Compression precracking methods to generate near-threshold fatigue-crack-growth-rate data

This paper summarizes some of the fatigue-crack-growth-rate data generated in the threshold and near-threshold regimes on two aluminum alloys (7075-T651, 7075-T7351), a titanium alloy (Ti–6Al–4V β-STOA), a high-strength 4340 steel and a nickel-based superalloy (Inconel-718) using compression precracking constant-amplitude (CPCA), compression precracking load-reduction (CPLR), and the ASTM E-647 load-reduction (LR) test methods. Tests were conducted over a range in stress ratios (R = 0.1, 0.4 and 0.7) on compact specimens. One of the aluminum alloys (T651) and the 4340 steel showed very little difference between the methods; however, the other three materials showed significant differences with the compression precracking test methods giving lower thresholds and faster crack-growth rates than the load-reduction test method. Materials that have shown significant differences exhibited either rough crack-surface profiles (7075-T7351, Ti–6Al–4V β-STOA) or produced fretting debris along the crack surfaces in the threshold and near-threshold regimes (Inconel-718).