Threshold for Fatigue Crack Propagation and the Effects of Load Ratio and Frequency

The threshold in the alternating range of stress intensity factor for fatigue crack growth is investigated for 2024-T3 aluminum. Testing techniques provided displacement control at frequencies up to 1000 Hz. The threshold is seen to decrease with increasing frequency at a fixed load ratio, dropping by nearly a factor of 2 from 342 to 832 Hz. The threshold is also observed to decrease with increasing load ratio in such a way that is tentatively explained by considerations of crack closure. Some confirmation of this is obtained from direct strain gage measurement of the closure stress intensity, Kcℓ, which agrees with that predicted from trends in the threshold data with respect to load ratio.