Pipe Deformation During a Running Shear Fracture in Line Pipe

Under sponsorship of the American Iron and Steel Institute, U. S. Steel Research has been conducting full-scale burst tests of large-diameter submerged-arc-welded line pipe to determine the toughness required to arrest running shear fractures for different design conditions. As part of that program, the pipe were instrumented with crack detectors, strain gages, and pressure transducers to determine the crack velocities and the actual pipe deformation and strain fields associated with the shear fracture propagating along the top of the pipe. This paper summarizes the test data that document the manner in which the pipe deforms during this type of crack propagation. The data show that for a propagating shear fracture, each of four different locations along the pipe length (relative to the crack tip) has a distinctive type of pipe deformation. For a location many pipe diameters ahead of the crack tip, the circumferential strain first decreases because of flexural waves associated with the initiation process and then continues to decrease in proportion to the local gas decompression; however, the longitudinal strain continuously increases because of a longitudinal “tongue” of tensile straining on the top of the pipe caused by pressure-induced opening of the flaps of the pipe on both sides of the fracture behind the crack tip. At a distance about two diameters ahead of the crack tip, the pipe cross section becomes oval, and in the presence of this deformation the strain field is no longer determined by the local pressure; in fact, the circumferential strain is near zero at a distance two diameters ahead of the crack. The oval pipe shape ahead of the crack tip is caused by the venting of the gas behind the crack tip which creates a downward reactive force on the bottom portion of the pipe. Opening at the crack tip is the result of tensile straining caused by circumferential and radial displacement of the flaps behind the crack tip. Thus it is believed that the action of the pipe-wall flaps behind the crack tip provides the primary force driving the crack down the top of the pipe.