The prime objective of the proposed work is to discern in a quantitative fashion the fundamental mechanisms responsible for the hydrogen-induced degradation of both model and industrially-relevant engineering materials using a fully integrated, combined applied mechanics/modeling and materials science/microstructural approach. The approach combines quantitative, large-scale numerical simulations with novel experimentation and in situ imaging to identify the salient physical micro-mechanisms and characteristic microstructural size-scales involved in the local fracture events. Since such fracture events are stochastic, the analysis is statistical in nature, formulated for real microstructures and based on the operative physical micro-mechanisms. The principal outcome of this work will be the establishment of physically based engineering criteria for hydrogen-related fracture in both ductile and brittle metallic materials, where the primary mechanisms of hydrogen degradation, specifically decohesion and shear localization, are active. Because of their industrial significance, these materials include low and ultrahigh strength steels and a Ni3Al high-temperature intermetallic. A second outcome will be the training of students in materials science, but in the context of mechanics-based analyses of material behavior. Graduate and undergraduate students who work on the project are exposed to both the materials science and applied mechanics cultures; thereby, they will acquire an essential capability required of modern researchers in fracture. Since the study focuses largely on structural materials of real engineering significance, students are being educated in an interdisciplinary way on the mainstay of structural engineering. %%% Meeting these goals provides answers to numerous pressing scientific issues associated with hydrogen embrittlement. These include (i) the relative significance of brittle decohesion versus hydrogen-assisted shear localization mechanisms, (ii) the process by which hydrogen enhanced localized plasticity promotes localized fracture, (iii) the relevance of equilibrium vs. non-equilibrium decohesion theories, and (iv) the role, and potential synergism, of solute impurities in varying material microstructures. The work also has a significant technological impact on the general operation of materials under severe environmental conditions. Moreover, it represents an enabling technology for the successful application of advanced materials such as intermetallics in any environment, since so little is known about the criteria for environmentally assisted local fracture events in these materials.
