Environmentally Influenced Near-Threshold Fatigue Crack Growth in 7075- T651 Aluminum Alloy

摘要

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 deltaK(th) 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 deltaK(th) values in vacuum, intermediate with an intermediate da/ dN and deltaK(th) in the aqueous 3.5% NaCl solution, and fastest with the greatest da/dN and smallest deltaK(th) values in laboratory air. In laboratory air and aqueous 3.5% NaCl solution, deltaK(th) initially decreased with increasing R until a value of 0.5 was reached, and then leveled off or decreased slightly. The detlaK(th) values for these two environments appear to converge at a higher R. On the other hand, in vacuum, the deltaK(th) increased linearly with increasing R. In addition, at lower R, a greater resistance to near-threshold 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.