TC4钛合金三点弯曲超高周疲劳性能研究

针对航空发动机压气机叶片复杂载荷环境下的超高周疲劳问题，开展了TC4钛合金三点弯曲超高周疲劳试验，研究其在弯曲加载下的超高周疲劳破坏行为。疲劳试验结果表明：在两种应力比（R=0.3、0.5）下，当循环次数超过107次时，试件仍发生疲劳断裂，S-N曲线均呈现双线性特征；SEM断口分析表明，随着最大应力的降低，裂纹萌生位置由试件表面向次表面转移，疲劳裂纹萌生是表面滑移和内部解理断裂之间相互竞争的结果；基于疲劳寿命建立模型分析了应力比对2种裂纹萌生机制之间竞争行为的影响。采用红外热像仪监测试件表面的温度，高周疲劳试件的温度变化分为四个阶段：稳定升高、稳定降低、快速升高和快速降低阶段，而超高周疲劳试件的温度变化分为三个阶段：稳定升高、快速升高和降低阶段。最后，阐述了疲劳过程中热产生和传热的特点，并分析了温度变化与应力分布的关系。

Study on Very-High-Cycle-Fatigue Properties of TC4 Titanium Alloy under Three-point Bending

Aiming at the problem of 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 S-N curves present a bilinear characteristic. SEM analysis indicates, 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 is proposed to describe the competition between the two mechanisms, which is in agreement with the experimental results. The temperature of the specimen surface is monitored by infrared camera. It can be divided into four stages in high-cycle-fatigue (HCF) regime: steady rising, steady decreasing, fast rising and fast decreasing stage. While it can be divided into three stages in VHVF regime: steady rising, fast rising and decreasing stage. Finally, The characteristics of heat production and transfer are described, and the correlation between temperature and stress distribution is analyzed.