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The scope of this paper is to determine the fatigue-crack growth rates in the near-threshold regime for both an aluminum and titanium alloy using the compression precracking constant-amplitude (CPCA) test method on compact specimens. Tests were conducted over a wide range of stress ratios (R=Pmin∕Pmax=0.1 to 0.9). Results were compared with threshold and near-threshold data generated on the same materials using the ASTM E647, “Standard Test Method for Measurement of Fatigue Crack Growth Rates,” load-shedding test procedure. On the 7075-T651 alloy, very little difference was observed in threshold values between the load-shedding and CPCA test methods. In contrast, the titanium alloy (Ti-6Al-4V) showed very large differences between the CPCA and load-shedding test results in the near-threshold and threshold regimes for the low stress ratios. Results under high R conditions (R≥0.7) agreed well between the two-threshold test methods for both materials. On the titanium alloy, the load-shedding test method also produced a specimen width effect on near-threshold behavior, whereas the CPCA test method produced results that were independent of specimen width and produced “steady-state” constant-amplitude data in the near-threshold regime, after the crack had grown several compressive plastic-zone sizes.
fatigue, threshold, crack growth, residual stress, aluminum, titanium, compression loading
Ruschau, John J.
University of Dayton Research Institute, Dayton, OH
Newman, James C.
Mississippi State University, Mississippi State, MS