Comparison of critical circumferential through-wall crack lengths in welds between pieces of straight pipes to welds between straight pipes and bends, with and without internal pressure at force- and displacement-controlled bending load

Abstract

In many research projects methods to calculate critical circumferential through-wall cracks have been developed and verified. During the last years, the differentiation between force- and displacement-controlled loading has been shown to be of significant importance. So it was looked at with more interest in new analytical methods to calculate the critical crack length. Most of the approaches applied in the safety analysis of piping systems assume defect at welds connecting pieces of straight pipes. But in nearly all cases in modern power plants the true position of the welds in the piping system is not correctly represented, as in those systems only few welds connect parts of straight pipes. Most of the connections are situated between pipes and bends, bends with elongated ends, nozzles or T-parts. This paper presents a non-linear finite element (FEM) study covering an essential part of the relevant piping parameters of nuclear power plants primary and secondary system. It compares defects in circumferential welds between straight pipes to those joining pipes to elbows. In the case of displacement controlled loading, e.g. as due to restrained thermal expansion, which is one of the most severe load cases for most of the welds, we find, that the calculated J-integral values, and so the critical crack length are of comparable size. At force-controlled loading the codes require stronger limitations to the allowable forces and moments. In the regime of allowable loads, we find that the critical crack sizes in welds near bends are not significantly longer than the ones connecting straight pipes. In the cases where we have to consider in the safety analysis of piping systems, it is a realistic approach to use the methods accepted for welds between pipes to calculate the critical crack length in welds near bends.