An Examination of the Fatigue Characteristics of Aluminum Alloy Composite High Pressure Tanks : The Effect of Prestrain on the Fatigue Characteristics of Age-Hardened Aluminum Alloy

Spinning process and pretension treatment are used for Al-Alloy composite tanks. The spinning process develops coarsened grains and the pretension treatment shears precipitation particles. These changes of material properties is considered to affect the fatigue characteristics of age-hardened metals, such as Al-alloy 6061-T6. In this study, the effect of the prestrain and grain size on fatigue crack growth characteristic of age-hardened Al-alloy 6061-T6 was investigated. Fatigue tests were performed with a rotating bending fatigue test machine. The specimen with an artificial hole were prepared to observe mode I fatigue crack. Coarse-grained plain specimens were used for observation of mode II fatigue crack. Mode I fatigue crack growth rates were decelerated a little by the pretension in the stable crack growth region. But in the case of mode II fatigue crack, the crack growth rates of prestrained specimens became about ten times faster than those of the non-prestrained specimens. The observation of the fatigue process showed that the slip bands due to prestrain accelerate the mode II crack growth. The specimens taken from a real high pressure tank showed similar crack growth characteristics.