Relationships Between Mechanical Properties and the Extension and Arrest of Unstable Cracks in Line Pipe Steels

With the increasing need for high-strength, high-pressure, large-diameter, gas-transmission lines, considerable attention has been given, in recent years, to the aspects of fracture initiation, propagation and crack arrest in line pipe. This paper presents an overview of the interrelations between material properties and design parameters that can lead to the initiation of a running fracture and the interrelationships which are necessary to arrest a running fracture. It is shown that if the pipe has ductility such that CVN/YS ≥ 0.6 ft-lb/ksi, further increases in Charpy toughness would not have a significant effect upon the critical crack size because fracture initiation becomes flow-stress dependent. Moreover, the length of a stable through-the-wall crack at operating conditions would be about two orders of magnitude longer than the current rejectable weld defect length specified by API. For “conventional” transmission-line applications CVN ≥ 0.024 σ_h^1.5D^0.5 assures arrest of running shear fractures.