Abstract
The resistance to fatigue crack growth in a high-humidity (relative humidity RH90%–25°C) air was evaluated for 7000 series aluminum alloys with a higher strength above that of 6061-T6. The alloys, 7N01 and 7003 with temper in T6, T5 or T7, respectively, showed a medium resistance between those of 7075-T6 and 6061-T6. The fatigue crack growth rate da/dN in the humid air increased compared with that in dry nitrogen gas, where 7075-T6 exhibited the highest values of da/dN and the rate-ratio of 2 at ΔK=12 [MPa√m]. Despite the enhanced crack growth due to the reduction in fracture surface roughness in the humid air, the decrease in frequency from 10 Hz to 0.1 Hz gave rise to a decrease in da/dN for 7075-T6, which is attributed to the microscopic crack deflection induced by rough zigzag facets formed on a transgranular crack surface. It is presumed that these rough facets result from the concentrated or localized slip at crack tip enhanced by hydrogen effects.
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