Equipment and procedures used in performing tension, fracture mechanics, fatigue, and creep tests on metal specimens in high-pressure hydrogen are described. Important test parameters, in particular, hydrogen purity and methods of achieving high purity are considered. The interpretation and use of data obtained from the various tests in high-pressure hydrogen are discussed.

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made a considerable contribution to our knowledge of the effect of high-pressure hydrogen on sustained-load crack growth in steels. However, we believe that the data on threshold stress intensity for sustained-load crack growth in hydrogen (KH) for the so-called resistant steels and possibly some of the moderately susceptible steels must be applied with caution to pressure vessels. In service, pre-existing cracks in hydrogen pressure vessels will be loaded in the hydrogen during pressurization. In the tests conducted by Loginow and Phelps, precracked specimens were loaded in air and thus the crack surface was covered with oxide prior to exposure to hydrogen. We have performed tests in which precracked specimens were loaded in hydrogen to determine KH-Loginow and Phelps found no crack growth in type 304 stainless steel in 10,000 psi (69 MN/m 2) hydrogen at a stress intensity of 62 ksi \/yn. (68 MN m~3' 2). In tests on a similar stainless steel, type 321, in 5000 psi (34,5 MN m-3/2) hydrogen, we noted crack growth on loading in hydrogen at a stress intensity of 28 ksi Vm. (31 MN m-3/2). The crack growth was small, since the crack soon arrested because of blunting due to rounding of the crack front. Crack growth can be reinitiated by loading to a stress intensity higher than previously, after which the crack grows for only a few minutes and again arrests, but at a stress intensity higher than the previous arrest stress intensity. This process of crack growth initiation and arrest at succeedingly higher stress intensities was continued up to a stress intensity of 111 ksi VST. (122 MN m-3/2). A similar behavior was found for the nickel-base alloy Inconel 625, except that rapid crack arrest resulted from crack branching rather than crack rounding. We suggest that crack growth did not initiate in the steels called resistant steels by Loginow and Phelps because of the presence of the oxide on the crack surface. In light of our results, granting that we tested different materials, did Loginow and Phelps see any correlation between KH and the stress intensity at initial loading for the moderately susceptible steels? It should be noted that as a crack grows through a pressure vessel wall, the stress intensity is continually increasing. It is possible for materials in which cracks tend to blunt or branch in hydrogen that although cracks will arrest at a relatively high stress intensity in a decreasing stress intensity test, a crack may begin to grow at a lower stress intensity when loaded in hydrogen, and the crack may continue to grow when the stress intensity is continually increasing. Tests of this nature are required.