FATIGUE-CRACK GROWTH OF BETA-21S TITANIUM-ALLOY UNDER CONSTANT AMPLITUDE AND MINITWIST FLIGHT SPECTRA AT 25-DEGREES-C AND 175-DEGREES-C

beta-21S titanium alloy sheet fatigue crack growth behavior was invest igated at 25 degrees C and 175 degrees C under constant amplitude (R = 0.1 and 0.5) and miniTWIST flight spectra. Based upon nominal Delta K values, constant amplitude fatigue crack growth behavior at 175 degre es C was either similar to (R = -0.1), or slightly better than (R = 0. 5) 25 degrees C. With crack closure taken into account, the fatigue cr ack growth curves at 175 degrees C, plotted as a function of E-eff, we re shifted to the left of the fatigue crack growth curves at 25 degree s C at near threshold values. Under flight spectra conditions, fatigue crack growth life at 175 degrees C was 40 to 90 percent longer than a t 25 degrees C. Flight spectra life calculations using NASA/FLAGRO bas ed upon constant, amplitude fatigue crack growth data, were primarily conservative but in good agreement with experimental data. Fatigue cra ck growth was transgranular with crystalline facets and striations tha t were evident at higher constant amplitude fatigue crack growth rates and with the miniTWIST spectra. Striations were observed to a limited extent at threshold and near threshold conditions at 25 degrees C, bu t not at 175 degrees C. Based upon desirable constant and variable amp litude fatigue crack growth and fatigue/fracture crack morphology, thi s beta-21S sheet alloy appears to be an acceptable material for damage tolerant aerospace situations between 25 degrees C and 175 degrees C.