Numerical Simulations Of Ultra Fine Grained Material’s Mechanical Behavior Under High Strain Rates

Usually, nano materials (or nanocrystalline, NC) are defined as the the materials with theaverage grain size less than100nm. If the average grain size of the materials are between100nm~1μ m, the materials are defined as ultrafine grained (UFG) materials. Materials with averagegrains more than1μ m are called coarse grain materials (Coarse Grained, CG). With manyexcellent mechanical properties such as the low melting point, the high toughness and hardness, etc,UFG material had aroused the scientific workers’ strong interest.In this paper, we mainly focused on the mechanical behaviors of UFG materials at high strainrates. The main task is to explore the grain size effects in shear zone formation, and themechanisms of shear localization and failures of UFG materials under high strain rate loadings.Taking typical metal materials as the research object, the one dimensional framework with thecharacteristic line method was studied. The effects of the initial defects and the strain rate on shearlocalization were investigated. Based on this foundation, the formation of adiabatic shear band(ASB) in ultrafine grained materials had been studied. Furthermore, we numerically studied thepropagation behavior of adiabatic shear band in a metallic strip that is constantly sheared at upperand lower boundaries. The Johnson-Cook constitutive relation incorporating a damage evolutionmodel was used to describe the deformation and failure properties of the material.ABAQUS/explicit was used to simulate the ASB propagations in the strip under different strainrates.The results of the study conclude:(1) The method of characteristics can effectively simulate the formation of ASB for materialsat a high strain rates in a one-dimension framework.(2) For the case of single band formation, the influences of initial temperature disturbances orgeometry defects were studied. For a prescribed initial disturbance, the relationship of the criticalstrains at which shear localization occurs and varying strain rates are showed U type.(3) For the case of multiple band formation with random initial geometric imperfection, theaverage shear band spacing is found to be dependent on the prescribed strain rate and thecharacteristic distributions of initial disturbances. If the initial disturbances were space distributedrandomly, the average spacing between multiple shear bands agrees with the Grady-Kipp formula. (4) For the case of multiple band formation with periodic initial geometric imperfection, theinvestigations confirmed that the periodic distributed defects in the slab affect the formation ofmultiple shear bands. Under certain strain rate region, the formation of multiple shear bands wascompletely controlled by the periodic defects.(5) For the case of single band formation in Ultra fine grained materials, it indicated that theevolution of variable was in fact equivalent to geometric imperfection effect. And it reached1.0(when the grain reorientation saturates) before the formation of ASB; strain hardening stronglyretarded or suppressed ASB formation; the mechanical properties of UFG Fe were dependent of itsgrain size; critical shear strain rate for severe adiabatic shear localization was existed in UFGmaterial Fe with a given length as conventional metals did.(6) For the case of ASB dynamic propagation process, it was shown that the averagepropagation speed of the ASB was dependent on the prescribed boundary velocity.