Abstract:
Strain-controlled fatigue tests were performed on 6061-T6 and 7075-T651 aluminum alloys as well as additional specimens of the same materials after exposure to a temperature of 538ºC for 1 hour and cooling to room temperature. The fatigue behavior of all four materials was characterized using traditional strain-life analysis. The fatigue data was found to show no appreciable effect of the heat exposure in low cycle fatigue, and a decrease in fatigue life in high cycle fatigue in both 6061 and 7075. In addition, scanning electron microscopy was employed to determine that crack initiation occurred at Fe-rich particles and subsequence analysis was used to measure crack growth rates. The fatigue cracks were found to initiate at cracked intermetallic particles in the peak-aged alloys and at de-bonded particles in the heat exposed alloys. Using standard metallographic techniques, microstructural characterization including grain size, intermetallic particle size, and nearest neighbor distance were quantified. As such, this characterization data was implemented into a microstructure-sensitive fatigue model resulting in a unique set of modeling parameters to capture the experimental results of both the peak-aged and thermally-affected 6061 aluminum alloy and 7075 aluminum alloy data.
