Comparison of fracture criteria for cracked steel pipelines by finite element method

The work is devoted to the simulation of pipeline’s fracture. Application of modern viscous pipe steels does not completely solve the problem of brittle fracture of these structures, because the desire to increase the thickness of the wall and the operating pressure and the need to use a pipe in a cold climate shifts the fracture mode toward brittle - but, of course, does not make it completely brittle and require the application of nonlinear fracture mechanics. Simulation of the fracture by formation of large longitudial cracks should take into account the inertial forces of the pipe material coming into motion at crack propagation, and the possibility of a pressure drop in the pipe at the loss of its impermeability. These features of the problem can be modeled using finite element package LS-DYNA. However, LS-DYNA offers a limited set of a fracture criteria, which does not include the classic criteria of nonlinear fracture mechanics. The aim of the work is the selection of a criteria that is implemented in the LS-DYNA program and at the same time is in good agreement with the experimental data. As such a criterion equivalent plastic strain is considered - with the proviso that the plastic deformation near the crack tip is calculated using a weighted averaging procedure on some representative volume to exclude effect of the finite element sizes on results. Parameters of the weighting function are selected to ensure that the loads corresponding to the beginning of the crack growth are the same for the model of viscous fracture and for the nonlinear fracture mechanics. It is shown that the problem of parameter selection is ill-conditioned. However, the obtained values of parameters will subsequently allow to perform the dynamic calculation of longitudial crack propagation in pipes taking into consideration the effects treated by the LS-DYNA code - inertia forces of the material of the pipe wall, velocity of the perturbation wave in the gas or liquid filling the pipe and decompression.