Prediction of titanium alloy Ti-6A1-4V lifetime under consecutive shock-wave and gigacycle fatigue loads

Abstract The influence of consecutive shock-wave and gigacycle fatigue loads on the lifetime of titanium alloy VT6 (Ti-6A1-4V) was investigated. An experimental methodology for estimating the ultra-high cycle lifetime was developed with reference to accidental high-speed collision of solid particles with fan blades and subsequent fatigue failure under the flight cycle conditions normal for operating aircraft engines. The program of ultra-high cycle loading (the number of cycles is 107-109) of specially made specimens was implemented experimentally. In these experiments, massive flat targets made of titanium alloy VT6 after shock-wave loading were impacted by a flat striker (explosive generator method), which initiated controlled damage in the material. Ultra-high cycle loading tests were carried out on the Shimadzu USF-2000 ultrasonic testing machine, which made it possible to conduct fatigue loading tests up to 108-1010 cycles and frequency of 20 kHz. The "in situ" method of determining the accumulation of irreversible fatigue damage was applied. It is based on the analysis of nonlinear features of the feedback signal in a closed system of an ultrasonic fatigue installation, which allowed us to relate the microscopic mechanisms of fatigue with model representations, as well as to consider the stages of damage evolution based on the nonlinear kinetics of defect accumulation during cyclic loading in high - and gigacycle fatigue modes. In the mode of ultra-high cycle (gigacycle) fatigue, the material exhibits abnormalities of elastic properties, which lead to the effect of non-linearity in the oscillation amplitude.