Slow fatigue crack growth in 2024-T3 and Ti-6Al-4V at low and ultrasonic frequency

Abstract

Fatigue crack growth in 2024-T3 has been studied in ambient air and in vacuum at load ratios R = -1, R = 0.05 and R = 0.5 using ultrasonic equipment (cycling frequency 20 kHz) and servo-hydraulic equipment (20 Hz). In vacuum, no strain rate influences were found and similar growth rates and threshold stress intensities were measured at both frequencies. In ambient air, threshold stress intensities were similar at 20 Hz and 20 kHz and were 53 - 62 % of the respective values measured in vacuum. Above threshold, fatigue crack growth rates at ultrasonic frequency are slower (at R = -1) or similar (at R = 0.5) to growth rates at 20 Hz. Ultrasonic fracture mechanics tests in Ti-6Al-4V at load ratios R = 0.1, R = 0.5 and R = 0.8 in ambient air delivered threshold values similar to cycling frequency 50 Hz, whereas growth rates above threshold are approximately a factor of 3 higher at 20 kHz. The compressive part of a load cycle under fully reversed loading condition causes additional fatigue damage, and the maximum stress intensity factor at threshold is lower at R =-1 than at R = 0.05 or R = 0.1.