Integrity Assessment of Mechanical Components Subjected to Mixed Mode Cracking

This paper addresses a mixed mode driven cracking and its integrity assessment for applications in aging nuclear power piping. Following our earlier discussion on the use of mode-I based criteria in the current R6-method-based practice of integrity assessment, case studies conducted using finite element analysis are conducted and examined: (1) A plate with a single and multiple central crack(s) under tension; (2) A full-scale laboratory test of a straight pipe with an obliquely inserted crack in a dissimilar metal weld. Our results confirm the following earlier observations: 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 an “effective stress intensity factor” based approach; (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; (iii) The effect of multiple cracks can be significant and an assessment by only examining one crack, which is a common practice today, may not be fully conservative.