Role of crack closure mechanisms on fatigue crack growth of Ti-62222 under constant-amplitude and transient loading at-54, 25, and 175°C

Fatigue crack growth behavior of Ti-62222 in sheet form was investigated at three temperatures under various load histories with a specific emphasis on crock opening measurements and mechanisms. Constant-amplitude R = 0.1 near-threshold results showed that based upon applied Delta K, fatigue crack growth rates were lower at the two temperature extremes in comparison with those at room temperature. Based upon Delta K-eff, a moderate temperature effect was observed. The various shifts in the Delta K-eff curves were a result of different controlling crack closure mechanisms at each temperature. Single tensile overloads at 2.5 x K-max produced a delay in fatigue crack growth and was most significant at -54 degrees C. Remote crack opening measurements showed no appreciable change during the delay period due to the small region along the crack front that experienced crack closure. Near crack tip opening measured for various combinations of overloads, underloads, and constant Delta K at 25 degrees C revealed a general trend for higher near tip opening following a tensile overload. However, a sufficient number of measurements were inconsistent and thus only minimal confidence could be placed with the method. Scanning electron microscopy was used to evaluate surface fatigue crack growth profiles and fatigue crack growth fracture surface morphology.