Multi-scale Propagation Model Of Fatigue Crack In Polycrystal

The failure behavior of polycrystalline materials is mostly fatigue failure,and the fracture caused by fatigue failure is one of the important causes of serious accidents.In recent years,domestic and foreign scholars have done a lot of research on polycrystalline materials based on the micro-damage fracture mechanism and macro-mechanical theory of fatigue crack propagation.A series of theoretical models of fatigue crack propagation have been proposed,but they are not applied to the cyclic tensile and compressive loads.After the passivation of the lower crack tip,the process of sudden crack propagation gives a reasonably accurate explanation.Based on the previous researches,this article uses the finite element software ABAQUS as the analysis platform to simulate the fatigue crack propagation behavior according to the characteristics of crack initiation and abrupt expansion after the crack propagation process.The conclusions are as follows:Taking the coarse-grained stainless steel round bar specimen as the research object,based on the virtual crack closure technique(VCCT)and the extended finite element method(XFEM),the finite element model of the cracked round bar was established.The simulation results show that both methods can be used to calculate the simulated crack propagation rule.However,compared with VCCT,XFEM can simulate the crack propagation in any direction without pre-cracking path.The meshing is simple and the result is more accurate.Since the crack tip of the damage model under static load is not passivated during the fatigue crack propagation process,although the crack tip of the damage model under the fatigue cyclic load is passivated,a process of crack tip passivation cannot be reasonably explained.Both models cannot independently and accurately simulate the fatigue crack propagation rule.Therefore,a hypothesis is proposed.When the fatigue crack tip is continuously expanded,the failure of the crack tip material is controlled by the material inelastic hysteresis mechanism in the fatigue damage.Under the fatigue load,the crack tip is passivated.If the loading continues,the failure process of the crack tip material is equivalent to the fatigue failure process under static load.The failure behavior of the crack tip material follows the material damage criterion under static load.When the fatigue crack tip is discontinuously expanded,it appears as crack tip passivation.After the medium-duty fatigue loadcycle,we suspect that the failure of the material in the passivation zone is controlled by the maximum principal stress under static load,assuming the above two materials the damage mechanism alternately controls the actual expansion behavior of the fatigue crack tip.Based on this conjecture,an attempt was made to develop a fatigue crack propagation model with a hybrid damage mechanism.Aiming at this CAE model with mixed damage mechanism,a numerical simulation method is proposed to predict the fatigue crack growth life,in order to improve the fatigue life prediction accuracy in engineering applications.Using the multi-scale calculation method of fatigue analysis,the numerical results of sub-model and sub-submodel are established for the fatigue crack propagation adjacent region and crack tip unit respectively.It is shown that the sub-model technology is more accurate and efficient by comparing the global model with the introduction of sub-model technology.The introduction of sub-submodels improves the accuracy of XFEM's description of crack tip cells.Through the establishment of the global model to the sub-model and then the sub-submodel,the convergence,transformation and inheritance of the mechanical information and behavior of the polycrystalline material on the adjacent scale are realized,and the polycrystalline material inherits from the macroscopic to the mesoscopic external load.A numerical simulation method is provided for the meticulous to macroscopic crack extension behavior upload.