Hydrogen gas container is one of the critical components for fuel cell electric vehicle (FCEV) , which is expected for CO2-free personal transportation. In order to choose an appropriate material for its metal boss or liner, crack growth resistance should be evaluated for various aspects such as stress corrosion cracking (SCC) and fatigue crack growth (FCG) in any environments for the purpose of commercial vehicle use. In the present study, characteristics of SCC and FCG in humid air and in NaCl solution were obtained for aluminum alloys of A6061, A6066 and A6351 with attention to the testing methods. Threshold K value for SCC,KISCC , was determined by two types of tests under K-increasing and D-constant (K-decreasing) conditions, where Kand Dare stress intensity factor and specimen displacement, respectively. In the former test, partial unloading was conducted at every 1000 s in order to monitor the crack growth by unloading compliance method. Crack growth rate of SCC, da⁄dt, is enhanced by the partial unloading in the K-increasing test, while measurable da⁄dt is much smaller in the D-constant test. KISCC of A6351 is much lower than those of A6061 and A6066 in humid air, while it was not able to determine in NaCl solution due to the anodic dissolution for all the materials. Grain size indicates minor influence on SCC tested by the K-increasing test. Conservative FCG characteristics are obtained by Kmax-constant test, where the maximum stress intensity factor,Kmax , should be less than the KISCC in order to avoid the SCC contribution. Crack growth rate of FCG, da⁄dN, is similar in humid and laboratory air environments, while it is higher in NaCl solution. Larger grainsized materials show lower da⁄dN both in humid air and in NaCl solution.
