Near threshold fatigue crack growth at positive load ratio in aluminium alloys at low and ultrasonic frequency: influences of strain rate, slip behaviour and air humidity

Fatigue crack growth near threshold has been studied in 2024-T3 and in over-aged 7075 (7075-OA) using servo-hydraulic equipment (cycling frequency 20 Hz) and ultrasonic equipment (20 kHz). Experiments have been performed at load ratio R=0.05 and R=0.5 in ambient air and in vacuum. Below 10−9 m/cycle, fatigue crack growth in vacuum is not influenced by the cycling frequency and no strain rate effects are found. Air humidity accelerates near threshold fatigue crack growth, and threshold stress intensities at 10−10 m/cycle in ambient air are approximately 50–60% of those measured in vacuum. Similar threshold stress intensities at 20 Hz and 20 kHz, however, higher crack growth rates at the lower frequency at stress intensities above threshold were found in ambient air, where the frequency influence was most pronounced for fully reversed loading. Near threshold 2024-T3 shows planar slip behaviour and crystallographic (stage I-like) fatigue crack growth in vacuum, whereas stage II fatigue crack growth is found in ambient air. 7075-OA shows homogeneous slip properties and stage II fatigue crack growth in both environments. No apparent influences of load ratio and cycling frequency on fracture surfaces produced at very low growth rates are visible.