Long-running ductile fractures have been observed in high-pressure gas pipelines. These fractures propagate at nearly constant speeds and are accompanied by a large amount of plastic deformation of the pipe wall. In particular, the pipe wall displacement is large, and of the order of one pipe radius. The phenomenon of ductile fracture propagation is very complex. In consequence, most of the existing theoretical investigations into ductile fracture propagation have used an energy balance method as a first approach. Various energies involved in the fracture propagation process depend strongly on the mode of pipe wall deformation and in particular on the geometry of the deformed pipe wall. The major theme of this paper is a proper consideration of the pipe wall deformation during the propagation of a constant-velocity ductile crack. Axial stretching is considered to be the prime mode of deformation of the pipe wall. It is shown that the mode of deformation can be adequately described by two parameters which define the pattern of the pipe wall mid-surface stretching. A shell theory approach is then used to obtain the deformed geometry of the pipe wall in terms of these two parameters; and physical models of the deformed pipe wall are constructed in order to study the large displacement of the pipe wall.

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