Comparison of fracture criteria for cracked steel pipelines by finite element method
Keywords:
pipeline, longitudinal crack, modeling, fracture mechanics, plasticity, nonlocal criterionAbstract
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 doesnot 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 isconsidered – 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.
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