Ti-6Al-4V钛合金振动弯曲超高周疲劳及寿命预测模型研究

On the Very High Cycle Fatigue and Life Prediction Model of Ti-6Al-4V Titanium Alloy Subjected to Vibration Bending

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。

Vibration bending fatigue experiment of Ti-6Al-4V titanium alloy were carried out under 300Hz high cycle and 20kHz very high cycle condition respectively, based on ultrasonic fatigue test system and high frequency vibration bending fatigue test system. In this paper, the fracture morphology of specimen was observed by Hirox optical imaging system and scanning electron microscope; a method was proposed to characterize stress size of specimen by using the product af of vibration amplitude and resonance frequency; then, based on af value, a prediction model of vibration fatigue life under very high cycle bending vibration condition was established and verified. Results indicate that when fatigue cycle times exceed 10^7, Ti-6Al-4V titanium alloy still presents fatigue fracture, there is not conventional fatigue limit; comparing with axial tension-compression fatigue, the stress level in vibration bending is obviously smaller than that in tension-compression loading; the experimental data obtained under 300Hz and 20kHz frequency are in good agreement, which indicates that the loading frequency has not obvious effect on fatigue performance. All cracks initiate at specimen surface due to the gradient distribution of stress in specimen. The fracture surface of specimen is flat and smooth, and the crack source did not show ＂fish eye＂ features. The fatigue life prediction model based on af value can well predict bending fatigue life in very high cycle vibration bending.