| Granular medium are omnipresent in the natural word,and our daily life,as well as in industrial and agricultural production,many natural phenomena(earthquake,avalanche)are related to the changes of structure and dynamics caused by disturbance in granular materials.Due to the sensitive penetration inside elastic media,valuable information of the internal structure of a compressed granular medium can be extracted by studying the elastic wave.As a response to the mechanical properties of the media,elastic waves offer a very useful,and sometimes unique,technique for investigation of granular assemblies through which they propagate.The study of acoustic properties in granular materials is of great significance to eliminate geological disasters and reveal material properties.In this paper,the discrete element method(DEM)is used to calculate the sound velocity of spherical particle system under different normal stresses,and the relationship between stress and sound velocity of particle system is investigated.On the basis of the consistence between simulation and experiment results,the microscopic parameters,which are necessary for the calculation of sound velocity by Granular Solid Hydrodynamics(GSH)and the effective medium theory(EMT),are obtained by DEM.Furthermore,the relationship between sound velocity and stress is explained by the two theories.On this basis,the influence of particle shape and boundary condition on the sound velocity is analysed.The grid method is used to determine the contact points among non-spherical particles and different-shaped boundaries.Under different normal stresses,the velocities of spherical and super-ellipsoidal assemblies confined in cylinder and spiral tube are calculated.Both in cylinder and in spiral tube,the sound velocity of the super-ellipsoidal particle system is greater than that of the spherical particle system.Both in spherical assembly and in super-ellipsoidal assembly,the sound velocity of cylindrical boundary is greater than that of spiral tube boundary.The influence of shape-induced anisotropy on sound velocity is explained from the perspective of contact force distribution,and the magnitude of anisotropy reflects the change of sound velocity.Finally,GSH theory is used to calculate the changes of stress and sound velocity with strain in triaxial compression particle system.With the increase of strain,the stress firstly increases and then decreases and finally tends to be stable,while the sound velocity decreases rapidly and then tends to be stable,which is in agreement with the experimental finding.GSH theory,combined with DEM,is a powerful method to understand the relationship between the formation of shear band and the change of sound velocity.This paper provides references for probing the structure and property of granular material by elastic wave. |
