In order to investigate the effect of hydrogen on the fatigue strength and the crack propagation behavior of an austenitic stainless steel JIS SUS304, bending fatigue tests were carried out on specimens with small twin holes in a low pressure hydrogen gas. Fatigue process was in-situ observed using a scanning laser microscope. Non-propagation phenomena of a crack from holes are observed not only in air also in hydrogen. Fatigue limit in hydrogen gas is rather higher than that in air and fatigue crack growth rate near fatigue limit in hydrogen gas is lower than that in argon gas and in air in spite that hydrogen gas enhance the crack growth rate in high stress intensity range. These phenomena are discussed through crack closure.
