Microscopic Numerical Simulation Of Deformation And Failure Mechanism Of Metallic Materials Under Various Loads

With the rapid development of modern industry,metal materials are widely used in the fields of aviation,aerospace,weapons and equipment,mechanical equipment and other key components,It is inevitable that these components will be deformed or even damaged in service.The macroscopic mechanical properties of materials depend on their own microstructure In general,the working loads borne by metal materials can be summarized into three types:uniaxial tension,cyclic load and impact load.At the same time,metal materials inevitably produce micro-defects in the manufacturing process,such as micro-holes,micro-cracks and so on.Therefore,in order to analyze the causes of macro-failure of metal materials,In this paper,molecular dynamics method is used to study the micro-deformation and failure mechanism of metal materials under three different loading forms.The main contents and conclusions of this paper are as follows:The uniaxial tensile process of single crystalα-Ti with voids is simulated by molecular dynamics method,and the effects of model size,strain rate and volume fraction of voids on the mechanical properties ofα-Ti single crystal are discussed.It is found that the stacking fault caused by twinning deformation and dislocation slip is the cause of double yield ofα-Ti stress-strain curve under 8×10⁸s^-1 tension.The initial yield strength and initial yield strain of the materials decrease with the increase of model size and void volume fraction,but increase with the increase of loading strain rate.The Young’s modulus of the materials is only sensitive to pore volume fraction and decreases with the increase of pore volume fraction.The fatigue crack propagation behavior of single crystal Ni₃Al under cyclic loading is studied,and the effects of crack initial configuration and strain ratio on the central fatigue crack propagation mechanism are discussed.The study found that:（110）[1^<sub>10]the crack propagation rate is the fastest because the atomic bond fracture directly propagates in cleavage mode;（111）[1^<sub>10]the crack appears severe passivation phenomenon at the crack tip,the crack width increases gradually,the crack propagation rate is the slowest,but it has the best plasticity;（010）[100]cracks are passivated at the crack tip.The stress concentration near the crack tip makes the atomic bond fracture appear micro-pores,the pores gradually form as sub-cracks,and finally the sub-cracks propagate through the mother crack.（101）[010]the crack is passivated because of the active crack tip of the slip zone.With the increase of cycle times,the crack propagates along the sliding direction of the slip band in I-II mixed fracture mode.The parameter identification and micro-mechanism of single crystal Ni damage model under impact loading are studied.It is found that when the temperature of the system is1350 K close to the melting point of single crystal nickel,the pressure-time curve of the system has two abnormal minimum values.According to CNA,RDF,the potential energy analysis:The first minimum value corresponds to the long-term ordered loss,and there are no voids in the system.The second minimum value corresponds to the fracture of atomic bonds caused by stress concentration.At this time,the system has void nucleation and initial growth.The optimal fitting parameters of NAG damage model at different temperatures were obtained by using improved genetic algorithm.It was found that the nucleation,growth threshold and viscosity coefficient of monocrystalline nickel decreased with the increase of temperature.Under the triaxial tensile action of high strain rate,the stress concentration causes the fracture of atomic bonds,which results in nucleation and rapid growth of voids.When the system pressure is less than the void growth threshold,the dislocation slip between the voids causes the atomic structure to be reordered,resulting in a{111}stacking fault,At this time,void coalescence is the main way of void volume increase.