Study On Mechanical Response And Acoustic Wave Propagation In Granular Solids

Granular material shows discrete,macroscopic,and multi-body characteristics;it's one of the most widely existence material aggregation on our earth.Its discrete property makes it very different from the common material states;the granular material manifests a lot of peculiar phenomena,and it can show the properties like usual solid,liquid and gas,but not exactly the same as them,granular material arouse people's interests.Recently,the research on granular solid,especially for the jamming transition,achieves much more progress.Granular material in jammed state can resist shear at certain degree,and keep the solid state unchanged;when the jamming transition occurs,the granular material shows flow characteristic.These researches are helpful to deeply understand the accidents involving granular solid in industry(collapse of materials packing for example)and the prediction of geological disaster.The focus of present thesis mainly on two aspects: 1)the mechanical responses of granular solids sample under triaxial test;2)the experimental exploration of the factors on the sound propagation in granular solid sample,and also discuss the exponent transition of sound velocity pressure dependence in granular solid sample.Firstly,many previous works have already dedicated to the mechanical responses of granular solid sample under triaxial test.In our work,using different granular material(including the difference of the particle's size and the size distribution),different sample preparation procedures,and different control steps of loading,we explore the responses of stress-strain under triaxial compression,creep,and stick-slip phenomena.Owing to the granular solid sample consist of a large number of discrete particles,the properties of particles,such as roughness and polydispersity,would affect the macroscopic responses of granular solid sample(stress-strain curve for example);we have also observed the confining pressure,shear rate,and even sample preparation history,take effect on the stress-strain curve.In our experiments,we find the critical state of granular solid sample is similarly like the classical solid-liquid transformation point,and which is not influenced by the sample preparation.When the loading is less than the yield strength of granular solid sample,the creep phenomenon appears,and the axial strain increases slowly till to another steady solid state;if the loading is larger than the yield strength,the granular solid sample is compressed rapidly,and the sample presents like common liquid which cannot resist any shear.We realize this jammed state to unjammed state(fluidization)of our sample through triaxial compression,we think the fluidization in granular solid sample is not for the whole sample,but for some local area in the sample,which is not easy to dynamically observe and research for 3D granular solid system so far.Shear Transformation Zone theory(STZ)is the one that can be used to explain this local fluidization of granular solid system.Because of our sample is 3D system,we cannot obtain the inner information;and basing on some rough assumption,we attempt STZ to explain our experimental results.Secondly,we take the ultrasound wave as a tool to probe the inner structure and information of granular solid sample,and think the small amplitude ultrasound wave does not take essential influence on the skeleton of granular solid sample.We experimentally explore the sound propagation in granular solid sample under different conditions,such as different sample thickness,different state of strain,and different boundary conditions.We find the sample thickness contribute to the sound velocity-pressure dependence;as for a thicker sample,the sound velocity-pressure exponent is larger,this phenomenon is more obvious for shear wave.As for exploring the influence of sample strain on the sound propagation,we find the sound velocities increase at the beginning of the increase of sample strain,and then tend to steady values;so if we measure sound velocities at the beginning of granular sample strain,this would lead to the experimental results deviating from the steady values.This enlightens us to think that the sample density may take effect on the deviation of sound velocity-pressure exponent between the prediction of EMT and the measurements in low pressure range;this also advises us to properly prepress our sample before sound measurement.As for the different boundary conditions of granular solid samples,the hardness of the boundaries and the different pressures of the boundaries putting on the sample would affect the shape of coherent part of shear wave,but not change the sound velocity-pressure exponent for the initial loading.Finally,because of the shear wave velocity usually used as a symbol for the formation of steady shear band in granular solid,and the reason of sound velocity-pressure exponent deviation from EMT still indefinite,we specially focus on the sound velocity in granular solid sample.We adopt different cyclic loadings and boundaries in our experiments,and find,in uniaxial condition,with the cycle loading increases,the sound velocity-pressure exponent transition shows more obvious,this is not happened in isotropic condition.We take the sample density,coordination number and shear stress ratio into consideration in EMT,and find the deviation between calculated results based on the amendatory EMT and our experiments still exists.The source of the complexity of sound propagation in granular solid sample is from the sensitivity and complexity of inner contact structure network in granular solid sample,and also from its heterogeneous of force chains.When loading on the granular solid sample and cause one strong force chain broken,many weaker force chains may form at the same time to support the sample in another metastable state;and many contact points,which do not contribute to sound propagation before,start joining to propagate sound wave.This is a nonlinear process,and EMT is not enough to explain.