Effect of Humidity on Fatigue Strength of Notched Age-Hardened Al Alloy

In order to investigate the effect of humidity on fatigue strength of an extruded and age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out using plain specimens and circumferential notched ones with notch radii of 1.0, 0.1 and 0.05mm in environments of relative humidity 25% and 85%. Fatigue strength was decreased by high humidity, except for notched specimen with notch radius of 0.1mm. Fatigue limit for notched specimen with notch radius of 0.05mm was very high in low humidity. Notch sensitivity for crack initiation was low and the one for crack propagation was reversely high in high humidity. Branch points were 0.05-0.1mm in low humidity and about 0.3mm in high one, respectively. Crack in sharply notched specimen tended to propagate in shear mode in low humidity with decreases in stress level, though all of crack propagations were in tensile mode in high humidity. This was a main reason for high fatigue limit in notched specimen with notch radius of 0.05mm in low humidity.