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运用超声疲劳试验系统和高频振动弯曲疲劳试验系统开展了Ti-6Al-4V钛合金在300Hz和20kHz下的高周和超高周振动弯曲疲劳试验。使用Hirox光学成像系统和扫描电镜观察了试样断口形貌,提出了使用振动幅值和共振频率的乘积af表征试样应力大小的方法,建立了基于af值的超高周弯曲振动疲劳寿命预测模型并进行了验证。结果表明:疲劳循环周数超过107以后,Ti-6Al-4V钛合金试样依然会发生疲劳断裂,不存在传统意义上的疲劳极限;同轴向拉压疲劳相比,相同疲劳寿命下,振动弯曲的应力水平显著小于轴向拉压加载;300Hz和20kHz两种频率下的试验数据吻合良好,加载频率对材料的疲劳性能没有明显影响。由于试样中应力梯度分布,裂纹均萌生于试样表面,试样断口平坦,裂纹源并未呈现“鱼眼”特征。基于af值的疲劳寿命预测模型能够很好地预测超高周振动弯曲疲劳寿命。
Ultrasonic fatigue test system and high-frequency vibration bending fatigue test system were used to study the high-cycle and ultra-high-cycle vibration bending fatigue tests of Ti-6Al-4V titanium alloy at 300Hz and 20kHz. Using the Hirox optical imaging system and scanning electron microscope to observe the fracture morphology of the specimen, a method of using the product of vibration amplitude and resonance frequency af to characterize the stress of the specimen was proposed. The fatigue life prediction of ultrahigh-cycle flexural vibration based on af The model is validated. The results show that the fatigue fracture of Ti-6Al-4V titanium alloy specimens still occurs after fatigue cycles more than 107 cycles, and there is no fatigue limit in the traditional sense. Compared with the same fatigue life, the vibration The bending stress level is significantly less than the axial tension and compression load; the experimental data at 300Hz and 20kHz are in good agreement, and the loading frequency has no obvious effect on the fatigue properties of the material. Due to the stress gradient distribution in the specimen, the cracks are all initiated on the surface of the specimen, and the fracture surface of the specimen is flat. The source of the crack does not exhibit the “fish-eye” feature. The fatigue life prediction model based on af value can well predict the bending fatigue life of UHV vibration.