Micromechanics of hydrogen-induced crack initiation in pipeline steels and subcritical crack growth

The technology of large scale hydrogen transmission from central production facilities to refueling stations and stationary power sites is at present undeveloped. Among the problems which confront the implementation of this technology is the deleterious effect of hydrogen on structural material properties. 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 order to study hydrogen embrittlement of pipeline steel, we investigate the interaction of hydrogen transport with material elastoplasticity in the neighborhood of an axial crack on the inner diameter surface of a steel pipeline. Given that the transport pressure will be about 15 MPa, 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. Next we examine the use of laboratory fracture specimens to assess the resistance to hydrogen embrittlement of a hydrogen pipeline. We find that one can ascertain the compatibility of a steel pipeline with hydrogen through using a laboratory specimen tested in hydrogen gas and subjected to the same intensity factor and hydrostatic constraint, T-stress, as the real-life pipeline.;In the second part of the thesis, we study hydrogen-induced crack initiation and crack propagation by coupling the transport model with micromechanics of the fracture mechanism as has been observed in low and medium strength steels. First, we study the hydrogen-induced ductile fracture assisted by MnS inclusions. We investigate how the thermal expansion coefficient mismatch and applied stresses affect decohesion along the inclusion/matrix interface in the neighborhood of a crack tip and how this process as well as subsequent void growth and coalescence are influenced by hydrogen. We model the MnS/ferrite matrix interface with cohesive elements whose traction-separation law accounts for the thermodynamics of hydrogen-induced decohesion. Considering the interaction of multiple voids (nucleated at MnS inclusions) with the crack tip in the presence of hydrogen-induced softening, we study fracture initiation under plane strain conditions.;To further explore the influence of hydrogen on ductile fracture, we model sustained-load cracking in the iron-base superalloy 1N903 at hydrogen pressures at which fracture is governed by plasticity. Through a micromechanics analysis, we quantify the void growth dependence on stress triaxiality and hydrogen-induced material softening as a function of position ahead of a crack tip. Correlation of the calculated void diameters with experimentally measured ones leads to the identification of a microstructural length that characterizes the onset of hydrogen-induced cracking. Lastly, to analyze the mechanics of sustained-load cracking at pressures greater than 100 MPa for which experiments suggest that hydrogen promotes failure by intergranular cracking, we simulate crack propagation by cohesive finite element methodology based on hydrogen-induced decohesion thermodynamics. The results reveal a number of issues related to the complexity of the failure mechanism and the robustness of the cohesive element approach.