The purpose of this study is to clarify the influence of the maximum pressure on the lifetimes of compressed hydrogen tanks during ambient temperature pressure cycling tests. We varied the maximum pressure from 100% to 200% of designed filling pressure (FP) in five levels.
Type 3 (Fully wrapped composite tank with metal liner): The tank's lifetimes, i.e. the numbers of cycles up to Leak Before Break (LBB), decreased with increasing maximum pressure. We observed that the internal surface of the liners had linear flaws resulting from the manufacturing process. Cracks causing LBB seemed to initiate from these flaws. Striation marks clearly appeared at the fracture surface of LBB cracks when the maximum pressure exceeded 125% of FP. Therefore, we suggest that LBB cracks were caused by a similar process of crack propagation in this range.
In spite of the maximum pressure changes (100% to 200% of FP), tank strain was proportional to pressure at all times. Hence the strain / pressure ratio was constant in this pressure range where circumferential tensile stresses generated in the aluminum alloy liners exceeded their proof stress. In fact, however, residual compression stress of the liner induced by autofrettage decreased the tensile stress. Thus the tank didn't expand beyond the elastic region under these test conditions.
Therefore, the metal liner fatigue is a determining factor in the lifetime of a Type 3 tank.
Type 4 Fully wrapped composite tanks with non-metallic liners - The lifetimes of tanks were very long, but in some cases the tanks' dome areas around the end boss ruptured without LBB.