Experimental observations of environmental contributions to fatigue crack growth

Aluminum 2024-T3 and 7075-T6 center-cracked panels were fatigued in a dry air environment of 10 ppm water vapor. Blocks of 100 cycles having 1, 3, or 5 cycles at 0.02 Hz and the remainder at 20 Hz were employed. Fractographic observations at crack lengths between α = 4mm and α = 12 mm revealed striation formation corresponding to the slow cycles followed by a transient region containing approximately 15 additional striations formed at 20 Hz. These striation bands were then separated by a flat featureless region. For crack lengths less than α = 4 mm extensive striation formation was found, while for crack lengths greater than α = 12 mm only striations corresponding to slow cycles were found. An analysis of the experimental results supports a mechanism of adsorption of water vapor, disassociation to produce free hydrogen, and subsequent surface migration of hydrogen to the crack tip. It is felt that diffusion of hydrogen into the plastic zone ahead of the crack tip does not adequately explain the environmental effect as represented by the presence of striations. A more plausible explanation appears to be the lowering of delta /SUB critical/ required for bond rupture of atoms at the crack-tip surface