Influence of loading frequency on the high cycle fatigue properties of AlZnMgCu1.5 aluminium alloy

Fatigue investigations of AlZnMgCu1.5 aluminium alloy have been performed with conventional testing equipment (cyclic frequency 100 Hz) and with ultrasonic equipment (20 kHz). No statistically significant influence of cyclic frequency on lifetimes was found in the investigated regime, i.e. cycles to failure above 105. Different heat treating influenced lifetimes and near threshold crack growth properties, and fatigue properties of AlZnMgCu1.5-T6 were superior to AlZnMgCu1.5-T66 and AlZnMgCu1.5-T64. Fatigue crack propagation in the range of approximately 10−9 m/cycle and below measured at ultrasonic frequency is affected by air humidity, since growth rates are below the mean diffusion distance of hydrogen during one cycle. In ambient air, a minimum growth rate of a propagating crack of approximately 10−10 m/cycle was found, whereas crack propagation rates may be as low as 10−12 m/cycle in a vacuum. Threshold values of AlZnMgCu1.5-T6 in ambient air and in a vacuum are 1.5–1.55 MPa√m and 2.7–2.95 MPa√m, respectively.