ENVIRONMENTAL EFFECTS ON FATIGUE CRACK GROWTH IN 7075 ALUMINUM ALLOY

The fatigue behavior of aluminum alloys is greatly influenced by testing environment. In this paper, the role of dry air and highly purified moisture on fatigue crack growth behavior of 7075 Al alloy was investigated. Fatigue crack growth rates were measured under various partial pressures of dry air and water vapor, and were compared with a baseline obtained in ultra high vacuum (UHV) (total pressure of 10(-9) torr). Standard compact tension specimens along the L-T orientation were used, and testing was performed under load ratios of R = 0.8. The microstructure and morphology of the fracture surfaces were examined by scanning electron microscopy (SEM) and correlated with the crack growth behavior. It was observed that a sticking phenomenon between crack faces took place in UHV and dry-air, but not in moisture. The moisture is believed to lubricate the crack surfaces. The mechanisms for sticking are described in detail.