Abstract
Gigacycle fatigue properties of a storage cylinder in hydrogen station, which had been subjected to 4 years verification tests, was examined in both longitudinal and circumferential directions. The storage cylinder was made of SCM435 steel. Fatigue tests were carried out on hydrogen-charged and uncharged specimens by using ultrasonic and servo-hydraulic fatigue testings. In the uncharged specimens, only surface fracture occuned 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 slender MnS inclusions in the longitudinal and circumferential directions, respectively. The fatigue strength of the hydrogen-charged specimens was lower in the circumferential direction than in the longitudinal direction, while the difference was small.
