| Ti40 alloy is a new burn resistant titanium alloy designed for aeroengine in case the titanium burning under friction at high temperature. It's nominal composition is Ti-25V-15Cr-xSi. Study of the fatigue crack propagation (FCP) behaviors is meaningful for the damage tolerance designing of components used in airplane. Considering the practical use, experiments here are about researches on FCP behaviors of Ti40 alloy at different temperatures, frequencies and stress ratios respectively. The phenomena and regulations of FCP behaviors are revealed and the essential reasons for the effects of different load conditions are determined. And at the same time, the theories and views used in dealing with experimental data are also validated. Results are drown as follows:1) The FCP rate of Ti40 alloy increases with increment of temperature from room temperature to 600℃. The material constant parameters( C and m ) in Paris formula increase or decrease monotonously. The FCP rate curves at differenttemperatures crosse at a point, at which △K is 45.5 MPaw1/2 and da/dN is1.85 ×10-3 mm/cycle. It is the point that the mechanism controlling the FCP behavior of Ti40 alloy changes from incomplete ductility to whole ductility.2) Effects of temperatures on the FCP behaviours of Ti40 alloy are of the changes of elastic modulus, yield strength, toughness and the oxidation at the tip of crack. From room temperature to 300℃, the decrease of yield strength and elastic modulus and the rise of toughness cause little difference between the FCP rate at room temperature and 300℃ . From 300℃ to 600℃, the influence of oxidation is stronger and stronger. So the FCP rates of Ti40 alloy increase significantly. The apparent active energy also increases when temperature rise. Whereas it changes unpercepertaly when the temperature is higher than 500℃. All these indicate that 500℃ is of a transformation point for some properties of Ti40alloy.3) The microstructure characteristics of Ti40 alloy cannot bring the effective resistance to FCP rate. While it's good physical properties and mechanical properties make the FCP rate lower than that of p21S. Compared with Ti6A14V alloy with lamella structure, there is no predomentance for Ti40 alloy in physical properties and mechanical properties, which induces comparatively higher FCP rate than Ti6A14V. When the temperature is high, the stronger oxidation of Ti40 alloy leads to a little quicker FCP rate at 600 °C than that of (321Sat65O°C.4) The lower the frequency, the higher is the FCP rate of Ti40 alloy. At room temperature, FCP rate changed little with frequency increasing from lHz to20Hz when AK is less than 15MPaVm . When AK is higher than 15MPaVw , itappears cycle-time dependence under the frequency of about 1 Hz and appears cycle dependence under the frequency of about lOHz. When the temperature is at 500 °C, the sensitivity of FCP rate to the load frequency decreases.5) The effects of frequency on Ti40 alloy's FCP behaviour rest with the change of viscidity-plasticity strain range. The lower the frequencies the more importantly do the strain range and viscidity-plasticity strain range play the role. When the temperature is at 500°C, oxidation is essential for the effect of frequency on FCP rate of Ti40 alloy. The main reason is oxidation will lead to high yield strength and then decreases the range of strain and viscidity-plasticity strain, which is the reason of the lower sensitivity of FCP rate of Ti40 alloy on frequency.6) The effect of stress ratio on FCP rate of Ti40 alloy is of the different tress field according to various stress ratios. AK and Kmax play different role on the contribution to FCP rate. For Ti40 alloy, when the stress ratio is from 0.1 to 0.5, AK and Kmax control the FCP rate together and AK is more important. When the stress ratio is above 0.8, Kmax become the most important reason. Crack growthtrajectory of map shows clearly two mechanism of controlling FCP of Ti40 alloy when stress ratio is different, which explain the different contribution to FCP rate of AK and Kmax. The character of the curves in the map is similar to that of other titanium, just the degree of the deviation to Kmax axis is stronger. This phenomenon indicates that Ti40 alloy is easier to be affected by oxidation and the time of FCP by the mechanism of facet is longer. 7) According to the features of FCP curves under different stress ratios at roomtemperature, non-existed "reverse plastic aera" at crack tip is found, which isthe same with the conclusion drawn by K.Sadananda et al.
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