The goal of this division is to provide the basic science that enables optimization of the cost, performance, and safety of pressurized hydrogen containment systems. In particular, the objectives include: development and use of advanced methods for experimentally characterizing the effects of hydrogen on the fatigue, fracture, and tribological properties of materials; development of models of hydrogen-affected fatigue, fracture, and tribo-interfaces; and development of next-generation monolithic and functionally graded materials having lower cost and improved performance (e.g., higher strength) while retaining resistance to hydrogen-induced degradation.
● RESEARCH HIGHLIGHTS
The research accomplishments from the Hydrogen Structural Materials division are in accord with the short-, mid-, and long-term technology and research objectives. An overview of progress toward objectives in the first two time horizons is provided in this section, and two corresponding significant accomplishments are detailed in the following section.
The short-term technology objective addressed by this division is deploying safe and reliable components (valves, piping, seals, etc.), vessels, and compressors for hydrogen gas storage and distribution. The work described here was motivated by several basic research objectives derived from the technology objectives, including elucidating the salient variables and mechanisms governing hydrogen-induced degradation in structural metals and optimizing methods for characterizing hydrogen-induced degradation in materials. The former research objective related to elucidating mechanisms of hydrogen-induced degradation pertains to fatigue and fracture as well as friction and wear.
One of the significant accomplishments described in the following sections relates to research objectives in the short-term horizon. These results from Prof. Robertson’s team provide new insights into the nature of cyclic deformation-induced microstructure evolution during fatigue crack growth. In addition, the results show that such deformation-induced microstructure evolution is notably influenced by hydrogen. These elusive observations were only possible because of the development of advanced material characterization methods. These accomplishments recently documented in the International Journal of Fatigue demonstrate that the activity reported last year has progressed toward a clear story. Complementary results were produced by other research teams on different material systems. For example, Prof. Kubota’s team demonstrated that hydrogen affected cyclic deformation in copper as well. In this new activity, it was observed that hydrogen significantly enhanced the development of slip bands. Although copper is not a high-priority structural metal for hydrogen technology, it is a model material for gaining fundamental insights about hydrogen-deformation interactions. In addition, Prof. Yamabe proposed that the mechanism of intergranular hydrogen embrittlement in high-strength steels includes the interaction of deformation twins with grain boundaries. This mechanistic interpretation was published in the International Journal of Fracture during 2012.
Two other activities represent an extension of work reported last year. In the 2012 report, Dr. Somerday indicated that the mechanism of hydrogen-assisted fatigue crack growth in low-strength steels involves intergranular fracture. Since then, Dr. Somerday has been collaborating with Prof. Takaki’s team to design experiments that can test proposed mechanisms. These experiments require model Fe-C alloys, which Prof. Takaki’s team have procured and characterized. In the friction and wear activities supervised by Prof. Sugimura, the notable result in last year’s report was that hydrogen uptake into steel is blocked by oxide films formed under sliding at low contact pressure. The progression of this work has revealed that other variables contribute to surface oxide formation and its effect on hydrogen uptake, including contact pressure and temperature. These efforts to characterize the basic processes and variables affecting hydrogen uptake during sliding and normal contact between surfaces complements work on characterizing hydrogen uptake at crack tips to define basic mechanisms of hydrogen-affected fatigue and fracture.
● FUTURE DIRECTIONS
The future directions for Hydrogen Structural Materials are oriented toward the short-, mid-, and long-term technology and basic research objectives identified for the division. The common thread for these future activities is an emphasis on collaboration. These collaborative relationships will be fostered within the division as well as across division and institutional boundaries. Examples of collaborative activities that impact the short- and mid-term objectives are highlighted below.
(i) Short term
Future directions that impact the short-term horizon focus on the basic research objective of elucidating salient variables and mechanisms governing hydrogen-induced degradation in steels under mechanical loading conditions that induce fatigue, fracture, friction, or wear. By way of example, a new collaboration that fuses expertise from the fatigue/fracture (Prof. Robertson team) and friction/wear (Prof. Sugimura team) technical areas will determine how the microstructure evolution under wear conditions is influenced by the presence of hydrogen. The collaboration has already been established and characterization of samples with known wear properties has started. Another collaborative activity that addresses the short-term research objective of elucidating salient variables and mechanisms governing hydrogen-induced degradation in steels involves Dr. Somerday and Prof. Takaki’s team. This collaboration represents an extension of work previously reported by Dr. Somerday, in which evidence from fatigue crack growth tests conducted on low-strength steels in hydrogen gas indicated that hydrogen embrittlement involved intergranular fracture. Dr. Somerday proposed several mechanistic scenarios for the hydrogen-induced intergranular fracture, and these will be tested by performing experiments on model Fe-C alloys. The first experiments will be conducted on two Fe-C alloys with varying grain size, which the Prof. Takaki team has already procured and characterized. The initial fatigue crack growth tests on these alloys will be a joint effort between Dr. Somerday and a member of Prof. Takaki’s team (Dr. Arnaud Macadre) at Sandia National Laboratories in late summer/early fall.
(ii) Mid term
Future directions that impact the mid-term horizon focus on the basic research objective of developing models that predict the relationship between hydrogen-induced degradation and material characteristics at different length scales. In pursuit of this objective, Prof. Sofronis’ team will facilitate a collaboration to finalize the new hydrogen-mediated crystal plasticity constitutive model. While the model framework now exists due to efforts by WPI Prof. Aravas, further progress depends on insights that must be derived from experiments. These experiments are extremely complex, involving compression tests on hydrogen-exposed single-crystal metals. Both the mechanical loading and hydrogen concentration boundary conditions must be accurately controlled. Design and execution of the experiments will be led by Dr. Somerday and WPI Prof. Kubota.