On the small scale character of the stress and hydrogen concentration fields at the tip of an axial crack in steel pipeline: Effect of hydrogen-induced softening on void growth

Gaseous hydrogen transport at pressures of 15 MPa is envisaged as a means of hydrogen delivery from central production facilities to refueling stations for the planned hydrogen economy. The study of the hydrogen embrittlement of medium or mild strength steels, which are under consideration for pipeline materials, has not as of yet led to methods to design safe and reliable pipelines. The most important failure modes in hydrogen containment components are due to subcritical cracking. However, current design guidelines for pipelines only tacitly address subcritical cracking by applying arbitrary, conservative safety factors on the applied stress. In the present work, we investigate the interaction of hydrogen transport with material elastoplasticity in the neighborhood of an axial crack in a steel pipeline. For all practical purposes, we find that the stress, deformation, and hydrogen fields exhibit a small scale character which allows for the use of the standard modified boundary layer approach to the study of the fracture behavior of steel pipelines. The approach is based on constraint fracture mechanics methodology whereby a two-parameter characterization - the stress intensity factor K_Iand the T-stress - is used to describe the interaction of the stress and deformation fields with the diffusing hydrogen under conditions of hydrogen uptake from the crack faces and outgassing through the outer boundaries, as in the pipeline. Employing the Rice and Tracey model of void growth, we find that hydrogen-induced softening can accelerate void growth in a small region confined at the crack tip by as much as 70 % relative to the case of a hydrogen-free material. We close by suggesting that one can ascertain the hydrogen effects on fracture at an axial pipeline crack with the use of a laboratory fracture mechanics specimen tested in hydrogen gas and subjected to the same intensity factor, K_I, and hydrostatic constraint, T-stress, as the real-life pipeline.