Study Of Multiscale Numerical Simulation On Macro-meso Responses Of Materials Under High Velocity Impacts

Research on transient responses of materials under high velocity impacts is of great significance both in military and civilian fields.However,since high velocity impacts have extremely short time scales,complex loading conditions such as high temperature and high pressure,current experimental and theoretical studies on the transient responses of materials are difficult.Although there is unique advantage in solving aforementioned issues by using numerical simulation,it still faces challenges in predicting complex responses of materials and coupling the responses of materials between macro and meso scales.In this paper,some improvements on the existent numerical simulation methods were presented,and a multiscale numerical simulation method was developed.By using these optimized and developed numerical simulation methods,responses of material on three typical high velocity impact were investigated,including solid-solid impact,fluid-solid impact,as well as fragments formation and dispersion.This work provides an effective approach to further reveal the inner mechanism of material responses under high velocity impact conditions.A macro-to-meso multiscale numerical simulation method was developed,which was based on the idea of loading information transmission.Using of the platform of LS-DYNA software,loading information was extracted from the micro region of the macroscopic model and applied to the mesoscopic model.Thus,the responses of materials from macroscopic to mesoscopic can be correlated directly.Using this method,the adiabatic shear deformation process of Ti-6Al-4V microstructure was simulated successfully.The morphology of adiabatic shear band as well as the dynamic recrystallization temperature were predicted,which were consistent with the corresponding experimental results,illustrating the reliability and efficiency of this method.For the solid-solid impact problem,a stress-strain coupling failure method was proposed,which took into account the cumulative damage effect,and the corresponding failure criterion was established.On this basis,the penetration process of 30 mm Ti-6Al-4V target by 12.7mm armor piercing projectile was simulated,and typical failure features of the target,such as cratering,ductile hole enlargement and back spalling,were reproduced successfully.The results showed that the cratering and back spalling of the target were caused by the accumulation of tensile stress,while the ductile hole enlargement of the target was caused by the accumulation of plastic strain.The formation mechanism of the periodic adiabatic shear bands in the Ti-6Al-4V titanium alloy target was revealed.The simulated results showed that the strain of the elements along the central axis of the target at different locations increased periodically,which was consistent with the experimental observed periodic adiabatic shear band both in positions and in cycles.Further analysis showed that the effective stress of these corresponding elements was basically identical,while the hydrostatic pressure presented a similar distribution to the plastic strain.So,the periodic loading-unloading cycle of the hydrostatic pressure leads to the formation of periodical adiabatic shear bands in Ti-6Al-4V target.For the fluid-solid impact problem,the SPH method was introduced to solve this issue,and some improvements,such as model construction,quantity control of interacting SPH particles,artificial viscosity,contact and failure of SPH particle and so on,were conducted.Based on the optimized SPH method,formation and penetration process of the shaped charge jet of tungsten-copper liner were simulated.The results showed that the kinetic energy and tip velocity of the tungsten-copper jet reached 549.5KJ and 5845m/s,respectively.The metal target has a limited defense ability on jet.After penetration,the tip velocity of the jet declined about 45m/s and the kinetic energy of the jet remained up to 439 KJ.Besides,the morphology of the jet remained intact.In contrast,the explosive reactive armor had good protection performance on jet.After penetration,the jet was disturbed and its tip collapsed seriously.Although the kinetic energy of the jet was still high,the penetration capability of jet decreased sharply.By using the constructed multiscale numerical simulation method,the compatible deformation mechanism of tungsten phase and copper phase during liner collapse was investigated.The results showed that the deformation behavior of the tungsten phase and the copper phase were significantly different.The morphology of the tungsten phase changed a little and its mean strain was only about 0.12;the copper phase was stretched to a narrow strip and its mean strain reached 3.5.After deformation,the temperature of copper phase rapidly increased to 500℃~600℃,while the temperature of tungsten phase reached to 400℃~500℃.These results indicated that the copper phase had reached its dynamic recrystallization temperature while the tungsten phase had not.The recrystallization of copper phase provided optimum condition for compatible deformation of tungsten phase and copper phase.For the fragments formation and dispersion problem,the fragment formation method was optimized by node-split algorithm.Besides,the basic idea of fragment identification and the statistics of its quantity,mass,as well as velocity was also proposed.Using the optimized fragment formation method,the explosive and fragment formation process of the 40 CrMnSiB steel cylinder was simulated,and the results recaptured the typical features of the expansion and dynamic fracture of the cylinder under inside-explosive loading successfully.Based on the proposed idea of fragment identification and statistics,a second development of LS-DYNA simulated results was conducted by APDL language.In the reconstructed fragment field in ANSYS,all the elements were searched and assigned a fragment number.By this means,all fragments were identified and the exact quantity of the fragment was also obtained.The mass and velocity of each fragment were calculated by mass summation of each element and resultant velocity vector of each node that attached to the fragment,respectively.The results showed that,at the time of t=200μs,the number of the fragment in the calculated fragment field was 3840,and the maximum velocity of the fragments was 1517m/s.The statistical results of numerical simulation were quite agreement with the fragmentation test results of cylinder,and the reliability and efficiency of this statistical method were also verified.By using the established multiscale numerical simulation method,the microstructure responses of 40 CrMnSiB steel under explosive loads was investigated.The results showed that the microstructure of 40 CrMnSiB steel had good deformation capacity,which was mainly attributed to the soft ferrite phase and the dispersed precipitation of the fine carbide particles.For the enormous difference in mechanical properties between the ferrite phase and carbide particles,the distributions of stress and strain in the microstructure of 40 CrMnSiB steel were inhomogeneous.At the time of t=3.0μs,the effective stress in the ferrite phase was about 617MPa~958MPa,and the effective strain was about 2~3.In contrast,minimal deformation occurred in carbide particles,but the effective stress reached 1100 MPa.The study also found that the distribution of carbide particles had a significant effect on the deformation pattern of the microstructure of 40 CrMnSiB steel.These isolated carbide particles distributed in the ferrite matrix would interrupt the uniform deformation of the ferrite phase,and the serious strain concentration occurred near the ferrite phase/carbide particle interface,which would lead to cracking of materials.