The conservatism of the net-section stress criterion for the failure of cracked stainless steel piping

Abstract

A standard procedure for predicting the failure of cracked stainless steel piping is based on the assumption that failure conforms to a net-section stress criterion, using as input an appropriate value for the critical net-section stress, together with a knowledge of the anticipated loadings. The stresses at the cracked section are usually calculated via a purely elastic analysis based on the piping being uncracked. However, because the piping is built-in at its ends into a larger component, and since the onset of fracture at a crack tip requires some plastic deformation, use of the net-section stress approach can lead to conservative failure predictions. This paper examines the extent of this conservatism, and shows that the degree of conservatism depends on the material ductility and a length parameter L∗, which is a measure of the elastic flexibility of a piping system. With a sufficiently ductile material and with a stiff piping system, definite advantages can be gained from allowing for the built-in characteristics of a piping system and for plasticity at the cracked section. There is an additional measure of conservatism due to the fact that unstable failure need not necessarily be associated with the onset of crack extension.