镍基单晶合金复杂应力状态低周疲劳寿命预测

根据损伤应变能释放率的定义表达式，将各向同性材料应力二三轴性因子拓展到正交异性材料，定义了含有3个弹性常数的镍基单晶应力三轴性因子。用它修正Mises应变范围作为疲劳损伤参量，可以显著消除晶体取向和多轴载荷对疲劳寿命的影响。用损伤应变能释放率作为热力学广义力描述正交异性材料的疲劳损伤过程，引入取向函数和损伤驱动力循环特征参数反映晶体各向异性对疲劳损伤的非线性影响以及循环载荷的交变特性，提出了单晶合金低周疲劳损伤模型。利用CMSX-2镍基单晶合金薄壁圆筒试样的拉一扭循环载荷低周疲劳试验数据和DD3镍基单晶合金缺口试样的低周疲劳试验数据，运用多元线性回归分析方法拟合模型的材料常数，试验所得数据分别落在2．5倍和2．0倍偏差分布带内。

Low Cycle Fatigue Life Prediction for Single Crystal Nickel-Based Superalloy

According to the expression of strain energy release ratio, a triaxiality factor, an expanding factor from isotropic material to orthotropic materials, for single crystal superalloy that contains three elastic constants was defined. By using the triaxiality factor to modify strain range as a failure parameter, effects of crystallographic orientation and multiaxial loading on fatigue life can be eliminated remarkably. Applying strain energy release ratio to describe fatigue damage course of orthotropic materials and taking account of nonlinear effect of crystal anisotropy of single crystal superalloys versus fatigue damage and alternate changing characteristic of cyclic loads by introducing an orientation function and a cyclic characteristic parameter for damage force, a low cycle fatigue damage model for single crystal superalloy was proposed. The low cycle fatigue test data correlation to DD3 and CMSX-2 single crystal superalloy with the parameter by using multivariate linear regression analysis is good and the all test data fall into the factor of 2.0 and 2.5 scatter band, respectively.