Proposal of Simplified J-Integral Evaluation Method for a Through Wall Crack in SFR Pipe Made of Mod.9Cr-1Mo Steel

Abstract

A simplified J-integral evaluation method applicable to unstable failure analysis in Leak Before Break (LBB) assessment of Sodium-cooled Fast Reactor (SFR) in Japan was proposed. Mod.9Cr-1Mo steel is supposed to be a candidate material for the coolant systems of SFR in Japan. This steel has relatively high yield strength and poor fracture toughness comparing to those of conventional austenitic stainless steels. In addition, SFR pipe has small thickness and large diameter. Furthermore, in SFR, primary stresses are insignificant and displacement controlled secondary stresses are predominant. Therefore, the load balance in such piping system changes by crack extension and R6 (2-parameter) method (hereinafter “2-parameter method”) [1] using J-integral is applicable to unstable failure analysis for the pipes under such loading conditions. As a J-integral evaluation method for circumferential through-wall crack in a cylinder, EPRI has proposed a fully plastic solution method. However, the geometry of SFR pipe and material characteristics of Mod.9Cr-1Mo steel exceed the applicable range of EPRI’s method. Therefore, a series of elastic, elasto-plastic and plastic finite element analyses (FEA) were performed for a pipe with a circumferential through-wall crack to propose a J-integral evaluation method applicable to such loading conditions. J-integrals obtained from the FEA were resolved into elastic, local plastic and fully plastic components. Each component was expressed as a function of analytical parameter, such as pipe geometries, crack size, material characteristics and so on. As a result, a simplified J-integral evaluation method was proposed. The method enables to conduct 2-parameter method using J-integral without any fracture mechanics knowledge.

Effect of local plastic component on crack opening displacement and on J-integral of a circumferential penetrated crack

“…In such materials with small work hardening, the local plastic component has a large influence on the evaluation of J-integral and COD even when used below the yield stress. In order to evaluate local plastic component accurately, the authors divided the evaluation formulas for J-integral and COD into elastic, local plastic and fully plastic components, and formulated them as follows (Wakai et al, 2014(Wakai et al, , 2018.…”

“…where, subscript E, LP and FP show elastic, local plastic and fully plastic components, respectively. Since elastic and fully plastic components have already been proposed in previous studies and verification with the GE-EPRI method has also been completed (Wakai et al, 2014(Wakai et al, , 2018, the local plastic component in the evaluation of Jintegral and COD was highlighted in this study, and an evaluation method was developed especially for small work hardening materials whose influence strongly appears. Elastic, plastic and elastic-plastic FEA were performed and Jintegral and COD due to the local plastic component were obtained by subtracting the results of elastic and plastic analysis from the results of elastic-plastic analysis.…”

“…In the calculation of J-integral, virtual crack extension method was employed. Analytical function of the code has been verified in a previous report (Wakai et al, 2014(Wakai et al, , 2018. Isoparametric 20-node three-dimensional hexahedral element (HEX20) was used in the analyses.…”