面心立方晶体单晶材料多轴低周疲劳寿命的估算方法

Evaluation Method of Multiaxial Low Cycle Fatigue Life for Face-centered Cubic Crystal Single Crystal Material

用立方晶体单晶材料的等效应变和等效应力作为参量,考虑正交各向异性材料偏轴受载时存在正应力和切应力的耦合效应,引入k参量描述非对称循环载荷对疲劳寿命的影响,建立工程上实用的幂函数形式的面心立方晶体单晶材料多轴低周疲劳寿命预测模型。将立方晶体单晶材料屈服准则及其弹塑性本构模型集成到ANSYS软件中,对DD3单晶合金在680℃温度下的低周疲劳缺口试样进行非对称载荷循环应力应变分析。对不同的模型参量进行多元回归相关分析,发现用立方晶体单晶材料等效应变和等效应力作为模型参量拟合的回归曲线的相关系数最大,k参量与循环次数之间呈幂函数关系。利用CMSX-2镍基单晶合金薄壁圆筒试样的拉—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对模型进行验证,试验所得数据分别落在2.5倍和2.0倍偏差分布带内。

By taking into account the coupling effect of normal stress and shear stress on orthotropic material when the applied loading deflects from the principal axes directions of material coordinate system, formulas of equivalent strain and stress for cubical single crystal material are used together with a variable k, which is introduced to express the effect under asymmetrical cyclic loading on fatigue life, to put forward a low cycle fatigue （LCF） life prediction model for face-centered cubic crystal （FCC） single crystal in multiaxial stress states. Based on the yield criterion and constitutive model of cubical single crystal material, a subroutine to calculate the thermo elastic-plastic stress-strain of the material on ANSYS platform is developed. The cyclic stress-strain of DD3 notched specimens under asymmetrical loading at 680 ℃ is analyzed. Different failure parameters of the model are analyzed with multiple regression correlation analysis. The equivalent strain and stress for cubical single crystal material as failure parameters have the largest correlation coefficient. A power law exists between k and the failure cycle. The model is validated with LCF test data of CMSX-2 and DD3 single crystal nickel-based superalloys. All the test data fall into the factor of 2.5 for CMSX-2 hollow cylinder specimens and 2.0 scatter band for DD3 notched specimens, respectively.