HIGH-TEMPERATURE LOW-CYCLE FATIGUE AND LIFETIME PREDICTION OF TI-24AL-11NB ALLOY

The influence of hold time on low-cycle fatigue (LCF) of Ti-24Al-11Nb was studied at 650 degrees C. At 0.167 Hz, the alloy exhibits cyclic h ardening at all strain levels studied and obeys the well-known Manson- Coffin behavior. A 100-second hold at peak tensile or compressive stra in at +/-0.6 pet strain has no observable effect on cycles to failure. For hold times at +/-0.5 pet strain, however, the fatigue lives are n early halved and specimens show secondary cracking normal to the stres s axis. The increase in inelastic strain as a result of hold time appe ars to be primarily responsible for the observed loss in fatigue life. A linear life fraction model, which considers both fatigue and creep damage, is found to provide good correlation of measured lives with pr edictions. For the range of test conditions employed, the total and th e tensile hysteretic energy per unit volume, absorbed until fracture, remain nearly constant. The tensile hysteretic energy appears to be a more useful measure of fatigue damage for life prediction.