Fatigue life scatter in 7xxx series aluminum alloys

Specimens of Al 7175-T7452 and Al 7075-T6511 were fatigue tested under several spectra to obtain estimates of the standard deviation of the log of the fatigue life and to determine if there was bimodality as was found for Al 7050-T7451. The 7175 alloy showed no sign of being bimodal when tested under a spectrum which included large compressive loads. The measured standard deviation in the log of the fatigue life was 0.105, which is greater than the value obtained for the 7050 alloy, but not significantly so. The 7075-T6511 alloy tested under both a spectrum with almost no compressive loads and a filtered spectrum derived from this, showed no indication of being bimodal. The measured standard deviation of the log of the fatigue life was found to be 0.138, which is significantly higher than that obtained from the 7050 alloy, and noticeably higher than the value of 0.10 used in determining safe life limits for aircraft aluminum components. These results clearly indicate that the practice of using a single scatter factor for all aluminum alloys is potentially unconservative.