The Study Of Multi-scale Method For Dynamic Fracture Behavior Of Nickel-based Materials Based On MD And CFEM

The failure behavior of metallic materials is a complex multi-scale problem,which is cross microscopic,mesoscopic and macroscopic.In recent years,it has attracted extensive research interest of scholars at home and abroad.An ideal research consensus is to study the failure process of metal materials at different scales.Finally,through a multi-scale method,the analysis results at different scales are coupled on a common analysis platform,and the numerical simulation process from micro to macro scale data flow and continuity can be effectively realized.However,after years of joint exploration and research by scholars at home and abroad,we all agree that to achieve such an ideal goal,there are still many key theories and methods to break through.Therefore,this paper adopts a cross-scale analysis method based on the combination of molecular dynamics analysis?MD?and cohesive finite element analysis?CFEM?.This paper takes nickel-based alloy?GH4169?as the research object.First,the molecular failure method is used to study the deformation failure mechanism,dislocation reaction process and TS curve of metal materials at the microscopic scale.Cohesive finite element method is used to simulate the random fracture process of metal materials and its dynamic fracture toughness at macro scale.Finally,the dynamic fracture toughness experimental value obtained by dynamic fracture experiment verifies the correctness of the multi-scale method.On aspect of experiments,three groups of three-point bending fracture experiments of Ni base alloy GH4169 with different impact velocities have been carried out by using the split Hopkinson pressure bar apparatus.In the experiment,the time history curve of impact load is obtained by using two pieces of strain gauges symmetrically distributed in the middle of the incident dry.At the same time,the macroscopic strain rate corresponding to different velocity is obtained according to the relevant compression test data,which provides the boundary conditions for the later finite element calculation and molecular dynamics simulation.The starting time of the specimen at different speeds was obtained by using the strain gauge at the position perpendicular to the crack and 2 mm away from the crack tip;the strain gauge at the position at 60°angle between the crack and 5 mm away from the crack tip was used to obtain the strain data of the dynamic fracture toughness calculated by the strain gauge method,and the experimental value of the dynamic fracture toughness of GH4169 was finally obtained.The effect of void size on the mechanical properties of twin nickel and its dislocation reaction process are studied by molecular dynamics method.The results show that with the increase of void radius,the effect of void size on the yield stress of twin nickel will decrease gradually,and the dislocation density will decrease.The dislocation reaction is mainly the reaction of two partial dislocations of Shockley,which results in lomer Cottrell dislocation.With the increase of strain,the dislocation of twin nickel is decomposed into Shockley dislocation for further plastic deformation.Then,the corresponding relationship between the strain rate in the micro molecular dynamics method and the strain rate in the macro experiment is studied by using the effect of aluminum rate ambiguity.It is found that 10¹⁰s^-1 in the molecular dynamics corresponds to 1000s^-1 in the macro experiment and 10⁶s^-11 corresponds to 0.1s^-1 in macro.Under 106s-1,it corresponds to the quasi-static state in the macro,and there is a one-to-one correspondence between the order of magnitude of strain rate.This study finds the correspondence between the strain rate in the molecular dynamics and the strain rate in the macro experiment,which provides the basis for the selection of the strain rate of the T-S curve obtained by the molecular dynamics method.Finally,in order to obtain the T-S curve of GH4169,three models of single crystal nickel,bicrystal Ni+Ni3Al and Ni/Ni3Al were established to represent the grain,grain boundary and two-phase particle interface of GH4169.According to the corresponding relationship between strain rate in molecular dynamics and strain rate in macro experiment,the molecular dynamics strain rate corresponding to the experimental strain rate is found to conduct micro simulation,and the T-S curves of three models are extracted to provide the material parameters for the subsequent cohesive finite element method.Taking the parameters of T-S curve as a bridge,molecular dynamics method and cohesive finite element method are used to simulate the random dynamic fracture process of GH4169 and obtain its dynamic fracture toughness simulation value,which is compared with the dynamic fracture toughness experimental value obtained in the previous experiment to verify the accuracy of the simulation results of multi-scale method.At the same time,the multi-scale method is also used to study the static fracture behavior of B₂-NiAl alloy.The simulation results of the static fracture toughness are compared with the literature results,and the results are in good agreement,which further verifies the reliability of the multi-scale method.