Chemically short-crack behavior of the 7075-T6 aluminum alloy

Commercial and military aircraft are exposed to deleterious environments (such as salt spray) that can enhance the fatigue-crack-growth (FCG) rates in structural components. This phenomenon, generically termed corrosion fatigue (CF) can have a large effect on their fatigue lives. CFCG behavior may be separated into two regimes: a chemically long-crack regime and a chemically short-crack regime. FCG rates in the chemically long-crack regime for the 7075-T6 aluminum alloy are approximately one order of magnitude greater than those in a dehumidified environment and exhibit growth rate similitude under a constant crack driving force ($D$K). FCG rates in the chemically short-crack regime exhibit a breakdown in growth rate similitude at crack lengths shorter than 7 mm. The growth rates at 0.5 mm are 2 times greater than those in the long-crack regime and decay with increasing crack length to the long-crack rates at approximately 7 mm. The extent of the chemically short-crack behavior in the 7075-T6 aluminum alloy depends on $D$K, crack length and dissolved oxygen concentration in the aqueous solution.; Mathematical modeling of the mass transport of dissolved oxygen to the crack tip region shows a good correlation between the chemically short-crack behavior and the amount of dissolved oxygen at the crack tip. The short-crack behavior is not observed in a deaerated solution.; The enhanced growth rates are caused by hydrogen embrittlement. Fracture in dehumidified air occurs along the $100$ fracture plane while cracking in the chemically short and long-crack regimes occur on $100$ and $110$ (flat brittle type fracture) planes with the fractional area of the $110$ fracture decreasing with increasing crack length in the chemically short-crack regime. Predicted and actual FCG rates correlate well showing that the enhancement of growth rates in the deleterious environment depends on the amount of $110$ fracture.; The chemically short-crack behavior may reduce the FCG life of a structure by one-half depending on the local stresses, environment and the FCG properties of the material. A one-half reduction in the FCG life is substantial when considering life management of aging aircraft and the design of structures with a 20 to 30 year design life.