Instability of a circumferential crack in a piping system subject to bending and tensile loadings

The problem of the instability of crack growth in a pipe fabricated from a ductile material has received extensive consideration, both theoretically and experimentally, during the last few years. This consideration has been motivated, to a large extent, by the technological problems posed by the intergranular stress corrosion cracking of Type 304 stainless steel piping systems in Boiling Water Reactors. The cracks have been observed in the heat-affected zones of girth welds and they form at the inner surface of a pipe; they are oriented circumferentially and are able to grow slowly in service by an environmentaly assisted mechanism. From a safety perspective, an important question is whether a severe overload, due to for example severe earthquake, will convert a part-through stress corrosion crack into a through-wall crack, which will then spread unstably around the pipe circumference and so cause complete pipe severance (guillotine rupture). Instability can be predicted by assuming that failure follows a net-section stress criterion, using as input a knowledge of the critcal net-section stress value and the anticipated loading. (1) Application of this criterion defines the conditions for failure for load-control deformation.