Hydrogen gas accelerates fatigue crack growth and reduces fracture toughness in ferritic structural materials such as pipelines and pressure vessels. The extent to which the crack growth rates are accelerated depends upon environment, mechanical loading conditions, and material. In this work, the effects of loading conditions and environment, specifically oxygen impurities, are examined on an X100 pipeline steel in high pressure hydrogen gas. Fatigue crack growth rates were measured in a gas mixture consisting of nominally 100 ppm O2 in a balance of H2 gas to evaluate the effects of pressure and load ratio (R-ratio) on the manifestation of hydrogen-accelerated fatigue crack growth (HA-FCG). Tests were performed at 21 MPa, 2.1 MPa, and 1.4 MPa and at load ratios of 0.5 and 0.1. The onset of HA-FCG was observed to be dependent on both absolute pressure and load ratio and it will be shown that a critical combination can result in complete mitigation of HA-FCG over the stress intensity factor range (ΔK) examined. Tests were predominantly performed at 10 Hz; however, a single test was performed at 1 Hz which exhibited negligible HA-FCG compared to a test at 10 Hz which did exhibit HA-FCG. Rising load fracture toughness tests were conducted via constant displacement rates to generate J-R curves in both pure H2 and 100 ppm O2 mixed gas. At similar absolute pressures, fracture toughness was measured to be greater in the 100 ppm O2 mixed gas compared to the pure H2. Hydrogen-assisted fracture was completely alleviated at pressures below 2.1 MPa in the 100 ppm O2 mixed gas, in which fracture toughness values were consistent with tests in air.

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