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ABSTRACTThe near-threshold fatigue crack growth behavior of a 7075-T651 aluminum alloy was studied in laboratory air, vacuum, and an aqueous 3.5% NaCl solution. Results obtained indicate that raising the stress ratio R enhanced the near-threshold fatigue crack growth with a greater crack growth rate da/dN and smaller threshold stress intensity range AK" in laboratory air and an aqueous 3.5% NaCl solution. However, the reverse was observed in vacuum. The near-threshold fatigue crack growth was most sluggish with the smallest da/dN and greatest AK^, values in vacuum, intermediate with an intermediate da/dN and AK^, in the aqueous 3.5% NaCl solution, and fastest with the greatest da/dN and smallest AK," values in laboratory air. In laboratory air and aqueous 3.5% NaCl solution AK", initially decreased with increasing R until a value of 0.5 was reached, and then leveled off or decreased slightly. The AK ft values for these two environments appear to converge at a higher R. On the other hand, in vacuum, the AKth increased linearly with increasing R. In addition, at lower R, a greater resistance to nearthreshold fatigue crack growth was detected in the aqueous 3.5% NaCl solution than in laboratory air. This is attributable to crack closure, induced by corrosion product at the crack tip. 15. SUBJECT TERMS fatigue, near-threshold fatigue crack growth, stress ratio, vacuum, laboratory air, 3.5% NaCl solution, threshold stress intensity range, maximum stress intensity 16.
Abstract:The near-threshold fatigue crack growth behavior of a 7075-T651 aluminum alloy was studied in laboratory air, vacuum, and an aqueous 3.5% NaCl solution. Results obtained indicate that raising the stress ratio R enhanced the near-threshold fatigue crack growth with a greater crack growth rate da/dN and smaller threshold stress intensity range AK t h in laboratory air and an aqueous 3.5% NaCl solution. However, the reverse was observed in vacuum. The near-threshold fatigue crack growth was most sluggish with the smallest da/dN and greatest AK t h values in vacuum, intermediate with an intermediate da/dN and AK t h in the aqueous 3.5% NaCl solution, and fastest with the greatest da/dN and smallest AK t h values in laboratory air. In laboratory air and aqueous 3.5% NaCl solution AK t h initially decreased with increasing R until a value of 0.5 was reached, and then leveled off or decreased slightly. The AK t h values for these two environments appear to converge at a higher R. On the...