Effects of crack morphology parameters on leak-rate calculations in LBB evaluations

Abstract

LBB analyses typically involve a two-step process. The first step is the determination of the through-wall crack length that leaks at the minimum detectable rate at normal operating conditions. In the second step, a safety factor is applied to this leakage detectable flaw length (i.e. the flaw length is increased by a factor) and then the stability of the resulting flaw at transient loads (i.e. seismic) is calculated. Probabilistic LBB analyses have shown that the leakage flaw size is more important to the failure probability than the fracture mechanics analysis at the seismic loads. The crack morphology is an important parameter in the calculation of the leakage flaw size. This paper contrasts results from using various crack morphology assumptions in leak-rate calculations. The results show that using a statistical analysis of surface roughness and numbers of turns from cracks removed from service can result in leakage crack sizes that are greatly different for different cracking mechanisms. Using improper morphology parameters can lead to large errors in leakage crack size and can produce non-conservative margins on the LBB critical crack size.