Study On Multiaxial Low Cycle Fatigue Damage Of Single Crystal Nickel-based Superalloy

Single crystal nickel-based superalloy became OK material for the hot sections of aircraft gas turbine engine due to its high performance of the anti-fatigue/creep under elevated temperature. Low cycle fatigue damage is an important factor to effect the life of gas turbine blades made of single crystal nickel-based superalloy, but so far the many problems in this field are not deeply studied all over the world. With the way of theory analysis, computer simulation and experimental test, the thermo elastic-plastic behaviour and low cycle fatigue performance of single crystal nickel-based superalloy was deeply studied, and the constitutive equations and life predictive equations were built correspondingly. Then the three-dimensional nonlinear cyclic stress-strain analysis of a gas turbine rotor disk-blade system was carried out with ANSYS, and the low cycle fatigue life of the single crystal gas turbine blade was predicted.The thesis is made up of two parts. First part researches into yield behaviour of single crystal nickel-based superalloy and its elastic-plastic constitutive equations, from chapter 2 to chapter 4. In the other part, from chapter 5 to chapter 8, based on the energy dissipation theory and the non-reversible thermodynamics respectively, two kinds of model for low cycle fatigue life prediction are presented, which are used for the calculation of true structures after evaluating the model constants by using the experimental data.First of all, it is conceived that the yielding function is constituted by linear combination of the invariable of certain deviatoric stress tensors. By considering the coupling effect as an invariable, which is the quadratic product item of components of deviatoric stress tensor, the Hill's yield criterion is modified to develop a new yield criterion for engineering. The equivalent stress and the equivalent strain for this new yield criterion are redefined and they can also be deduced to uniaxial stress and strain. For isotropic material, the new yield criterion and its equivalent stress and equivalent strain could be reduced to Von Mises yield criterion and the corresponding equivalent stress and equivalent strain. Based on the Drucker rule, using the associated flow rule andisotropic hardening model the elastic-plastic constitutive equations for single crystals superalloy are built and the corresponding elastic-plastic matrix are deduced. The new yield criterion and the elastic-plastic constitutive equations are implemented into a finite element subroutine on ANSYS platform, and numerical simulation of thermo elastic-plastic stress-strain fields for single crystal superalloy was carried out.Then by analyzing main factors of affecting low cycle fatigue life of single crystal superalloy, the formula and computing method of cyclic plastic strain energy are researched and discussed. The low cycle fatigue life prediction model in which the cyclic plastic strain energy as a parameter is used is deduced based on the damage mechanics theory. Besides, based on the non-reversible thermodynamics theory, using a strain energy release ratio as a thermodynamics generalized force for describing fatigue damage course, a stress triaxiality factor for single crystal superalloys which contains three elastic constants are deduced. By introducing an orientation function which takes account of nonlinear effect of crystal orientation of material on fatigue damage and using the strain energy release ratio for defining cyclic characteristic parameter, an evolutional rule for single crystal superalloy is deduced and the anisotropic fatigue damage models under multiaxial stress state are built. The models can be not only applied to low cycle fatigue damage under full-reversal loads, but also to asymmetrical loads. Using ANSYS in constructing a three-dimensional cycle-symmetric finite element model for a gas turbine rotor disk-blade system, the three dimensional nonlinear cyclic stress-strain of the gas turbine rotor disk-blade assembly is analyzed. Finally the low cycle fatigue life of the gas turbine blade made of single crystal superalloy DD3 is predicted with the two life prediction models built in the thesis and the predicted results have been validated by test of the gas turbine blade components in some degrees.