Effect of remote hydrogen boundary conditions on the near crack-tip hydrogen concentration profiles in a cracked pipeline: Fracture toughness assessment

To investigate the hydrogen embrittlement of pipelines carrying gaseous hydrogen at high pressure, we quantify the hydrogen, stress, and deformation fields in the fracture process zone near the tip of an axial crack on the inner diameter (ID) surface. Under hydrogen pressure 15 MPa, we first explore how these fields evolve with time under conditions simulating either hydrogen outgassing (zero hydrogen concentration) through the outer diameter (OD) surface or an oxidized/coated and therefore impermeable OD surface (zero hydrogen flux). Next we investigate the development of the same fields through a modified boundary layer (MBL) formulation pertaining to laboratory specimens in which the domain is loaded remotely by the stress intensity factor and the T-stress the real-life pipeline experiences and under the same hydrogen boundary conditions. For all practical purposes, we find that the steady state hydrogen concentration fields in the crack tip region in both real-life pipeline and MBL formulation are nearly the same and independent of the boundary conditions prescribed respectively at the OD surface of the pipeline or the remote boundary of the MBL domain so long as small scale yielding conditions are prevalent - which is the case for axial crack depths less than 40% of the pipeline wall thickness. We also find that the near tip results in the MBL formulation are independent of the size of the domain of analysis if small scale yielding conditions prevail. We conclude that one can study the hydrogen effects on fracture at an axial crack on the inner surface of a pipeline through a laboratory fracture mechanics specimen subjected to the same stress intensity factor as the real-life pipeline and exhibiting the same hydrostatic constraint.