缺口半径对疲劳寿命影响的有限元分析

针对Mod.9Cr-1Mo铁素体钢缺口件进行了非比例应变路径低周疲劳试验,采用直流电位差法测量裂纹萌生寿命,比较了缺口半径和应变路径对疲劳裂纹萌生寿命的影响.试验结果表明:缺口件裂纹萌生寿命占疲劳寿命的比例与应变路径和缺口半径相关.采用ANSYS软件进行模拟计算,材料弹塑性特性采用von Mises屈服准则、多线性运动硬化律和单轴循环应力应变曲线描述.模拟结果表明:各应变路径下缺口根部处产生了明显的应力集中,等效应力最大值均发生在缺口根部处,随着离缺口根部距离的增加,应力随之而降低.但不同缺口半径下,离缺口根部不同距离处的应力梯度变化趋势随应变路径不同而不同.基于模拟结果,采用Smith-Watson-Topper(SWT)模型进行疲劳寿命预测.结果表明SWT模型针对大缺口半径的预测结果较好,大部分点位于2倍分散带内,但针对小缺口半径预测结果普遍偏低. 

Finite element analysis for the effect of notched radius on fatigue life

A series of tests for low cycle fatigue were conducted for the notched specimens of Mod.9Cr-1Mo ferrite steel under nonproportional loading. The crack initiation life was measured by the direct-current(DC) potential difference method. The effects of the notched radius and strain path on crack initiation life were discussed. The test results show that the ratio of the crack initiation life to the fatigue life is related to the strain path and notched radius. ANSYS software was used for simulation, while the von Mises yield criterion, multlinear kinematic hardening rule, and uniaxial cyclic stress-strain curve were employed to indicate the material elastic-plastic behavior. The simulation results show that the stress concentration at the notched root occurs significantly for any strain paths. It is obvious that the maximum equivalent stress lies in the notched root. But the stress decreases with the increase of distance. For different notched radiuses, the changing trend of the stress gradient with the distance to the notched root differs with the strain paths. Smith-Watson-Topper (SWT) model was used to predict the fatigue life based on the simulation results. The better prediction results are gained for the larger notched radius and most results are in a factor-2 scatter band. But for smaller notched radius, the predictions are conservative.