Small surface fatigue crack growth tests were performed on a 7075-T6 aluminum alloy under constant amplitude loading in laboratory air, argon and nitrogen gas to investigate the influence of gaseous environments. Crack length(and depth) and crack closure load were measured by unloading elastic compliance method using back face strain gage. Fracture surfaces were observed with scanning electron microscope (SEM).
The growth rates of small surface fatigue crack were well described by the effective stress intensity factor rang accounting for crack closure phenomenon for the various load ratio and the gaseous environments. The crack aspect ratio, a/c was insensitive to the load ratio and the gaseous environments and was found to decreased linearly with a/t.
Fracture surfaces were scarcely influenced by the gaseous environments tested. Fractographical observations show that surface fatigue crack extends through propagation and coalescence of several surface cracks initiated along the initial circular notch front.
It was concluded that small surface fatigue crack growth behavior of 7075-T6 aluminum alloy was insensitive to the three gaseous environments tested.
