For the past several years the Pipeline Research Committee of the American Gas Association has sponsored research at Battelle's Columbus Laboratories with the objective of obtaining a better understanding of the behavior of defects in pressurized pipe. This objective is being pursued by means of full-scale experiments on line pipe specimens containing both artificial and actual defects. These experiments have led to the development of semiempirical equations for predicting the ductile failure stress levels of through-wall flaws and surface flaws. Although these equations have been presented before, the supporting data and analyses are described in this paper. The through-wall flaw equation is analogous to fracture mechanics criteria for plane stress fracture; but because it has been adapted to ductile line pipe materials, it contains the Dugdale Model for plastic flow in the material and a correction for the bulging stress resulting from pressure acting on the curved pipe walls. The surface flaw equation evolved from the experimental results on surface-flawed pipe specimens. It accounts for both length along the axis of the pipe, depth through the wall, and the bulging which also takes place at surface flaws. Both equations have been shown to give reliable prediction of failure stress levels for not only steel line pipe materials but for stainless steel and aluminum pressure vessels as well. The usefulness of these equations extends over a wide range of material toughness and strength levels, because they embody both tensile strength parameters and the notch-toughness as determined from the ductile shelf energy of Charpy V-notch impact specimens. The experimental results upon which the equations are based are presented and discussed herein as are the utility and degree of reliability of the equations.
Presented is a discussion of an hypothesized analytical explanation of ductile fracture initiation, propagation, and arrest in cylindrical pressure vessels and piping. The hypothesized analytical treatment is an attempt to predict initiation and arrest conditions for ductile fractures using Charpy V-notch plateau energy as a means of determining the toughness of the material. Data from a number of full-scale experiments on gas transmission pipe, nuclear reactor piping, and other cylindrical vessels are presented and are shown to be in agreement with the hypothesis.
The program of research on line pipe under the sponsorship of the A.G.A. Pipeline Research Committee is a comprehensive effort to investigate the important properties of pipe used in gas transmission. Several different phases are involved in this project, ranging from fundamental laboratory studies to fracture-behavior experiments on large-diameter pipe. This paper discusses the full-scale experimental parts of the program in which the fracture toughness of line pipe is being studied. Some of the factors that influence full-scale fracture behavior are discussed—material properties, fracture speed, temperature, wall thickness, nominal stress level, and type of backfill. Laboratory fracture tests that are being run and correlated with full-scale behavior are also described.
The results of extensive research on piping and pressure vessels of plain carbon and low-alloy materials is providing an understanding of the significance of impact test results in terms of the full-scale fracture initiation and propagation characteristics. The impact tests employed in the research work were the Charpy V-notch impact test and the drop-weight tear test (DWTT). The impact tests have been correlated with full-size cylindrical vessel tests in which the initiation and propagation characteristics have been determined under controlled conditions. The results of the research indicate that, for the materials studied, the impact test fracture appearances relate to the structural fracture appearance with certain limitations. Specifically, the DWTT when conducted on material of the same thickness as the structure will predict directly the fracture propagation transition temperature. The Charpy V-notch test fracture appearance can be correlated to the structural fracture propagation transition temperature but is offset generally on the temperature scale because of the difference in thickness between the Charpy specimen and the structure. Correlations have been attempted between Charpy or DWTT energies and the fracture initiation characteristics determined from full-scale cylindrical vessel experiments. These correlations to date between the plane stress-stress intensity factor Kc and the plateau impact energies have exhibited considerable scatter, and no consistent trends are indicated. Also, correlations were attempted between the impact energies and the fracture propagation behavior, but no consistent relationship was found.
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