The influence of air humidity on near-threshold fatigue crack growth of 2024-T3 aluminum alloy

Abstract The environmental influence of humid air, dried air and vacuum on the fatigue crack growth behavior of the aluminum alloy 2024-T3 was measured in the very low threshold regime (down to approximately 10−13 m per cycle), using the high frequency (20 000 Hz) technique of ultrasonic resonance fatigue. The crack growth curves, (Δa/ΔNvs. Kmax relationship) obtained in humid air were characterized by a plateau-like regime (regime with reduced slope of crack growth curve) between 10−9 and 10−10 m per cycle and a threshold value of 2.1 MPa m 1 2 . The threshold in vacuum was 3.3. MPa m 1 2 and no plateau-like regime occurred. In dried air, the curve was very close to that in vacuum, when the Kmax values were high enough; at the very lowest Kmax values, however, the curve approached that for humid air, and an almost identical threshold stress intensity value (2.3 MPa m 1 2 ) was found. The fracture morphology reflected the changing cracking mechanisms: ductile fracture with more plastic deformation and some crystallographic and intercrystalline features were observed for “high” Kmax values, whereas less plastic deformation and no crystallographic or intercrystalline features were characteristic of the threshold regime. Hydrogen embrittlement is assumed to be the main mechanism responsible for the observed corrosion fatigue behavior of alloy 2024-T3 in humid air.