Abstract
The effects of microstructure and axial tension on the fatigue behavior of TC4 titanium alloy in high cycle (HCF) and very high cycle (VHCF) regimes are discussed in this paper. Ultrasonic three-point bending fatigue tests at 20 kHz were done on a fatigue life range among 105–109 cycles of the alloys with equiaxed, bimodal and Widmanstatten microstructures. Experimental results without axial tension show that three typical shapes of S-N curves clearly present themselves for the three different microstructures. Moreover, the crack initiation sites abruptly shifted from surface to subsurface of the specimen in the very high cycle fatigue regime for equiaxed and bimodal microstructures. But for the Widmanstatten microstructure, both surface and subsurface crack initiation appeared in the high cycle fatigue regime, and the multi-points crack initiation was found in the bimodal microstructure. The subsurface fatigue crack originated from the αp grains in equiaxed and bimodal microstructures. However, it originated from the coarse grain boundary α in the Widmanstatten microstructure. Additionally, the S-N curve shape, fatigue life and fatigue crack initiation mechanism with axial tension are similar to that without axial tension. However, the crack origin point shifts inward with axial tension.
Highlights
Many studies have shown that there is no endurance limit for most materials in a very high cycle (VHCF) regime [1,2,3,4,5,6]
The results show that the fatigue S-N curves exhibit different characteristics under different stress ratios, and the fatigue life increases with increasing stress
13, 68experiment was carried out using an ultrasonic three-point bending fatigue test system (HC SONIC, Hangzhou, China) with a load frequency of 20 kHz
Summary
Many studies have shown that there is no endurance limit for most materials in a very high cycle (VHCF) regime [1,2,3,4,5,6]. According to the traditional endurance limit design theory, mechanical design cannot ensure safety. As the requirements for life of aero engines continue to increase, the number of stress cycles for rotating components has already exceeded 107. Bathias pointed out in his monograph that the number of cycles of gas turbine engine components can reach 1010 –1011 cycles [7]. Aero engines will face increasingly severe challenges in the field of very high cycle fatigue. It is necessary to study the VHCF performance of aero-engine blade materials
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