柴油机机体紧固面微动裂纹开闭现象的研究

Study on Opening and Closing Phenomenon of Fretting Crack on Fastening Surface of Diesel Engine Block

柴油机机体材料ZL702A铝合金的微动疲劳实验中,裂纹形核于接触后缘,且在拉应力下裂纹处于闭合状态,在压应力下裂纹处于张开状态。本文针对该特殊的开闭机制采用有限元进行分析。首先利用累计耗散能的概念确定了裂纹萌生的位置,运用SWT参数法和MTS准则法模拟得到的裂纹拓展路径,与实验对比发现MTS准则可以较为准确地模拟裂纹纹路径;其次运用MTS准则模拟裂纹拓展时,测量了裂纹表面的法向应力与切向应力,得出这种特殊的裂纹的开闭现象是由微动接触引起裂纹表面一侧变形造成的;最后,引入裂纹张开度参数RCOD与有效应力强度因子对Paris公式进行修正,得出具有裂纹开闭现象的微动疲劳寿命方程,对比之下发现:不考虑裂纹开闭的疲劳寿命预测值明显高于实验值,而考虑裂纹开闭的疲劳寿命预测值则与实验值一致性较好。

The fretting fatigue test of ZL702A aluminum alloy shows that the crack nucleates at the contact trailing edge,and it is closed under tensile stress and open under compressive stress.The finite element method is used to analyze the special opening and closing mechanism.Firstly,the concept of cumulative dissipation energy is used to determine the location of crack initiation.In the simulation of crack propagation path with Smith-watson-topper(SWT)parameter method and Max Tangential Stress(MTS)criterion method,MTS criterion can accurately simulate the crack pattern path comparing with the experiment.Secondly,when MTS criterion is used to simulate crack growth,the normal stress and tangential stress on the crack surface are measured,and the special crack opening is obtained.The closing phenomenon is caused by the deformation of one side of the crack surface caused by fretting contact.Finally,the ratio of crack opening dependency(RCOD)and the effective stress intensity factor are introduced to modify Paris formula,and the fretting fatigue life equation with crack opening and closing phenomenon is obtained.By comparison,it is found that the prediction value of fatigue life without considering crack opening and closing is significantly higher than the test value,while the fatigue life with considering crack opening and closing is significantly higher than the test value.The life prediction value is in a good agreement with the test value.