Study On Multi-axial Low Cycle Fatigue Of Aero-engine Single Crystal Blades

With superior high-temperature mechanical properties, single crystal blades have been introduced into most of the advanced aero-engines. Fatigue fracture is a pervasive problem because of elevated temperature, pressure and rotating speed and iterative fatigue loading. Taking aero-engine single crystal blade made by nickel-based single crystal superalloy DD3 as its objective, some main research on low cycle fatigue (LCF) in this paper is performed under the crystal plastic theory and non-linear dynamic hardening rule, by methods of theoretical analysis and numerical simulation combing with experiment research. After deep and systematic research on kinds of fatigues such as un-axial fatigue, notch fatigue and multi-axial fatigue, the writer tries to seek after a reasonable and effective aero-engine single crystal blade LCF constitutive model. The main works described in the paper are:(1) A more fitting yield rule (SC rule) was put forth to estimate yield stress along three major bearing orientations for single crystal based on Hill's yield rule. An aero-engine single crystal blad LCF constitutive model (NLDH model) was set up by crystal plastic theory and non-linear dynamic hardening rule, and the calculating methods of dynamic hardening back stress, reference shear stress and resolved shear stress in the model were given. The user subroutine UMAT based on the finite element program ABAQUS was compiled, and the numerical simulation tool about SC rule and NLDH model was empoldered.(2) The NLDH model was used to predict the LCF behavior along DD3 three crystal orientations with finite element (FE) method. And then relevant tests were performed. The FE result was in good agreement with that of the experiment, which indicated that the NLDH model was feasible in nickel-based SC un-axial LCF. Furthermore, the experiment results showed that DD3 had obviously anisotropic for un-axial LCF, [111] had the longest fatigue life, [001] took second place and [011] was the shortest.The effect of ratcheting on LCF of SC was first discussed.(3) The NLDH model was used to predict the LCF behavior of SC notched specimens under different conditions. Stress relaxation and ratcheting at notch tip was observed from the FE simulation under fatigue loading. Stress relaxation could decrease the rate of crack propagation and prolong fatigue life. The presence of ratcheting showed that crack at notch tip would grow till rupture when plastic distortion cumulating to some degree. And then relevant experiments were carried out, and the scanning electron microscopy (SEM) was employed to investigate the fracture mechanism. The experimental results showed that both stress concentration factor and loading conditions affected notched specimens LCF life under the same temperature and stress ratio, which was in good agreement with the results of FE. Thus, stress concentration and loading conditions should be considered together in the analysis of fatigue life at some especial position.The predicted life with simulation results was true. All of these proved the feasibility of developed model applying to multi-axial LCF for SC.(4) The paper presented orthogonal experimental design (OED) method for the thin-walled cylindrical tensile-torque experiment schemes of DD3 SC. It was the first time that the design and manufacture of non-standard thin-wall cylindrical specimens were performed. The tensile-torque experiments at elevated temperature on DD3 SC thin-wall cylinder were successfully completed for the first time, and the SEM was employed to investigate the fracture mechanism too. All experimental results were studied entirely.The NLDH model was used to predict the LCF behavior of thin-wall cylindrical specimens, and the simulation results reflected the LCF characteristics under tensile-torque loading to some degree, the life prediction was in good agreement with the test results, which further demonstrated the feasibility of NLDH model in multi-axial LCF.(5) An in-depth analysis of the blade crack was undertaken using some advanced XactLIFETM system and the NLDH model respectively. The two methods both predicted the exact fracture position at turbine blade, but the fracture modes were different:creep is the major driver of crack with XactLIFETM system, and LCF rupture with the NLDH model. The later agreed with the fracture appearance, which showed the NLDH model could depict the fracture mechanism of SC turbine blade more exactly.