Publications

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Following the ASME codes, the design of pipelines and pressure vessels for transportation or storage of high-pressure hydrogen gas requires measurements of fatigue crack growth rates at design pressure. However, performing tests in high pressure hydrogen gas can be very costly as only a few laboratories have the unique capabilities. Recently, Code Case 2938 was accepted in ASME Boiler and Pressure Vessel Code (BPVC) VIII-3 allowing for design curves to be used in lieu of performing fatigue crack growth rate (da/dN vs. ?K) and fracture threshold (KIH) testing in hydrogen gas. The design curves were based on data generated at 100 MPa H2 on SA-372 and SA-723 grade steels; however, the data used to generate the design curves are limited to measurements of ?K values greater than 6 MPa m1/2. The design curves can be extrapolated to lower ?K (<6 MPa m1/2), but the threshold stress intensity factor (?Kth) has not been measured in hydrogen gas. In this work, decreasing ?K tests were performed at select hydrogen pressures to explore threshold (?Kth) for ferritic-based structural steels (e.g. pipelines and pressure vessels). The results were compared to decreasing ?K tests in air, showing that the fatigue crack growth rates in hydrogen gas appear to yield similar or even slightly lower da/dN values compared to the curves in air at low ?K values when tests were performed at stress ratios of 0.5 and 0.7. Correction for crack closure was implemented, which resulted in better agreement with the design curves and provide an upper bound throughout the entire ?K range, even as the crack growth rates approach ?Kth. This work gives further evidence of the utility of the design curves described in Code Case 2938 of the ASME BPVC VIII-3 for construction of high pressure hydrogen vessels.

Limited fatigue data exists for small-volume welded austenitic stainless steel components typically employed in hydrogen infrastructure due to the difficulty of testing these components with conventional specimen designs. To assess the fatigue performance of orbital tube welds of austenitic stainless steels, a hole-drilled tubular specimen was designed to produce a stress concentration in the center of the orbital weld. Fatigue life testing was performed on welded and non-welded 316L stainless steel hole-drilled tubular specimens, and the effects of hydrogen were evaluated by testing specimens with no added hydrogen and with internal hydrogen introduced through gaseous precharging. When accounting for the differences in flow stress caused by microstructural variations and the presence of internal hydrogen, the total fatigue life and fatigue crack initiation life of the welded and non-welded tubes were comparable and were reduced by internal hydrogen. In addition, the fatigue life results produced with the hole drilled tubular specimens were consistent with fatigue life data from circumferentially notched stainless steel specimens that have a similar elastic stress concentration factor. To better understand the mechanics of this specimen geometry, mechanics modeling was performed to compare the stress and strain distributions that develop at the stress concentration in the hole-drilled tubular and circumferentially notched specimens during fatigue cycling.

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Tensile properties, fatigue crack initiation, fatigue crack growth rate, and fatigue life are evaluated in 304L austenitic stainless steel fabricated by directed energy deposition (DED). Large lack of fusion (LoF) defects (often >1 mm in length) significantly reduce ultimate tensile strength and ductility, as well as accelerate fatigue crack initiation and reduce fatigue life. In comparison, small spherical defects (<100 μm in diameter) have less effect on tensile and fatigue properties. Fatigue crack growth rate is less severely affected by defects than other properties, showing only local acceleration in the proximity of LoF defects. Therefore, shorter fatigue life is attributed to the role of LoF defects on facilitating fatigue crack initiation and to a lesser extent fatigue crack propagation. Additionally, the fatigue life can be normalized for defects by considering their effect on ultimate tensile strength, suggesting that in the limit of low defect population, the fatigue strength of additively manufactured stainless steel is similar to conventional wrought materials.

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