Publications

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Gaseous hydrogen is an alternative to petroleum-based fuels, but it is known to significantly reduce the fatigue and fracture resistance of steels. Steels are commonly used for containment and distribution of gaseous hydrogen, albeit under conservative operating conditions (i.e., large safety factors) to mitigate so-called gaseous hydrogen embrittlement. Economical methods of distributing gaseous hydrogen (such as using existing pipeline infrastructure) are necessary to make hydrogen fuel competitive with alternatives. the effects of gaseous hydrogen on fracture resistance and fatigue resistance of pipeline steels, however, has not been comprehensively evaluated and this data is necessary for structural integrity assessment in gaseous hydrogen environments. In addition, existing standardized test methods for environment assisted cracking under sustained load appear to be inadequate to characterize low-strength steels (such as pipeline steels) exposed to relevant gaseous hydrogen environments. In this study, the principles of fracture mechanics are used to compare the fracture and fatigue performance of two pipeline steels in high-purity gaseous hydrogen at two pressures: 5.5 MPa and 21 MPa. In particular, elastic-plastic fracture toughness and fatigue crack growth rates were measured using the compact tension geometry and a pressure vessel designed for testing materials while exposed to gaseous hydrogen. Copyright © 2010 by ASME.

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Fatigue crack growth rates and rising displacement fracture thresholds have been measured for a 4130X steel in 45 MPa hydrogen gas. the ratio of minimum to maximum load (R-ratio) and cyclic frequency was varied to assess the effects of these variables on fatigue crack growth rates. Decreasing frequency and increasing R were both found to increase crack growth rate, however, these variables are not independent of each other. Changing frequency from 0.1 Hz to 1 Hz reduced crack growth rates at R = 0.5, but had no effect at R = 0.1. When applied to a design life calculation for a steel pressure vessel consistent with a typical hydrogen trailer tube, the measured fatigue and fracture data predicted a re-inspection interval of nearly 29 years, consistent with the excellent service history of such vessels which have been in use for many years. Copyright © 2010 by ASME.

No concerns for Hydrogen-Enriched Compressed Natural gas (HCNG) in steel storage tanks if material strength is < 950 MPa. Recommend evaluating H{sub 2}-assisted fatigue cracking in higher strength steels at H{sub 2} partial pressure in blend. Limited fatigue testing on higher strength steel cylinders in H{sub 2} shows promising results. Impurities in Compressed Natural Gas (CNG) (e.g., CO) may provide extrinsic mechanism for mitigating H{sub 2}-assisted fatigue cracking in steel tanks.

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Threshold stress intensity factors were measured in high-pressure hydrogen gas for a variety of low alloy ferritic steels using both constant crack opening displacement and rising crack opening displacement procedures. The sustained load cracking procedures are generally consistent with those in ASME Article KD-10 of Section VIII Division 3 of the Boiler and Pressure Vessel Code, which was recently published to guide design of high-pressure hydrogen vessels. Three definitions of threshold were established for the two test methods: K{sub THi}* is the maximum applied stress intensity factor for which no crack extension was observed under constant displacement; K{sub THa} is the stress intensity factor at the arrest position for a crack that extended under constant displacement; and K{sub JH} is the stress intensity factor at the onset of crack extension under rising displacement. The apparent crack initiation threshold under constant displacement, K{sub THi}*, and the crack arrest threshold, K{sub THa}, were both found to be non-conservative due to the hydrogen exposure and crack-tip deformation histories associated with typical procedures for sustained-load cracking tests under constant displacement. In contrast, K{sub JH}, which is measured under concurrent rising displacement and hydrogen gas exposure, provides a more conservative hydrogen-assisted fracture threshold that is relevant to structural components in which sub-critical crack extension is driven by internal hydrogen gas pressure.

Carbon-manganese steels are candidates for the structural materials in hydrogen gas pipelines, however it is well known that these steels are susceptible to hydrogen embrittlement. Decades of research and industrial experience have established that hydrogen embrittlement compromises the structural integrity of steel components. This experience has also helped identify the failure modes that can operate in hydrogen containment structures. As a result, there are tangible ideas for managing hydrogen embrittement in steels and quantifying safety margins for steel hydrogen containment structures. For example, fatigue crack growth aided by hydrogen embrittlement is a key failure mode for steel hydrogen containment structures subjected to pressure cycling. Applying appropriate structural integrity models coupled with measurement of relevant material properties allows quantification of safety margins against fatigue crack growth in hydrogen containment structures. Furthermore, application of these structural integrity models is aided by the development of micromechanics models, which provide important insights such as the hydrogen distribution near defects in steel structures. The principal objective of this project is to enable application of structural integrity models to steel hydrogen pipelines. The new American Society of Mechanical Engineers (ASME) B31.12 design code for hydrogen pipelines includes a fracture mechanics-based design option, which requires material property inputs such as the threshold for rapid cracking and fatigue crack growth rate under cyclic loading. Thus, one focus of this project is to measure the rapid-cracking thresholds and fatigue crack growth rates of line pipe steels in high-pressure hydrogen gas. These properties must be measured for the base materials but more importantly for the welds, which are likely to be most vulnerable to hydrogen embrittlement. The measured properties can be evaluated by predicting the performance of the pipeline using a relevant structural integrity model, such as that in ASME B31.12. A second objective of this project is to enable development of micromechanics models of hydrogen embrittlement in pipeline steels. The focus of this effort is to establish physical models of hydrogen embrittlement in line pipe steels using evidence from analytical techniques such as electron microscopy. These physical models then serve as the framework for developing sophisticated finite-element models, which can provide quantitative insight into the micromechanical state near defects. Understanding the micromechanics of defects can ensure that structural integrity models are applied accurately and conservatively.

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Objectives are to enable development and implementation of codes and standards for H{sub 2} containment components: (1) Evaluate data on mechanical properties of materials in H{sub 2} gas - Technical Reference on Hydrogen Compatibility of Materials; (2) Generate new benchmark data on high-priority materials - Pressure vessel steels, stainless steels; and (3) Establish procedures for reliable materials testing - Sustained-load cracking, fatigue crack propagation. Summary of this presentation are: (1) Completed measurement of cracking thresholds (K{sub TH}) for Ni-Cr-Mo pressure vessel steels in high-pressure H{sub 2} gas - K{sub TH} measurements required in ASME Article KD-10 (2) Crack arrest test methods appear to yield non-conservative results compared to crack initiation test methods - (a) Proposal to insert crack initiation test methods in Article KD-10 will be presented to ASME Project Team on Hydrogen Tanks, and (b) Crack initiation methods require test apparatus designed for dynamic loading of specimens in H{sub 2} gas; and (3) Demonstrated ability to measure fatigue crack growth of pressure vessel steels in high-pressure H{sub 2} gas - (a) Fatigue crack growth data in H{sub 2} required in ASME Article KD-10, and (b) Test apparatus is one of few in U.S. or abroad for measuring fatigue crack growth in >100 MPa H{sub 2} gas.

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