ENVIRONMENTAL FATIGUE CRACK PROPAGATION OF ALUMINUM ALLOYS AT LOW STRESS INTENSITY LEVELS.
BOEING CO RENTON WASH COMMERCIAL AIRPLANE DIV
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The environmental fatigue crack propagation in 2024-T3, 7075-T6, and 7178-T6 was studied at low levels of cyclic amplitude of stress intensity, delta K. Both wedge force loading and remote loading techniques were employed to achieve the desired delta K levels, and preliminary experiments were designed to test their compatibility. Testing was carried out in humid air, distilled water, and 3.5 sodium chloride solution, and the observed crack growth rates compared with those in desiccated air. Later studies were also conducted in an inert reference environment with a total water content of less than 2 ppm. When the data are plotted as log delta K versus log d2adN, alloy 2024-T3 exhibits a marked slope transition, alloy 7075-T6 a slight slope transition, and alloy 7178-T6 a rectilinear behavior throughout the whole range of delta K studied. The basic shape of these curves is discussed in terms of state-of-stress conditions at the crack tip, frequency effects, environmental effects, strain rate sensitivity, and metallurgical structure. An attempt is also made to correlate the rate of fatigue crack propagation in a particular environment and at a particular delta K level with the fracture topography. Author
- Metallurgy and Metallography