First : When a crack exceeds a certain size, the maximum stress intensity factor () is used to explain the occurrence condition of brittle crack, and the limit condition of hydrogen assisted and stress corrosion crack propagation. The fatigue crack can be explained through the stress intensity factor range (). However, as the stress intensity factor deals with only linear elastic bodies, it is necessary for the nonlinear range of crack tips to be small enough. It is known that this condition is not satisfied in the case of the micro-crack problem, which became smaller than the threshold value of the long crack. This study evaluates the threshold stress intensity factor and fatigue limit of the input/output piping material STS316L for hydrogen storage tanks using the formula proposed by the Ando et al. to analyze the micro-crack problem uniformly and using the formula proposed by Tange et al.
Second : Linear fracture mechanics can quantitatively deal with the crack problem of metals and ceramics. In particular, the fracture toughness () and the threshold stress intensity factor () of fatigue crack propagation depend on the crack size. Linear fracture mechanics assumes that the plastic zone or nonlinear zone occurring at the crack tip is very small compared to the crack size. However, micro cracks do not satisfy this condition. This study evaluated the threshold stress intensity factor () and the fatigue limit () of short surface crack size due to fatigue load using the equation that considers the stress ratio and the plastic behavior of peculiar fatigue proposed by Ando et al. In addition, the through crack of the infinite plate was also evaluated for comparison.
Third : The threshold stress intensity factor and fatigue limit of hydrogen storage tank piping material (STS316L) were determined, and the fatigue limit and threshold stress intensity factor of micro crack were evaluated. In addition, the crack size evaluated at the maximum operating pressure (87.5 MPa) of the hydrogen storage tank, and the safety of the STS316L piping was confirmed.