In this report, we provide a framework for describing the permeability, solubility and diffusivity of hydrogen and its isotopes in austenitic stainless steels at temperatures and high gas pressures ...of engineering interest for hydrogen storage and distribution infrastructure. We demonstrate the importance of using the real gas behavior for modeling permeation and dissolution of hydrogen under these conditions. A simple one-parameter equation of state (the Abel–Noble equation of state) is shown to capture the real gas behavior of hydrogen and its isotopes for pressures less than 200
MPa and temperatures between 223 and 423
K. We use the literature on hydrogen transport in austenitic stainless steels to provide general guidance on and clarification of test procedures, and to provide recommendations for appropriate permeability, diffusivity and solubility relationships for austenitic stainless steels. Hydrogen precharging and concentration measurements for a variety of austenitic stainless steels are described and used to generate more accurate solubility and diffusivity relationships.
Seventeen metastable austenitic stainless steels (type 304 and 316 alloys) were tested in tension both with internal hydrogen and in external hydrogen. Hydrogen-assisted fracture in both environments ...is a competition between hydrogen-affected ductile overload and hydrogen-assisted crack propagation. In general, hydrogen localizes the fracture process, which results in crack propagation of particularly susceptible materials at an apparent engineering stress that is less than the tensile strength of the material. Hydrogen-assisted crack propagation in this class of alloys becomes more prevalent at lower nickel content and lower temperature. In addition, for the tests in this study, external hydrogen reduces tensile ductility more than internal hydrogen. External hydrogen promotes crack initiation and propagation at the surface, while with internal hydrogen surface cracking is largely absent, thus preempting hydrogen-assisted crack propagation from the surface. This is not a general result, however, because the reduction of ductility with internal and external hydrogen depends on the specifics of the testing conditions that are compared (e.g., hydrogen gas pressure); in addition, internal hydrogen can promote the formation of internal cracks, which can propagate similar to surface cracks.
The capabilities in the Hydrogen Effects on Materials Laboratory (HEML) at Sandia National Laboratories and the related materials testing activities that support standards development and technology ...deployment are reviewed. The specialized systems in the HEML allow testing of structural materials under in-service conditions, such as hydrogen gas pressures up to 138 MPa, temperatures from ambient to 203 K, and cyclic mechanical loading. Examples of materials testing under hydrogen gas exposure featured in the HEML include stainless steels for fuel cell vehicle balance of plant components and CrMo steels for stationary seamless pressure vessels.
The use of type 316 stainless steels in gaseous hydrogen infrastructure motivated this work on hydrogen-assisted fracture. The tensile ductility of type 316 stainless steel is reduced by internal ...hydrogen contents of 136
wppm that have been generated by thermal precharging in hydrogen gas, although these reductions of ductility tend to be modest for both annealed and strain-hardened microstructures. Consistent with the relatively high ductility of both hydrogen-precharged and non-charged specimens, the tensile fracture modes involve plasticity mechanisms. However, internal hydrogen enhances void nucleation by lowering the interface strength at particles and/or by promoting slip localization. High nickel content in type 316 stainless steels appears to offer greater resistance to hydrogen-assisted fracture; in particular, nickel plays an important role in deformation processes that affect hydrogen-assisted fracture. Carbon was found to have no measurable effect on hydrogen-assisted fracture, although it is expected to contribute to stabilizing type 316 stainless steel with respect to the formation of strain-induced martensite.
Slip blockage during slip band intersections impacts mechanical properties of crystalline materials. Molecular dynamics simulations of eight band intersections in Fe70Ni10Cr20 alloys revealed that ...secondary bands always transmit into ε bands more easily than into twin bands. While this is surprising because the ε-phase has a different crystal structure from the matrix, our finding that twins do not possess easy crystallographic pathways for transmission explains this phenomenon. We also found that the band intersection regions preferentially nucleate voids. These findings provide understanding of the deformation and damage behavior of FCC metals. For example, since hydrogen promotes ε-bands, our findings can help understand hydrogen compatibility.
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The objective of this study was to quantify the effects of mechanical and environmental variables on oxygen-modified accelerated fatigue crack growth of steels in hydrogen gas. Experimental results ...show that in hydrogen gas containing up to 1000v.p.p.m. oxygen fatigue crack growth rates for X52 line pipe steel are initially coincident with those measured in air or inert gas, but these rates abruptly accelerate above a critical ΔK level that depends on the oxygen concentration. In addition to the bulk gas oxygen concentration, the onset of hydrogen-accelerated crack growth is affected by the load cycle frequency and load ratio R. Hydrogen-accelerated fatigue crack growth is actuated when threshold levels of both the inert environment crack growth rate and Kmax are exceeded. The inert environment crack growth rate dictates the creation of new crack tip surface area, which in turn determines the extent of crack tip oxygen coverage and associated hydrogen uptake, while Kmax governs the activation of hydrogen-assisted fracture modes through its relationship to the crack tip stress field. The relationship between the inert environment crack growth rate and crack tip hydrogen uptake is established through the development of an analytical model, which is formulated based on the assumption that oxygen coverage can be quantified from the balance between the rates of new crack tip surface creation and diffusion-limited oxygen transport through the crack channel to this surface. Provided Kmax exceeds the threshold value for stress-driven hydrogen embrittlement activation, this model shows that stimulation of hydrogen-accelerated crack growth depends on the interplay between the inert environment crack growth increment per cycle, load cycle frequency, R ratio and bulk gas oxygen concentration.
The selection of austenitic stainless steels for hydrogen service is challenging since there are few intrinsic metrics that relate alloy composition to hydrogen degradation. One such metric, ...presented here, is intrinsic stacking fault energy (SFE). This work reviews the exiting literature to use estimated intrinsic SFE values, calculated with a sub-regular solution thermodynamic model, to compare the retention of tensile ductility of γ-austenitic stainless steels in the presence of hydrogen. The goal is to demonstrate SFE as a metric to screen γ-austenitic stainless steels that use diverse alloying strategies for hydrogen compatibility. A transition in the tensile reduction of area of both 300-series and manganese-stabilized stainless steels is observed at a calculated stacking fault energy of approximately 39 mJ m
−2
, below which pronounced hydrogen degradation on tensile ductility is observed. Calculated intrinsic stacking fault energy is demonstrated as a high-throughput screening metric for a diverse range of austenitic stainless steel compositions with regard to hydrogen compatibility.
The mechanical behaviors of biological soft tissues are challenging to describe abstractly, with each individual tissue potentially characterized by its own unique nonlinear, anisotropic, and ...viscoelastic properties. These complexities are exacerbated by patient to patient variability, both mechanically and anatomically, and by inherent constitutive heterogeneity. Despite these challenges, computational models of whole knee biomechanics can be instrumental in describing the onset and progression of injury and disease. In this work, a three-dimensional whole knee computational model was developed using patient-specific anatomy, containing tissues with constitutive relationships built from relevant experimental investigations. In an effort to address the common assumption of linear elastic descriptions of articular cartilage in whole knee models, this work investigates the implications, with respect to macroscopic kinematics and local deformation, of incorporating physiologically motivated and mechanically accurate constitutive heterogeneity in articular cartilage, highlighting the sensitivities of each corresponding level of constitutive complexity. We show how the inclusion of representative cartilage material models affects deformation distributions within the joint, as well as relative joint motion. In particular, the assumption of linear elasticity in articular cartilage results in an overprediction of joint motion and significantly affects predicted local cartilage strains, while full-field, mechanically heterogeneous cartilage descriptions have a less drastic effect at both the tissue and joint levels. Nonetheless, joints containing complete descriptions of articular cartilage heterogeneity may be an integral component in building comprehensive computational tools to advance our understanding of injury and disease mechanisms.
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•Measured crack growth resistance of welds at 223K with 140wppm H (gas charged).•H reduced fracture initiation toughness by >59% and altered fracture mode.•223K altered fracture mode ...but had no effect on JIC of precharged welds.•At 293K, microcracks initiate at δ-ferrite, and ferrite governed crack path.•At 223K, microvoids form at γ deformation band intersections near phase boundaries.
Effects of low temperature on hydrogen-assisted cracking in 304L/308L austenitic stainless steel welds were investigated using elastic–plastic fracture mechanics methods. Thermally precharged hydrogen (140wppm) decreased fracture toughness and altered fracture mechanisms at 293 and 223K relative to hydrogen-free welds. At 293K, hydrogen increased planar deformation in austenite, and microcracking of δ-ferrite governed crack paths. At 223K, low temperature enabled hydrogen to exacerbate localized deformation, and microvoid formation, at austenite deformation band intersections near phase boundaries, dominated damage initiation; microcracking of ferrite did not contribute to crack growth.
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