Crack growth in IMI 829 at 550°C under combined high and low cycle fatigue

Abstract

Fatigue crack growth rates have been determined for the near-alpha titanium alloy IMI 829 at an elevated temperature of 550°C using corner notched test pieces. Combined low cycle fatigue (LCF) and high cycle fatigue (HCF) tests simulate stresses induced by the start-to-stop operation, and the in-flight vibrations, of aero-engines. Growth rates from these tests were compared to those from purely low cycle fatigue in order to determine the onset of HCF crack growth. This onset was safely predicted by the fatigue threshold value determined by the load-shed method for HCF cycles having a stress ratio of 0.90. The unsafe prediction for a stress ratio of 0.82 is connected with a HCF/LCF load interaction which is apparent in the vicinity of the onset condition, particularly in tests employing the higher ratio of HCF to LCF cycles. The introduction of superimposed high cycle fatigue was reflected in the appearance of the fatigue fracture surface.