关键词: 单晶材料, 低周疲劳, 多轴应力, 非对称循环载荷, 正交各向异性

Abstract: 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.

Key words: Asymmetrical cyclic load, Low cycle fatigue, Multiaxial stress, Orthotropy, Single crystal material