Volume 5, Issue 7 (July 2008)
Compression Precracking to Generate Near Threshold Fatigue Crack Growth Rates in an Aluminum and Titanium Alloy
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.