Chemical and metallurgical aspects of environmentally assisted fatigue crack growth in 7075-T651 aluminum alloy
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INTRODUCTION
THE resistance of high-strength aluminum alloys to crack growth under fatigue loads is known to be influenced by moisture in the surrounding air. t~-SjWater vapor can significantly accelerate fatigue crack growth and thereby speed the failure of structural components. This effect of water vapor is strongly coupled to the frequency of load fluctuations over a critical range of water vapor pressures. Fatigue crack growth can also be influenced by alloy composition and microstructure, the presence of oxygen, temperature, load ratio (R), material thickness (or thickness in relation to plastic zone size), stress intensity range, and the processes used in preparing the alloys. It is recognized that the interactions among these variables complicate the proper interpretation and extrapolation of experimental data and introduce additional uncertainties with respect to damage-tolerant design and failure analysis. Recent fracture mechanics and surface chemistry studies on a 2219-T851 aluminum alloy and on high strength steels tl-7] have shown that the rate of fatigue crack growth is a function of water vapor pressure and of the time available for reaction; that is, of exposure (pressure x time). These results are consistent with the earlier findings by Bradshaw and Wheeler tSJ and Hartman et al. [91 The enhancement of fatigue crack growth was shown to be dependent either on the rate of transport of water vapor to the crack tip or on the surface reaction kinetics, and was attributed to embrittlement by hydrogen produced by the reactions of water vapor with the new crack surfaces. [~-7] With identification of the rate controlling processes and the development of chemical data, tl-7] a quantitative model of corrosion fatigue crack growth was proposed, tl'3'S1 This model provided a formal framework for estimating the freMING GAO, Visiting Scholar, and R. P. WEI, Professor of Mechanics, Department of Mechanical Engineering and Mechanics, are with Lehigh University, Bethlehem, PA 18015. P.S. PAO is with Mechanics of Materials Branch, Code 6384, Naval Research Laboratory, Washington, DC 20375. Manuscript submitted February 17, 1986. METALLURGICALTRANSACTIONS A
quency and pressure dependence of the cycle-dependent component, (da/dN)r of corrosion fatigue crack growth rate in gaseous environments. In the model, (da/dN)c/ is assumed to be proportional to the amount of hydrogen produced by the surface reaction during each fatigue cycle, which is proportional in turn to the crack area produced during the prior loading cycle and to the extent of surface reaction. For a highly reactive material-environment system, such as aluminum alloy-water vapor, the corrosion fatigue crack growth rate is governed by the rate of transport of the deleterious environment to the crack tip. To verify the adequacy of the proposed model for aluminum alloys with different composition and microstructure, and to develop further quantitative understanding of the mechanism for embrittlement, a broad program of study was undertaken. The program w
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