颗粒样品形变对声波传播影响的实验探究

摘要

为了更好地理解颗粒间接触结构的变化对通过颗粒介质中的声波的影响,本文利用单轴压缩实验,通过一系列增加的轴向压力使样品塑性应变不断增大,这在颗粒尺度上对应于颗粒间接触结构的改变.我们测量了此过程中通过颗粒样品的声波变化,结果表明颗粒体系内接触结构的变化对声波波形中的非相干波部分和频率有明显的影响,并且在样品接触结构变化的初始阶段声速是偏离有效介质理论的预测的.%Effective medium theory (EMT) predicts a scaling relation between sound velocity c and pressure P as c ∝(?Z) 13 (P/E0) 16 , where ? and Z are respectively the packing fraction and the mean coordination number of granu-lar material. In this relation, the granular contact network is represented via two simple parameters?and Z stemming theoretically from a strong approximation that microscopic and macroscopic granular displacements remain affine. This hypothesis simplifies tremendous computations for sound wave in a granular system, however some experimental results show that the scaling relation is recovered only for the case of very high pressure confinement (larger than 106 Pa for a glass bead system), but for the lower pressure case (less than 106 Pa) the relation does not hold. Owing to the fact that the change of microscopic granular displacement relates to the contact network variation of granular sample, and for better understanding the effect of the variation of contact network on the sound propagation in granular system, we conduct uniaxial shear experiments, in which the granular solid sample, composed of 0.28-0.44 mm glass beads, is cyclically compressed under a series of axial loadings (denoted as Pcomp). After these axial loadings, different contact networks of the sample are formed. Ultrasonic waves are then measured in the granular sample with these different con-tact networks under a constant axial pressure (denoted as Pobse). It is found that the axial deformation of the granular sample apparently affects the incoherent part of ultrasonic wave, but not the coherent part. A resemblant parameter is introduced to quantitatively discuss the variations of incoherent parts of sound waves in different axial deformations. In this paper, we also compare the frequency and the energy spectra of the sound waves, and find that the tendencies of their varying with the increase of axial deformation are nearly the same. This indicates that during the sound wave propagation in the granular solid sample, the processes of wave scattering and dissipation on particle contacted occur at the same time and the energy dissipation of sound wave in the air among particles can be neglected. In our experiments, compressional wave velocities based on time-of-flight method are also explored. The experimental results show that the velocity increases rapidly at the beginning of the axial deformation, and then tends to a steady value which is predicted by EMT. These illuminate that the variation of contact networks of granular sample may contribute to the deviation of velocity-pressure exponent from the prediction of EMT in low confining pressure.