Fatigue of 2024-T351 aluminium alloy at different load ratios up to 1010 cycles

Fatigue properties of 2024-T351 aluminium alloy are investigated in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. Endurance tests are performed with ultrasonic equipment at 20 kHz cycling frequency at load ratios of R = −1, R = 0.1 and R = 0.5 up to 1010 cycles. Additional servo-hydraulic tests between 8 and 10 Hz at R = 0.1 show no frequency influence on fatigue lifetimes. Linear lines in double logarithmic S–N plots are used to approximate data. Slope exponents of approximation lines increase with increasing numbers of cycles for all load ratios. Failures above 5 × 109 cycles (R = −1 and R = 0.1) or 1010 cycles (R = 0.5) occur, and no fatigue limit is found. Fatigue cracks leading to failures above 109 cycles are initiated at the surface or slightly below at broken constituent particles or at agglomerations of fractured particles, which are probably Al7Cu2(Fe, Mn). Specimens stressed with more than 1010 cycles at R = −1 without failure show several cracks starting at constituent particles. Maximum crack lengths are 30 μm, which is considerably below grain size.