A COMPARISON OF TEST METHODS FOR THE DETERMINATION OF FATIGUE CRACK-GROWTH RATE THRESHOLD IN TITANIUM AT ELEVATED-TEMPERATURE

The near-threshold fatigue crack growth rate behavior of Ti-8A1-1Mo-1V at elevated temperature was investigated using three different test methods and two different specimen geometries. The first two methods were performed on compact tension specimens where crack closure effects were accounted for by monitoring crack opening load using automated compliance measurements. The third method was performed on a cantilever bending specimen that was designed to minimize closure effects. Crack opening load was not monitored for the bend specimens. All tests were performed at 500°F (260°C) in a laboratory air environment. It was observed that the fatigue crack growth rate data for each of the first two methods collapsed to one curve when crack closure was incorporated into the analysis, with a unique value of threshold stress intensity range, ΔKth,eff. However, the value of threshold stress intensity range, ΔKth, determined from the third method differed slightly from the values obtained from the other two procedures. This suggests that a unique value of ΔKth may not exist for this material under these test conditions even when crack closure is accounted for, and that it may be a function of the test method and the specimen geometry that are used in the investigation. © 1990.