Modeling of the initiation conditions of cracks in a pipe under pressure of a hydrogenous medium

An actual problem of modern engineering about destruction of a pipeline as a result of influence of hydrogen contained in the transported products is considered. Hydrogen changes the mechanical properties of metal, affecting the stress-strain state of the pipe, which, in turn, affects the distribution of hydrogen in the pipe. The hypotheses about the nature of this relationship accepted in the paper allowed to explain the reason of circumferential crack formation in the pipe under the influence of hydrogen. An algorithm for iterative calculation of the stress-strain state of a tube containing a hydrogen-containing mixture inside the tube has been developed. The coupled problem of the theory of elasticity and diffusion in the planar axisymmetric formulation is solved. Since the interaction process of hydrogen and metal is very slow, it is considered in sequential static formulations. First, the Lame-type problem for a tube with the modulus of elasticity depending on the radial coordinate is solved. By the finite difference method the stress and strain fields of the pressurized pipe are found. Further, the concentration of free hydrogen in the pipe caused by its content on the pipe surfaces and its stress state is determined. The accepted hypothesis about the condition of hydrogen atoms embedding into the crystal lattice of metal allows to estimate the influence of hydrogen on mechanical properties of the pipe material at the next stage of the calculation. The calculation of stress and concentration fields is repeated again with already modified mechanical properties. The iteration process is stopped when the stresses in the tube reach critical values according to Mises criterion or when the mechanical properties of the pipe material stop changing. The calculations show that at some combination of hydrogen concentration and pressure on the pipe wall, zones of plastic deformation arise in the pipe, which can lead to delamination of the material in the circumferential direction. This result is consistent with known experimental data.