STRESS INTENSITY FACTOR AND ELASTIC COD FOR CIRCUMFERENTIAL THROUGH-WALL CRACKS IN THE INTERFACE BETWEEN AN ELBOW AND A STRAIGHT PIPE UNDER INTERNAL PRESSURE

Abstract

Leak-Before-Break (LBB) is one of important approaches applied to nuclear piping design. In the LBB assessment, it is important to evaluate crack instability and to predict leak rate based on a fracture mechanics concept, in which an idealized straight pipes with through-wall cracks (TWCs) are generally considered in the typical LBB analysis. On the other hand, in nuclear piping system, elbows are often connected with straight pipes by welding, in which cracks could occur as well known. Hence, accurate assessment needs to be performed for cracks in weldments joining an elbow and attached straight pipe. In the previous study, it has been revealed that crack instability of a TWC in the interface between an elbow and a straight pipe under bending moment could be different with that of a straight pipe with a TWC depending on a change of pipe thickness, radius of curvature and crack length. Especially, elbows attached to a pipe were more severe than straight pipes for relatively shorter crack length. Thus, a need of engineering solutions for cracks in the interface between an elbow and a pipe is raised for accurate LBB analysis on nuclear piping system. In this present study, stress intensity factor (SIF) and crack opening displacement (COD) are estimated via detailed 3-dimensional finite element (FE) elastic analyses for circumferential TWCs in the interface between an elbow and a straight pipe subjected to internal pressure. The geometric variables (pipe thickness, radius of curvature and crack length) affecting SIFs and CODs were systematically considered in order to cover actual ranges of geometric variables. Also, the effect of elbow on elastic fracture parameters was investigated by comparing the present results with the results from the previous straight pipe solution. Moreover, based on the present FE results, the shape factors (F, V) used for calculating SIFs and CODs are proposed for circumferential TWCs in the interface between an elbow and a pipe. The present results can be used to perform the accurate LBB assessment for nuclear piping system including elbows welded to a straight pipe.