solution for the equivalent stress-strain relationship has been obtained by assuming that the average resistance along a slip-line controls the plastic-stress and strain fields near the crack tip. Detailed formulae for the plane strain center cracked plate (CCP), three point bend (TPB), and compact tensile (CT) specimens have been derived on the basis of limit load solutions. The value of J has been then estimated using the `equivalent stress-strain relationship' and for the actual stress-strain data obtained from experiment in conjunction with the Nonlinear Finite Element Method. Both the results show good agreement, and since the approach is quite general it can be extended for the case of pipes involving fracture assessment.