Mechanisms of Slow Fatigue Crack Growth in High Strength Aluminum Alloys: Role of Microstructure and Environment |
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Authors: | S Suresh A K Vasudévan P E Bretz |
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Affiliation: | (1) Division of Engineering, Brown University, 02912 Providence, RI;(2) Alloy Technology Division, Aluminum Company of America, 15069 Alcoa Center, PA |
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Abstract: | The role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated
in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios.
Threshold stress intensity range, ΔK0, values were found to decrease monotonically with increasing load ratio for all three heat treatments fatigue tested in 95
pct relative humidity air, with ΔK
0 decreasing at all load ratios with increased extent of aging. Comparison of the near-threshold fatigue behavior obtained
in humid air with the data forvacuo, however, showed that the presence of moisture leads to a larger reduction in ΔK0 for the underaged microstructure than the overaged condition, at all load ratios. An examination of the nature of crack morphology
and scanning Auger/SIMS analyses of near-threshold fracture surfaces revealed that although the crack path in the underaged
structure was highly serrated and nonlinear, crack face oxidation products were much thicker in the overaged condition. The
apparent differences in slow fatigue crack growth resistance of the three aging conditions are ascribed to a complex interaction
among three mechanisms: the embrittling effect of moisture resulting in conventional corrosion fatigue processes, the role
of microstructure and slip mode in inducing crack deflection, and crack closure arising from a combination of environmental
and microstructural contributions. |
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