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Ductile crack propagation in pneumatically pressurized pipelines is an extremely complex process, involving interactions between the fracturing pipe, the escaping gas, the covering backfill, and mechanical crack arresters, if any are present. In order to extend the experience gained through testing programs to the design of pipelines, it appears to be necessary to develop theoretical models of the process based on simplifying assumptions which are consistent with the available experimental data. Over the past several years, the American Iron and Steel Institute has conducted a full-scale test program on propagating shear fractures in pneumatically pressurized linepipe, and many measurements have been made of crack motion, pipe wall deformation, pressure loading, fracture toughness, etc. The relevant data are reviewed, and some implications for modeling pipe wall plasticity, gas flow and escape, and backfill resistance are considered. Pipe wall deformations are assumed on the basis of test data, and the contributions to an overall energy balance equation due to individual deformation components are computed in the spirit of upper-bound calculations of plastic limit load analysis. It is concluded that for ductile materials the resistance of the pipe wall material to axial inplane stretching near the crack tip is a major contribution to the total structural and inertial resistance to the gas pressure driving force, and relative magnitudes of other resistance terms are also estimated for the case of no backfill. The capacity for soil backfill to dissipate the work of the driving force is also considered, and it is concluded that for soils with low cohesion the inertial resistance of the backfill is much greater than the resistance due to soil strength.
crack propagation, fracture mechanics, pipelines, plastic deformation, ductile fracture, gas dynamics, soil backfill
Professor of engineering, Brown University, Providence, R. I.
Assistant professor, Massachusetts Institute of Technology, Cambridge, Mass.