Study On Failure Mechanisms Of Pure Iron In High Speed Deformation

Pure iron,also known as wrought iron,has the advantages of high magnetic permeability and magnetic induction,low coercivity,low hardness and high plasticity.It has broad application prospects in important industrial fields such as high-speed rail,nuclear power,weapons,energy,power electronics,etc.During the high-speed deformation of pure iron,the deformation zone will undergo large strain,high strain rate and high temperature deformation,and the dynamic mechanical properties of pure iron will change.At a certain critical strain rate,its dynamic mechanical properties will tend to exhibit the characteristics of brittle materials,that is,the so-called plastic-brittle transition phenomenon occurs.At present,the Johnson-Cook(JC)damage model,which is widely used to characterize the strain-rate-related fracture failure behavior of plastic materials,has been proven to have a lower applicable strain rate range,and its strain rate function term can not accurately describe the rapid embrittlement of materials under ultra-high strain rate.Therefore,to study the relationship and characteristics between pure iron fracture failure strain and stress state,strain rate and temperature,to analyze the rapid embrittlement process of pure iron under high-speed impact load,and to establish a fracture strain model considering the plastic-brittle transition effect of materials at high strain rate.The fracture strain model is an important theoretical basis and premise for further analysis of the heat,force,elastoplastic coupling process and failure mechanism changes in the deformation zone of pure iron.Firstly,The pure iron specimens with different structures were subjected to uniaxial tensile,pure shear,low strain rate shear and Hopkinson pressure bar(SHPB)high-speed impact tests.With the help of scanning electron microscope(SEM),ultra-depth microscope,metallographic microscope and other experimental equipment,the dynamic mechanical properties and fracture characteristics of pure iron under different stress states,strain rates and temperatures are analyzed.Fully study the influence of different deformation conditions on the fracture strain and fracture behavior of pure iron.Secondly,based on the JC damage model,re-model the strain rate function terms,introducing the concepts of critical strain rate of plastic-brittle transition,embrittlement coefficient,embrittlement index.To make it quantitatively characterize the rapidembrittlement behavior of the material at the ultra-high strain rate stage,and construct a new phenomenal empirical material strain model.Based on the experiments,statistically obtain the fracture strain data of pure iron under different stress states(stress triaxiality),strain rate and temperature,and regression analysis and fitting calculation were used to determine the material constants of the newly constructed fracture model and JC damage model of pure iron.The numerical simulation calculation of the material deformation process was conducted by using MATLAB to compare the description capabilities of the two models in a large strain rate range.Finally,the coupling evolution data of the maximum temperature,flow stress,fracture strain,and heat distribution coefficient of the deformation zone of pure iron at different deformation speeds(strain rates)are analyzed.Further investigate the influence of the coupling between flow stress,strain rate,temperature and elastoplasticity on the transformation of pure iron failure mechanism,and divide the fracture model of pure iron within a large strain rate range.