Multiscale Dynamic Behavior Of Micron Scale Ductile Granular Materials Subjected To Impact Loading

The dynamic behavior of granular materials is involved in ammunition engineering,protection engineering,metallurgical industry and other applications,but many mechanical problems related to this process have not been completely solved,and some mechanical phenomena of granular materials are still controversial.In the macroscopic aspect of dynamic response of granular materials,the stress-strain relationship,overall deformation characteristics and motion law of granular system all affect the efficiency of engineering application.For example,the stress and deformation of granular system will affect the density and state of powder.In mesoscopic aspect,in the particle system and the structure,shape,distribution features of force chain affect the stability of the particle accumulation and force transmission in the macroscopic particles system,will affect the mechanical behavior of the system,in addition of mutual contact between the particles such as collision,such as adhesion,friction behavior is more complex,and affects the macroscopic behavior of the particle system and the micro deformation of particles;In the microscopic aspect,the elastic-plastic deformation and motion(translation,rotation,etc.)of particles also influence the distribution and stability of the chain.The aluminum powder with different particle size was loaded at different strain rates using split Hopkinson pressure bar.The stress-strain results show that aluminum powder has strain rate effect and size effect.When the particle size is smaller than 10 m,the strain rate effect of the powder is obvious.When the particle size is larger than 100 m,the strain rate of the powder is not obvious.It was found that the strain rate effect of the powder was related to the particle rearrangement,that is,when the loading speed was higher,the particles could not rearrange and reached a relatively dense state,and part of the load would be borne by the deformation of the particles.Therefore,the higher the loading speed,the greater the macroscopic stiffness of the powder,showing the strain rate effect.Under the same porosity,the rearrangement efficiency of powders with larger particle size is very low,and the load is all borne by the deformation of particles.Increasing the loading speed has little effect on the rearrangement of particles,so the strain rate effect of powders with larger particle size is not obvious.Under the same loading rate,the powder with smaller particle size is more likely to rearrange,so its macroscopic stiffness is lower than that of the powder with larger particle size,so the powder shows size effect.In the process of impact loading,a high-speed camera was used to capture the change of speckle position on the sample,and the strain field on the sample surface was calculated by digital image correlation method.At the initial loading stage,the deformation of the sample is relatively uniform,and the compressive strain and shear strain of the sample are small.At this stage,the pores in the sample are large,and the compression deformation of the sample is mainly completed by filling the larger pores with particles translational motion.With continuous loading,the compactness of the sample increases,the particle translational movement is limited,and the shear strain of the sample increases gradually,that is,the particle rearrangement caused by particle rotation and slip increases.The strain field shows that the deformation of the powder sample is gradually uneven and large strain appears locally.It is found that the particle rotation velocity increases obviously when the compaction wave front passes through.When the compactness of the sample is further improved,the contact between particles increases,the friction between particles and the deformation meshing of particles lead to the self-locking of particles,and the rotation speed of particles decreases.An infrared temperature measuring system was used to measure in situ the temperature variation trend of the sample during impact loading.The results show that the temperature variation of powder materials is different from that of solid materials.In the powder sample,due to the existence of a large number of pores,the particles will accelerate the movement after being pressed.At this stage,the impact energy of the rod is converted into kinetic energy of the particles,and the temperature of the sample rises slowly.After the sample is further compacted,the pores in the powder are reduced,the particle movement is limited,and its kinetic energy is transformed into deformation energy and friction energy,etc.At this stage,the energy in the particle increases,and the temperature of the sample rises rapidly.In solid materials,the sample is deformed immediately after being loaded,the internal energy increases,and the temperature rises,so the heating mechanism of powder and solid is different.In addition,the heat energy caused by the friction and collision between particles is only deposited on the surface of particles,and the temperature on the surface of particles is higher than the temperature inside particles,while the temperature measured in the experiment is the surface temperature,so the measured temperature is higher than the temperature calculated by the plastic work theory of samples.The powder was put into a PMMA sleeve and loaded by passive confining pressure.Under this condition,the sample was in a one-dimensional strain state.Three kinds of powder were used in the sample: aluminum powder,nonamide powder and silica powder.Under the passive confining pressure,the particles in the sample are easily deformed and meshed together to form a scab layer with relatively high density.The stress of ductile soft powder(nonamide powder)increases rapidly with the increase of strain when the powder density reaches a certain degree under high speed loading.Under the loading of brittle particles,the final strain of the sample is small,and the sample is difficult to compress.The propagation process of the compaction wave in the sample was photographed by a high-speed camera,and the propagation velocity of the compaction wave was calculated according to the position of the compaction wave front in the image.The compaction wave velocity is related to loading speed,particle size and porosity of the sample.In addition,with the increase of propagation distance,the energy of the wave front gradually loses,and the compacted wave speed decreases gradually.The powder samples were loaded with single-pulse split Hopkinson pressure bar,ordinary split Hopkinson pressure bar and passive confining pressure.After the experiment,the samples were recovered and the micromorphology of particles was observed by scanning electron microscope.Only under the normal split Hopkinson pressure bar loading,the sample particles were sintered together.Compared with the other two loading conditions,it was found that the friction between particles and the large deformation of particles provided energy for the particle sintering.Electron backscatter diffraction(EBSD)was used to characterize the internal microstructure of the samples.The results showed that recrystallization was found in the samples loaded by ordinary split Hopkinson pressure bar,and the sintering of particles was proved in the microstructure.In this paper,the dynamic response of ductile granular materials is studied from three aspects: macroscopic response,uneven deformation,uneven temperature distribution and sintering mechanism.The results are of great significance for establishing macroscopic constitutive relation of granular materials,promoting the development of powder metallurgy and improving the power of fire extinguishing bomb.