Critical circumferential through-wall cracks according to the unloading ofthe cracked section under displacement-controlled bending load

The evaluation of critical circumferential through-wall crack lengths in pi ping is usually performed by the flow stress concept. plastic limit load me thod or GE-EPRI procedures. Most of these methods treat the secondary stres ses, especially those caused by bending moments resulting from restrained t hermal expansion, as force-controlled loads. In reality, there is a movemen t of the piping into the direction of the prescribed displacement and, ther efore, a relaxation of the cracked section, which is due to the local rotat ion of the cracked section. Instead of the bending moment originating from the elastic analysis of the piping system there will be a reduced bending m oment, the load decreases, the real critical through-wall crack lengths due to this displacement-controlled loading are larger than those predicted by the load controlled methods. A corresponding analytical procedure taking i nto account this relaxation was developed and validated by a comparison wit h experiments as well as finite element calculations. The procedure can be used for the evaluation of the safety of piping systems (e.g. leak-before-b reak analyses), if the usual methods based on force-controlled loads give u nrealistic, conservative results. (C) 1999 Elsevier Science S.A. All rights reserved.