Crack initiation due to low cycle fatigue in X60 pipeline steel tested in high pressure gaseous hydrogen using hollow and conventional specimens

The present work compares strain controlled low cycle fatigue results using hollow (HS) and conventional specimens (CS) for a X60 pipeline steel in ambient air and 5 MPa hydrogen gas, Rε = -1, f = 0.1 Hz. When tested in air, the LCF lifetimes of CS and HS are comparable. When tested in 5 MPa gaseous hydrogen, the lifetimes of HS are higher by a factor of 2 to 4 presumably due to a delayed crack initiation in the fine-grained surface layer induced by the deep-hole drilling technique used for manufacturing. For the HS tested in ambient air, fatigue cracks initiate randomly at the outer and inner gauge surface. For the HS tested in hydrogen gas, fatigue cracks always initiated at the inner bore gauge surface. A finite element analysis of the elastic–plastic response of CS and HS at typical LCF strain amplitudes revealed no significant difference between the specimen types.