Crystal Plasticity Finite Element Simulation Of Lithium Fluoride During High Pressure And High Strain-rate Impact Deformation

Lithium fluoride(LiF)is common window material used in impact test.Due to the influence of its dynamic response under high-pressure loading conditions on impact measurement results of other specimen material being non-negligible,it is necessary to study the dynamic mechanical evolution rule of LiF.Since impact testing method lack of cognition for microscopic dynamic evolution mechanism,we employed crystal plasticity finite element method based on crystal microscopic structure of LiF in simulation researching its elastic-plastic large deformation behavior under high pressure and high strain-rate.In this work,we built a dynamic crystal plasticity finite element model,using equation of state to describe the nonlinear elastic relationship and using phenomenological hardening equation,which takes phonon drag mechanism in consideration,to describe the viscoplastic deformation of material under high pressure.Through numerically simulating the impact compression process of<100>LiF single crystal and LiF polycrystal respectively,we analyzed the physical information indicated by propagation processes of stress wave in specimen under various loading conditions,and further explained and predicted the dynamic response of LiF material from the view of continuum mechanics.Through the simulation of<100>LiF single crystal material under uniaxial impact compression,the results indicate as the following.The model we built can obtain the stress wave profiles in accordance with experimental results,which show characteristics of elastic-plastic two-wave response,stress relaxation and elastic precursor amplitude decay under impact pressure approximately lower than 20 GPa.The main factors controlling the stress wave profiles of LiF single crystal are impressed pressure,specimen thickness and material constitutive relation.Considering the stress relaxation phenomenon under impact loading,the continuum mechanical basic reason for which is the viscous plastic deformation of material during high pressure and high strain-rate impact deformation process.Based on the two-wave and one-wave response of stress wave profile obtained under shock pressure lower than about 20 GPa and beyond that range respectively,the third order derivative of pressure to time can be the criterion for estimation of the critical pressure of two-wave and one-wave response.It is also important to consider the temperature rise effect during dynamic deformation,and the simulation results indicate that the temperature rise mainly arises from elastic volumetric expanding.Through the simulation of LiF polycrystal material under uniaxial impact compression,the results indicate as the following.Accumulated slip rate rapidly increases up to 10~7/s at the initial stage of plastic deformation,and keeps increasing along with the decreasing width of plastic deformation concentration until the stress wave arrives at the free surface of the specimen,which is consistent with the dynamic deformation trend of single crystal.The main reason for inhomogeneous deformation of LiF polycrystal is orientation differences among crystals,and the major strain concentration area is grain boundary.Further increasing the impact pressure or loading rate,the stress wave profiles of LiF polycrystal show decreased elastic-plastic wave width,smoother boundary of deformation concentration area and more uniform stress distribution in completely deformed region.