Fatigue crack growth and thresholds at ultrasonic frequencies

The existence of a threshold of fatigue crack growth is treated for the aluminium alloy 7075-OA as well as Ti–6Al–4V. Results are reported on the influence of environment (vacuum and ambient air) and load ratio (R = −1 to +0.8) on fatigue crack growth in the near-threshold regime. The main results are: in vacuum, crack growth still takes place at lower rates than 10−10 m/cycle, and thus thresholds – if at all – are below approximately 5 × 10−12 m/cycle. No frequency influence on crack propagation is present for loading frequencies of 20 Hz and 20 kHz. In ambient air environment, thresholds are identical at 20 kHz and 20 Hz cycling frequency, but crack growth is slower at ultrasonic frequency in humid air than at 20 Hz. Time governing processes, like surface diffusion of water vapour to the crack tip, water vapour adsorption and formation of a mono-layer at the crack tip as well as diffusion of hydrogen in front of the crack tip are discussed. The influence of load ratio is shown and discussed.