Variable amplitude loading of Al 2024-T351 at different load ratios using ultrasonic equipment

Ultrasonic fatigue tests with variable amplitude are performed for the first time at positive load ratios. The method utilising ultrasonic equipment, a servo-hydraulic or electromechanical load frame and computer control is described. Aluminium alloy 2024-T351 is tested at approximately 20 kHz with variable amplitude in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime at load ratios R = −1, R = 0.1 and R = 0.5. Constant amplitude data are available and are used for Miner damage accumulation calculations. For load ratio R = −1, the mean damage sum decreases from 0.5 to 0.3 when lifetimes increase from the HCF regime to 109 cycles, and increases at higher numbers of cycles. For load ratio R = 0.1, the mean damage sum decreases from 1.1 to 0.2 when lifetimes increase from HCF to VHCF. At load ratio R = 0.5, the mean damage sum is approximately 0.1 in the VHCF regime. Cracks leading to VHCF failures start at broken constituent particles, probably Al7Cu2(Fe, Mn), or at agglomerations of fractured particles. Crack initiation occurs at the surface or less frequently in the interior.