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

In many research projects methods to calculate critical circumferential thr ough-wall cracks have been developed and verified. During the last years, t he differentiation between force- and displacement-controlled loading has b een shown to be of significant importance. So it was looked at with more in terest in new analytical methods to calculate the critical crack length. Mo st of the approaches applied in the safety analysis of piping systems assum e defect at welds connecting pieces of straight pipes. But in nearly all ca ses in modern power plants the true position of the welds in the piping sys tem is not correctly represented, as in those systems only few welds connec t parts of straight pipes. Most of the connections are situated between pip es and bends, bends with elongated ends, nozzles or T-parts. This paper pre sents a non-linear finite element (FEM) study covering an essential part of the relevant piping parameters of nuclear power plants primacy and seconda ry system. It compares defects in circumferential welds between straight pi pes to those joining pipes to elbows. In the case of displacement controlle d 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 th at the critical crack sizes in welds near bends are not significantly longe r than the ones connecting straight pipes. In the cases where we have to co nsider in the safety analysis of piping systems, it is a realistic approach to use the methods accepted for welds between pipes to calculate the criti cal crack length in welds near bends. (C) 1999 Elsevier Science S.A. All ri ghts reserved.