Very-High-Cycle-Fatigue Properties of TC4 Titanium Alloy Under Three-point Bending

Aiming at the very-high-cycle-fatigue (VHCF) of aeroengine compressor blades under complex loading, three-point bending ultrasonic fatigue tests of TC4 titanium alloy under stress ratio R of 0.3 and 0.5 were performed, and failure mechanisms of VHCF under three-point bending was also investigated. Test results show that when the number of cycles exceeds 107 cycles, specimens under the two stress ratios R of 0.3, 0.5 fail and S-N curves present a bilinear characteristic. SEM analysis indicates that the crack initiation sites are transferred from the surface to the sub-surface with the decrease of the maximum stress. Fatigue crack initiations are results of competition between surface slip and internal cleavage fracture. Then a model based on fatigue life was proposed to describe the competition between the two mechanisms, which is in agreement with the experimental results. The temperature of the specimen surface was monitored by an infrared camera. Its change can be divided into four stages in the high-cycle-fatigue (HCF) regime: steady rising, steady decreasing, fast rising and fast decreasing. While it can be divided into three stages in the VHCF regime: steady rising, fast rising and fast decreasing. Finally, the characteristics of heat production and transfer were described, and the correlation between temperature and stress distribution was analyzed.