| The service life and usability performance of the explosive is directly affected by the quality of explosive crystal.However,in the process of preparation and processing,In the process of preparation and processing,it is inevitable that heterogeneous molecules,micro-voids and other micro-damages will be introduced to degrade the crystal quality of explosives,which will destroy the integrity of crystal structure at the micro scale,and then affects its stability,safety,mechanical properties and sensitivity in the process of use,etc.Benefit from the rapid development of parallel computing technique and molecular dynamics(MD)methods,the macroscopic properties of HMX crystal can be studied through constructing the microscopic model of HMX crystal,and the structure changes at the molecular scale can be observed in real time.The changes of stability,sensitivity and mechanical properties of HMX crystal containing heterogeneous molecules(?/?/y-HMX,RDX)were calculated by using Forcite module in Materials Studio software and LAMMPS program was applied to study the influence of the micro-void on micro-structure evolution under tensile loading.The main contents and conclusions of this work are as follows:(1)In this paper,the simulation model of ?,?,?-/?-HMX biphasic mixture systems under different mole ratios are constructed by Materials Studio software.The results show that the order of the binding energies of ?/?-HMX,?/?-HMX and?/?-HMX is 1:1>1:3>1:5>1:7>1:11>1:9,which indicates that the mixture systems have a more stable structure at low mole ratio,especially at 1:1 molar ratio,the binding energy of all mixture structures attain the maximum values.After forming mixture structure,the excited bond length does not change significantly,but the cohesive energy density(CED)value decreases gradually with the increase of the?-HMX molecule content,indicating that the more disordered the explosive structure,the higher its sensitivity and the lower its safety.At the same time,when the molar ratio rises from 1:1 to 1:11,the E,K and G of the mixture system show an obvious downward trend,while the Cauthy pressure(C12-C44)and K/G increase,which indicates that the deformation resistance of the mixture explosive is weakened,and the ductility and toughness are enhanced.(2)For the mixture system of different explosives,the mixture model of ?-HMX and RDX at 1:1 molar ratio was constructed in this paper.Meanwhile,for comparison,the co-crystal structure of HMX/RDX with the same molecular molar ratio was constructed as well.The results demonstrate that as the temperature increases from 220K to 380K,the binding energy of HMX/RDX mixture and co-crystal system gradually decreases,thus the thermal stability deteriorates.At the same time,the binding energy of mixture is much lower than that of co-crystal,so the mixture structure is more unstable.With the increasing temperature,the maximum bond length of ?-HMX single crystal,mixture and co-crystal(LN-NO2)increase continuously,and the cohesive energy density(CED)decreases monotonically,which indicates that the temperature is negatively correlated with the sensitivity of the explosive,and the high temperature makes the sensitivity of the explosive increase and the safety becomes worse.Furthermore,comparing the values of CED and LN_NO2,the sensitivity of mixture is worse than co-crystal.The Young's modulus(E),volume modulus(K)and shear modulus(G)of the mixture and co-crystal models are all less than those of?-HMX and RDX single crystal in the whole simulated temperature range,so the mixture structure and co-crystal of HMX/RDX has weaker deformation resistance.Compared with co-crystal,mixture explosive has lower K/G and C12-C44 values,which implies that mixture explosive has worse ductility and toughness.Generally speaking,HMX/RDX mixture has worse performance,and co-crystallization of HMX and RDX can be considered to improve their performance.(3)Micro-void is one of the most common forms of damage in HMX explosive crystal.In this paper,the LAMMPS program is adopted to construct the damage model containing micro-void in HMX crystal.The molecular dynamic mechanics(MD)method is utilized to explore the effect of micro-void size on stress distribution,mechanical properties and breaking mechanism in explosive crystal under tensile loading.The results show that the existence of micro-void reduces the breaking energy barrier and maximum breaking stress of the system,and with the increase of the radius of the micro-void,the ultimate yield stress and yield strain of the model gradually decrease,which indicates that the increase of micro-void radius makes the crystal more likely to break.From the yield stage of the stress-strain curve,it can be seen that HMX perfect single crystal reaches the minimum stress value under a very small strain,while the damage models go through a relatively slow process.The results suggest that the perfect of single crystal breaking displacement is small,the breaking model is brittle break,while the values of the damage models are large,and the breaking mode is plastic break.With the increasing degree of stress concentration,micro-voids,plastic deformation areas and other microscopic damages begin to appear in the system.The micro-void grows and expands along the direction of stress concentration,and gradually coalesces with the main crack to accelerate the failure of HMX crystal.Under tensile loading,the porosity rate in the single crystal model remained unchanged until strain ?=0.10,while the damage model with r=3nm begins to grow from ?=0.035.Therefore,the larger the micro-void is,the more vulnerable the system is to break.(4)For damage model with double micro-voids,the breaking barrier,the maximum breaking stress and ultimate yield stress of the models all increase with the increasing micro-void spacing,indicating that the smaller the distance between the micro-voids,the more likely the HMX damage model is to break.The reason is that the model with smaller void spacing is more likely to gather stress around the micro-voids,which makes the internal stress of the system more easily concentrated.Moreover,the interaction between the micro-voids is enhanced during the tensile process,which makes the system more vulnerable to damage.It can be seen from the yield stage of the stress-strain curve that the stress decreases quickly with the changing strain as the spacing between the micro-voids increases,and the residual stress in the model with smaller void spacing is less after the system is completely failed.This is because the interaction of the micro-voids makes the system easier to penetrate.As the void spacing is large,a large number of plastic deformation areas generate in the system,which is the plastic break mode.Due to the continuous concentration of internal stress in the system,such micro-damages as micro-void and sub-crack are generated continuously along the crack propagation path,and grow and expand continuously under the following tension,and gradually coalesce with the main crack,accelerating the failure of HMX explosive crystal.When the system at the elastic deformation stage(?<0.05),the microstructural changes are mainly the formation of large number of point and linear plastic deformation areas.At the plastic stage,the stress concentration area around the micro-voids increases,and the concentration degree is higher.Meanwhile,the void fraction in the system increases rapidly.What's the difference is that as the micro-void spacing is larger,the interaction between the micro-voids is weaker,resulting in a large number of plastic deformation areas under tensile loading in the system,which makes the breaking mode mainly plastic breaking.(5)For HMX explosive crystals containing multiple micro-voids,the increase of the number of micro-voids makes the elastic modulus,ultimate yield stress,breaking barrier and maximum breaking stress of the system all decrease,demonstrating that the increasing number of micro-voids makes the system more prone to deform and break.What's the reason is that in the tensile process,the model with more micro-voids can accumulate stress around the micro-voids more easily and quickly,making the stress in the system more easily to concentrate.When the strain of the system attains a certain level,the stress concentration area around the micro-void will gather together.Under the further tensile loading,plastic deformation areas are easily formed,which accelerates the penetration between micro-voids and causes the formation of crack in the HMX crystal.In addition,micro-voids and sub-cracks are formed continuously along the crack propagation path,which grow and expand along the direction of stress concentration under subsequent tension and gradually coalesce with the main crack to accelerate the failure of HMX explosive crystal.At ?<0.03,the system is at the elastic strain stage,and the change of internal microstructure is mainly the formation of many plastic deformation areas in the style of point and line.At?>0.03,the five models gradually enter into the plastic deformation stage,and the stress concentration areas around the micro-voids in the system are more and more concentrated,which increases the void fraction in the system rapidly.As the system enters the yield stage,the models accelerate the breaking under tensile loading,and the breaking process is dominated by the perforation between micro-voids.Herein,the model with 3 micro-voids is the most prominent,which completely breaks when the strain is 0.26674. |
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