Consideration on hydrogen condition in fatigue crack growth test in hydrogen gas environment: Focusing on concentrated hydrogen zone near crack tip

In this study, in order to consider the validity of saving hydrogen pre-charge for the fatigue crack growth tests on the components exposed to hydrogen gas, two types of fatigue crack growth tests on pre-strained 0.13% carbon steel specimens were carried out; (1whether hydrogen in the plastic zone at a crack tip have the ability to accelerate the fatigue crack growth rate (FCGR (2whether the hydrogen charged specimen which has almost the same FCGR of uncharged specimen in the case of (1exists. Fatigue crack growth rate after alteration of environment from hydrogen to nitrogen kept higher level than that thoroughly in nitrogen gas. Fracture surfaces in both tests revealed almost ductile morphology. Highly hydrogen charged specimen revealed high FCGR. FCGR of lower content specimen closed to that of uncharged specimen in the test (1. And a high hydrogen content specimen showed a high area ratio of quasi-cleavage fracture surface but a low hydrogen content specimen showed a higher area ratio of ductile fracture surface. From the above results, it could be considered that hydrogen in the plastic zone at a crack tip has the ability to accelerate FCGR, and a hydrogen charged condition could exist, which has the ability to accelerate FCGR to almost the same degree but its hydrogen content is higher than the former. Therefore, it could be considered that saving the hydrogen pre-charge process is valid in fatigue crack test in hydrogen within some limit of testing condition.