Crack growth characteristics were investigated under static and cyclic loading conditions for aluminum alloys in humid and dry air environments in order to clarify the dominating crack growth mechanisms, i.e. stress corrosion cracking (SCC) and fatigue crack growth (FCG). The frequencies of the cyclic loading varied between 0.02 Hz and 20 kHz using three types of testing machine including ultrasonic fatigue. Crack growth mechanisms were discussed based on the traditional model proposed by McEvily and Wei. The results revealed that the crack growth mechanisms were dominated by SCC at 0.02Hz, while they were dominated by FCG at 20Hz and 20kHz.

Stress corrosion cracking (SCC) tests were performed on CT specimens of Aluminum alloys in humid air in order to clarify the effect of small load fluctuation on the threshold conditions of SCC crack growth. The results were compared with fatigue crack growth (FCG) characteristics obtained at 20 Hz and 20 kHz. Crack growth mechanisms were discussed based on the superposition model proposed by McEvily and Wei.

The Ultrasonic fatigue tests were performed at 20 kHz for three types of aluminum alloys under high stress ratio in order to clarify the effect of humid environment on the characteristics of fatigue crack growth. The test results showed that cracks grew at a very slow growth rate under a low stress intensity factor range regardless of the materials and the stress ratio. Based on the observation and elemental analysis by SEM, it was found that the fracture surface, where crack grew at a very slow growth rate, covered with aluminum oxide. Observations on the specimen surface showed that some powders were developed around the crack during the testing. These results suggest that the ultra slow crack growth behavior in humid environment occur by peeling off the oxide film near the crack tip due to the cyclic loads.

Ultrasonic fatigue tests at 20 kHz were performed for precracked Aluminum alloy specimens under high stress ratio in dry and humid air conditions in order to clarify the effect of environment on the threshold conditions of fatigue crack growth. The results were compared with those obtained by fatigue tests at 20 and 0.02 Hz as well as SCC tests. Crack growth mechanisms were discussed based on the superposition model proposed by McEvily and Wei.

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, it is required to evaluate the characteristics of stress corrosion cracking(SGCC) in humid air environment, which is expected in the hydrogen charging process. In the present study, the SCC evaluation method was investigated for the candidate aluminum alloys of Al-Mg-Si series. Two types of test method were applied for the materials under load-increasing and displacement-constant conditions and revealed the effect of Cu content on the SCC characteristics.