Micro And Macro Damage Mechanism For Typical Lightweight Alloys And Numerical Simulation

Lightweight materials,as aluminum and magnesium alloys,find limitations in industry casuse they tend to fracture under complex service conditions.Numerical simulation is often used to predict the failure behavior.The material model and fracture model are crucial in numerical simulation.Deforming and fracture behaviors are studied by the experimental and numerical methods for typical lightweight materials,as ZK60 magnesium alloy and 7003 aluminum alloy.Experimental data,including Microscopic Electron Microscopy and Finite Element Analysis,provides the basis for numerical simulation.Numerical simulations are conducted from micro-scale and macro-scale.Under micro-scale,the fractured surface presents dimple,characterizing as void nucleation,growth and coalasence.Current researches study the void coalasence behavior through unit cell model by strain gradient theory,explaining the failure behavior for lightweight alloy,analysing the effect of void geometry,stress state,size effect on void coalasence,providing micro-scale data for macro-scale continuum model.The macroscale deforming and fracture behaviors are studied through suitable constitutive model and parameter identification method under different stress state and strain rate.Engineering crashworthiness cases for typicle lightweight alloy are conducted to analysis impact behavior and the crash performance for automotive components.The main results for current paper are as follows:(1)Macro-and micro-fracture mechanism for lightweigth alloy under different stress states and dynamic strain rate.The fracture locus,which is the fracture strain vs.stress-state component relation,is nonlinear and presents ―W‖ shape.7003 aluminum alloy is dimple fracture mechanism under tensile dominated states,and turns into shear fracture mechanism undere shear dominated states.ZK60 magnesium alloy shows slip separation under pure shear,and turns into the mixed mode of slip separation and dimple fracture mechanism under uniaxial tension and multiaxial tension.The ductility enhances with increasing strain rate.Under high strain rate,the fracture mechanism presents plastic instability,the microscopic morphology reveals the dimple fracture.(2)Size-effect enhanced void evolution.Based on the viscoplastic strain gradient theory of Fleck and Hutchinson,a finite element method framework proposed by Nielsen and Niordson is choosen to study the void evolution mechanism.The results show:(a)For fiber-matrix model under cyclic loading condition,the maximum residual stress exists in the fiber-matrix interface.The residual stress increases with fiber radius,then decreases and reaches platues.(b)For void-matrix model under intense shear,the ductility enhances with diminishing void radius;Smaller void radius weakens the effect of aspect ratio,κ,on ductility.(c)For ellipsoidal void inder intense shear,the oblate void presents best ductility.The influence on ductility ranks: initial void volume fraction value,loading aspect ratio and size effect,void geometry and void orientation.The critical shear angle presents linear relationship with size effect and the power exponential relationship with loading aspect ratio.(3)Numerical characterization of fracture mechanics for lightweigh alloy under dynamic strain rates and stress states.The fracture mechanism considering different stress state and strain rate is studied.The modified Johnson-Cook model is used to characterize strain softening and strain rate effect.The GISSMO model is used to study the fracture mechanism under different stress states.An inverse finite element method is establized to determine the parameter in GISSMO model.The numerical method is validated through the consistency between numerical and experimental results for typical stress state test and three-point bending test.(4)Applications.Based on the constitutive model and corresponding parameters,this paper investigates the crashworthness for automotive components.The results show:(a)the impact simulation for ZK60 alloy indicates the maximum stress locates on the inner side of the spoke,which is consistent with the location on the experimental coating shedding.The initial velocity and initial angle change the fracture mode.(b)The optimization analysis of 7003 aluminum alloy bumper shows that the lightest bumper with the best impact performance.