Simulation Research On Crack Propagation Of Solid Material

Fracture Mechanics is an emerging science which has developed over the last half century and made important contributions to the classical continuum mechanics. Major study of Fracture Mechanics is the strength of solid with crack and the law for crack propagation, whose main task is researching the stress and strain of crack tip, mastering the crack propagation law in the load, understanding the carrying capacity of solid with crack, proposing design method of anti-crack and ensuring safety work of component.This paper summarizes and analyzes the research development and status quo at home and abroad, and does the following research on crack propagation micromechanism and it's law:This paper is divided into three parts, which are researching the tensile mechanical properties of fcc-Ag single crystal model without crack and crack propagation micromechanism of fcc-Ag single crystal model crack using molecular dynamics method and simulating mixed-crack(ⅠandⅡ) propagation forecast using commercial non-linear finite element software--MSC.MARC.The first part introduces the basic tenets of molecular dynamics and provides several methods of solving Newton's equation of motion with its advantages and disadvantages. Also there are several common atomic potential functions especially embedded atom method (EAM) which is the atomic potential function of this paper's. Then it illuminates how to impose the boundary conditions and chose the time step. At last it gives the methods of starting molecular dynamics simulation.Through the simulation of nano-crystal tension it provides the application details of molecular dynamics method. It shows specific process of modeling and simulation, choices of the Velocity-Verlet integration methods, and embedded atomic potential functions (EAM). It analyzes the model's stress-strain curve and atomic energy-strain curve, and reveals under the tensile state there are elastic stage and plastic stage in the stress-strain curve. In the elastic stage Hooke's law is still effective. In the plastic stage stress-strain curve produces fluctuations. The fracture of metal bonds leads the stress suddenly fall. Using the concept of macro-materials the stress ofε= 0.165 is seen as yield strength of Ag single crystal, and the stress ofε= 0.18 is seen as ultimate strength of Ag single crystal. Dislocations have produced in the atomic arrangement fig. Now the damage has already occurred in the crystal structure according with the stress-strain curve, the atomic average total energy-strain curve and the atomic average kinetic energy-strain curve.The second part simulates crack propagation mechanism of fcc-Ag single crystal model with crack using molecular dynamics method. In the simulation process the simulation chose embedded atom potential function (EAM) and the Velocity-Verlet integration methods, Atoms'interaction follow the Newton's law. In the simulation the model is relaxed at first to arrive at the lowest energy state.Then model is outputted the atomic arrangement fig. We can observe crack-tip passivation, disorderliness regional appearing. With continuing to load, the crack began to propagate, the crack propagates with the disorderliness regional propagating, and micro-empties appear. Then the stress-strain curve, the atomic average total energy-strain curve, atomic average potential energy-strain curve and the atomic average kinetic energy-strain curve are drawn and analyzed. The maximum stress of fcc-Ag single crystal model without crack is 13.9GPa in the simulation process. Ag single crystal structure occurred damage not-reached yield strength of the Ag single crystal model without crack(σs = 20.5GPa).So the crack impacts the structural strength greatly.The models of different simulation conditions (boundary conditions, loading rate and model size) are simulated. The stress-strain curve, the atomic average total energy-strain curve, atomic average potential energy-strain curve and the atomic average kinetic energy-strain curve are drawn. The conclusions according to the simulation results are as following:(1) Surface effect is one of key factors impacting nonmaterial's mechanical behavior and mechanical properties. For the atoms of free surface stores surface energy, initial energy is higher than that of the model which has one pair free surface and so does the stress. The atomic average total energy and atomic average potential energy is always higher than that of the model which has one pair free surface in the simulation.(2) For the same size model, The atomic average total energy, atomic average potential energy and the atomic average kinetic energy of the higher loading rate model is higher than that of the lower loading rate model. But the maximum stress is lower, and the increased extent is smaller.(3) For the same loading rate model, the atomic average total energy and atomic average potential energy of the smaller size model is higher than that of the bigger size model. This is caused by surface effect really.The third part introduces the basic tenets and development overview of the finite element method. It introduces the basic tenets of the commercial non-linear finite element software- MSC.MARC/MENT, the modules and the process of modeling and analysis. It studed the modeling process and the details of the model with eccentric crack, and simulates mixed-crack(Ⅰa ndⅡ) propagation forecast that. It forecasted crack propagation direction accurately. The model is failure of iridizations using other software.For lacking of systematic integritive theoretical knowledge and being restricted by time and conditions, there are some disadvantages in this paper which need further work to improve.The disadvantages of the molecular dynamics method are concentrated in the small size and the short time scale of the simulation system, the disadvantages of the algorithm and the theory at the present stage at the same time. If the grid size of finite element method is too big, the integral numbers are unstable. If it is too small, the CPU time increases greatly. It only can be used in small scale unless we use supercomputers.This paper is just the study of single-scale simulation. Multiscale method can reduce the numerical integration order of magnitude and the total CPU time, so it can be used in simulation of large structure deformation. Someone could study the Multiscale method through coupling the molecular dynamics method and the finite element method on the basis of paper, or do it through making the cohesive model as the combinational point of coupling two methods. Developing the theory about the solid damage multiscale mechanics is one of the most important research directions at present.