Fatigue Failure Mechanism And Strength-life Prediction Method Of TC4 Titanium Alloy For Compressor Blade In Very High Cycle Regime

As an important component of aircraft engine, compressor blades endure high temperature, high-frequency and ultra-long term cyclic loading in service, which often cause fatigue fracture accidents. Because of good properties such as low density, high specific strength, high temperature resistance and high corrosion resistance, TC4 titanium alloy is accepted as one of the most im portant structural materials for compressor blade in aircraft engine. Considering the long-term safety, intensive research on fatigue damage behavior and failure mechanism of TC4 titanium alloy in working conditions is of great significance for the reliability design and fatigue life prediction of compressor blade.Firstly, axial loading very-high-cycle fatigue(VHCF) tests of TC4 at room temperature under five stress ratios were carried out. Fractographs of specimens were taken by scanning electron microscope(SEM), followed by subsequent measurements and analyses. And the influences of stress ratio on the VHCF S-N properties as well as the crack initiation and propagation mechanisms were discussed. The results indicate that surface failure dominates under R =-1 whereas the duplex S-N property with both surface failure and interior failure occurs under R ≥-0.3. Both the threshold stress intensity factor ΔKth and fracture toughness Kc increased with the increase of stress ratio. Besides, three types of crack initiation mode s were observed at room temperature, which were explicitly classified as surface-slip, surface-cleavage and interior-cleavage, whose probabilities of occurrence were in relation to the stress ratio.Secondly, the axial loading VHCF tests at 150℃ and 200℃ were carried out, still followed by observation, measurement and analysis of the fractographs. And the influences of high temperature on the VHCF S-N properties as well as the crack initiation and propagation mechanisms were investigated. Four types of crack initiation mode were found at these two temperature, namely surface-slip, surfacecleavage, interior-slip and interior-cleavage. The fatigue strength of TC4 tends to decrease with the increase of temperature, this trend becomes less distinct with the increase of stress ratio however. In addition, the stress intensity factor(SIF) range ΔK of surface defect is irrelevant to temperature wh ereas ΔKth and Kc decrease with the increase of temperature.Thirdly, based on the SEM observation of specimens failed at room temperature, the fatigue lives consumed in crack initiation stage and crack propagation stage were investigated respectively with the two failure modes taken into account. A total life prediction model involving crack initiation and growth was developed then. The investigation reveals that the crack growth life is of significant portion of total life at higher stress level. On the contrary, the crack initiation life dominates at lower stress level. And the predicted results by using the model are in good agreement with experimental results where the error was within a factor of three.In the end, an energy model for fatigue strength prediction of materials with single S-N property was modified to be available for the TC4 titanium alloy with duplex S-N property. And the predicted results of fatigue strength and S-N curves are in good agreement with experimental results.