Gigacycle fatigue properties of a storage cylinder in hydrogen station, which had been subjected to several years verification tests, was examined in both longitudinal and circumferential directions. The storage cylinder was made of SNCM439 steel. Fatigue tests by using ultrasonic and servo-hydraulic fatigue testings were carried out on hydrogen-charged and uncharged specimens at stress ratio of R=-1. In the uncharged specimens, surface fracture was dominant in both directions. On the other hand, the hydrogen-charged specimens showed internal fracture and degradation of fatigue strength. The crack initiation sites of internal fracture were different in the two directions: globular Al_2O_3 and cluster of Al_2O_3 inclusions in the longitudinal and circumferential directions, respectively. The fatigue strength of the hydrogen-charged specimens was much lower in the circumferential direction than in the longitudinal direction.
