6061アルミニウム合金の疲労特性に及ぼす試験温度と試験環境の影響

Abstract

Hydrogen storage containers in hydrogen stations for fuel-cell vehicles undergo cyclic pressure change. Therefore, pressure cycle tests must be conducted up to 106 cycles at maximum. Reports have presented different cycles to failure, because different liquid pressure media have been used and temperature rise during the tests has taken place without control. In this study, a tensile fatigue test of a 6061 aluminum alloy with T6-tempered is carried out, which is an actual liner material for hydrogen storage containers. Fatigue tests are carried out under stress cycles in sine wave of 1 Hz with stress ratio, R, of 0.1 under the environments of tap water, de-ionized water and 50% ethylene glycol aqua solution at 25 and 75°C. The original surface adjacent to the fracture point as well as the fracture surface is observed in detail with SEM. Results elucidated the effects of the test temperature and environment on the fatigue behavior of the alloy. The number of cycles to failure increased with decreasing test temperature and decreasing stress amplitude. Moreover, the 50% ethylene glycol aqua solution was associated with more cycle to failure than de-ionized water and tap water. On the original surface of the specimens tested in de-ionized water and tap water, corrosion associated with an Al–Fe–Si secondary phase particle was observed at the crack initiation point, while no such corrosion was observed on the specimen tested in the ethylene glycol aqua solution. We therefore inferred that corrosion enhanced crack initiation in de-ionized or tap water. The ethylene glycol aqua solution with a concentration not less than 50% was considered to be desirable in the pressure cycle test of high-pressure hydrogen storage containers because of no influence of corrosion, and it was necessary to carry out a temperature control to avoid temperature rising.