Study Of Crack Propagation In Nickel-base Single Crystal Alloy

Due to its excellencent performance, nickle-based single crystal superalloy as animportant structural and functional material has been widely used for the blade ofaerocraft engines and gas turbines. When aerocraft engines and gas turbine start and stopfrequently, the blade is repeatedly dashed by high-temperature/pressure gases, renderingthermal cyclic stress to be applied to the blade. As a result, the blade usually suffers lowcyclic fatigue damage. Studying the fracture behavior and the fatigue crack propagationrate of nickel-based single crystal superalloy is of academic and technical significance,which is helpful to improve safe life and reliability of aerocraft engines and gas turbines.In this thesis, the crystal plasticity theory is adopted to describe the constitutivebehavior of Nickle-based single crystal superalloy, and the cohesive zone model isemployed to model crack initiation and propagation. To track crack propagating along itspossible paths, the cohesive zone elements are distributed on interfaces between FEmeshes. Main works and contributions include:(1) By using ANSYS and ABAQUS software, a mothod to model fracturebehavior with the cohesive zone model is suggested. The main idea is that byadopting APDL command provided by ANSYS, the geometrical model andfinite element model are created and broken up, then the cohesive elements areassembled between FE meshes and some components are defined. After theseoperations are finished, they are imported to ABAQUS to compute andpost-process.(2) With the software ABAQUS, propagation behavior of I-type crack inthree-dimensional compact tension specimen of nickle-based single crystalsuperalloy is computationally studied. The distributions of resolved shear stress,Mises stress and normal stress on slip plane and the slip system activation lawwithin crack-tip region are discussed in detail. Based on these, the strain energydensity criterion is employed to analyze the inherent mechanism of crackinitiation and its propagation, with a special concern over thought-thicknessvariation of these important physical parameters. (3) By using Cohesive Zone Model, the fatigue crack propogation intwo-dimensional compact tension specimen of Nickle-based single crystalsuperalloy is modeled. The effects of crystal orientation, stress ratio, loadingfrequency on the propagation rate of fatigue crack are studied, and then arelationship among stress ration, crack tip open displacement and thepropagation rate of fatigue crack is given.