Effects of Humidity on the Fatigue Crack Growth Rate in Aluminum Alloy 8090-T8771Thick Plate

摘要

The purpose of this investigation was to determine the effect of relativehumidity on the fatigue crack growth rate in aluminum alloy 8090 thick plate thermomechanically processed to the T8771 condition. High and low humidity fatigue crack growth rate data were generated for three load-ratios, 0.1, 0.33, and 0.7, from the near threshold through intermediate linear region. The test material showed a significant crack closure for load-ratios equaling 0.1 and 0.33. Above the near threshold region the saturated air environment caused an increased fatigue crack growth rate. For a laboratory air environment and an R-ratio equal to 0.1, the crack growth rate leveled off to a plateau, with the growth rate increasing gradually. The slowed growth rate was related to considerable fracture surface roughness; this effect diminished when the R-ratio was 0.33 and disappeared in the absence of crack closure when the R-ratio equals 0.7. When the load-ratio equaled 0.1 in the near threshold region, material tested in high humidity produced a larger threshold stress intensity range for fatigue crack propagation compared to laboratory air. The phenomena was attributable to additional environmental-generated debris bridging the fracture surfaces, which caused increased closure; the effect diminished with increased R-ratio where greater crack opening displacement reduced closure effects. (MM).