Impact Experimental And Nanoscopic Mechanical Simulation Investigation Of Tungsten Alloy For Penetrator

For the excellent physical and mechanical properties, tungsten alloy isunreplaceable in the material application of national defense. But when it is used inextreme conditions of high temperature and high pressure, the microstructure of thetungsten alloy will change obviously, which makes the material properties differentfrom those under normal conditions. And the tungsten properties of extremeconditions such as phase transition, microscopic effects and size effects cannot beobtained easily according to experiments. For a better understanding of the tungstenalloy properties under extreme conditions from atomic level and finding therelationship between microstructure and macroscopic properties, the previous studiesof macroscopic dynamic properties and microscopic damage theory of tungsten alloyhas been reviewed. Based on the experiment and molecular dynamic (MD) method,some significant issues of tungsten alloy materials under typically extreme conditionshave been usefully discovered and investigated. The details are listed as follows:(1) Some contrastive tensile experiment of tungsten alloy at quasi-static state anddynamic state with a room temperature were conducted on material test machineMTS810and Bar-bar tensile impact testing apparatus, respectively and the fracturemorphology of specimens was analyzed. It was shown from the investigations that:the tungsten alloys quasi-statically loaded at room temperature were brittlely fracturedwithout plastic deformation and the strength of the materials increased with theincrease of strain rate. When materials were dynamically loaded at room temperature,necking phenomenon appeared to the tungsten alloy which showed some plasticity oftungsten alloy and with the increase of strain rate, the strength and stiffness of thematerials were increased. In addition, the yield strength of tungsten with dynamicloads was obviously higher than that with quasi-static loads and a visible strain-rateeffect appeared to the tungsten alloy. On the basis of experimental investigations andcombining the EAM potential function with MD method, a tensile code of tungstenalloy was developed to simulate crystal structure, phase transition and yield strengthof tungsten alloy with different crystal structure and interacted by tensile andtemperature loads. It was found that the crystal structure, ductility and necking behaviors predicted by EAM potential function were in a good agreement withexperimental results which showed this potential was applicable to the tungstencrystal and EAM potential had a good extensive property. The extensive property oftungsten crystal was different between high strain-rate loading and low strain-rateloading and a unique double-peak phenomenon appeared to the stress-strain curve oftungsten crystal loaded by high strain rate. Also, the yield strength of tungsten crystalcorresponding to different loading rate, temperature, lattice size and lattice directionwas studied and the simulations were compared with experimental results. The MDsimulations of tungsten crystal with high strain-rate tensile loading were expanded toobtain the responding formula at high strain rate.(2) The crack growth and the fracture behaviors of tungsten alloy materials weresystematically tested by three point bending testing machine and scanning electronmicroscope (SEM). The grain refinement tungsten alloy specimen under tensile loadswas first plastically deformed. The bonding phases around tungsten particulates bearthe most loads while stress almost gathered in particulates. The defect aroundprecracks first developed to crack sources and then microcrack extension andevolution were developed in the bonding phase. Based on the above achievements, thecrack extension law in tungsten alloy was concluded. In addition, a MD code wasprogramed to investigate the I type crack growth model of tungsten alloy and analyzethe crack growth mechanism of tungsten alloy crystal under tensile loads. It wasshown that both the size and the critical stress of crack growth were anisotropic: thecrack growth along [123]lattice direction had the largest critical stress; the criticalstress of the crack growth along lattice direction [112] was a little lower and thatalong lattice direction [110] was the lowest one among the three critical stresses. Thecrack growth length along the lattice direction [100] was larger than those three alonglattice directions [123],[112] and [110], respectively and the following three growthlengths were approximately2/3of the growth length along the lattice direction [100].The anisotropy of crack growth critical stress and size was coursed by differentarrangement of atomic structure and glide resistance between lattice directions.(3) The impact melting behaviors of tungsten alloy were numerical simulated bya self-developed MD program. By Solving the MD code with an inserted multi-scale model, the investigations of impact dynamic behaviors of tungsten alloy wereoperated under a dynamic high pressure state. In the numerical simulation of tungstenalloy, the critical velocity of impact melting, the time dependent curves of pressure,temperature and volume and the Hugoniot curves of tungsten thermodynamicparameters were obtained. In addition, the relationship between impact velocity andparticle velocity was fitted. By combining the parameters obtained by MD simulationsand the derived formula, the corresponding parameters of cold energy curve weregiven and compared with the theoretical calculated results in citations. A goodagreement between the numerical predictions and experimental and theoretical resultswas reported, which meant that the method of combining MSST model with MDmethod to describe impact melting issues of tungsten alloy was reliability in a certainlevel. At the same time, influences of impact velocity and impact lattice direction onimpact melting were computed. It was found that the processes of impact meltingwere considerably different when the impact velocity and impact lattice directionwere different.(4) Based on the above experiments and numerical simulations of tungsten alloy,a MD code applied to hypervelocity impact of tungsten projectile on target wasprogrammed. First, a three dimensional model of hypervelocity impact of a projectileon target was setup to analyze the failure mode of the microstructure generated by atungsten alloy crystal projectile impacting on target. Then, studies on effects ofprojectile diameters, impact velocities, target thickness and material properties ondebris cloud were performed. Finally, numerical predictions were compared with highquality experimental results. The shape of simulated debris clouds and the velocitiesof characteristic points in the obtained debris clouds agreed well with theexperimental results, which proved that the improved EAM potential function wasapplicable for numerical simulation of hypervelocity impact issues and theeffectiveness MD method was verified further.