Although steels behave in a ductile manner in a simple tensile test, they sometimes crack in a brittle fashion when subjected to fairly low service stresses. In order to understand and to control this brittle or cleavage fracture in line pipe, a research program was initiated. At first, the program was directed toward the propagation aspects of the problem. This resulted in a series of large-scale tests using natural gas as the pressuring medium. In this series of tests, it was established that there is a definite relation between the velocity at which a crack travels through a pipe and the behavior...
Although steels behave in a ductile manner in a simple tensile test, they sometimes crack in a brittle fashion when subjected to fairly low service stresses. In order to understand and to control this brittle or cleavage fracture in line pipe, a research program was initiated. At first, the program was directed toward the propagation aspects of the problem. This resulted in a series of large-scale tests using natural gas as the pressuring medium. In this series of tests, it was established that there is a definite relation between the velocity at which a crack travels through a pipe and the behavior of a small coupon of the pipe steel in a notched-bar impact test. The later emphasis has been on the initiation phase of cleavage fracture. This emphasis was based on the belief, indicated by ship-plate steel fracture studies, that the stress-level barrier to crack initiation is greater than the barrier to crack propagation. Operational economics and the fact that a crack cannot propagate unless it is initiated indicated that an investigation of the factors involved in the initiation of cracks in line pipe would be valuable. Such an investigation should provide a sound foundation on which to base pipeline design, specification, and operation.