Finite Element Analysis of a Full-Scale Test of a Cracked Dissimilar Metal Weld for Aging Nuclear Power Piping and Fracture Failure Assessment

This report addresses a mixed mode driven cracking and its fracture failure assessment for applications in aging nuclear power facilities. Following our earlier discussion on the use of mode-I based criteria in the current practice of fracture assessments, a finite element analysis of a full-scale laboratory test, a Benchmark four-point bending test of a straight pipe with an obliquely inserted crack in a dissimilar metal weld of ferritic steel (A508) and austenitic steel (316L), together with weld (308L) and buttering material (309L/308L), is conducted. The behavior of the crack front at the load level, at which crack initiation is observed in the test, such as stress intensities (K_I, K_II, K_III), J-integrals and other relevant parameters along the crack front, are computed. Crack initiation assessments are thereafter made using three alternatives: (1) Mode I cracking; (2) Mixed mode cracking; (3) An empirical approach suggested for accounting the mixed mode effect using a so-called R6-method. The results confirm our earlier observation: For cases when mixed mode loading conditions are significant, (i) the fracture initiation predicted by using J-integral based mixed mode cracking criteria can approximately be achieved by using the R6-empirical approach for the mixed mode cracking; (ii) it is not conservative to use a purely mode-I based criterion for the evaluation of the fracture failure assessment for typical problems of mixed mode driven cracking.