垂直载荷下颗粒物质的声波探测和非线性响应

Acoustic detection and nonlinear response of granular materials under vertical vibrations

对于玻璃珠组成的颗粒介质样品,本文测量了横波和纵波声速,同时分析了剪切模量(G)与体积模量(B)的比值(G/B)随压强的变化规律.结果表明,在低压强下,颗粒体系的纵波声速(C_L)明显大于横波声速(c_T),且体系的CL,CT及G/B均随压强p变化呈幂律标度,即CL∝p^(0.3817),CT∝p^(0.2809)G/B∝p^(-0.4539),幂指数与文献[1]中预言的-1/2非常接近,暗示在我们实验压强范围内的颗粒样品处于L玻璃状态.此外,本文还利用快速傅里叶变换法测量了玻璃珠样品中的声学衰减特性及二阶谐波随压强的变化,发现:纵波声衰减系数(α)、接收端二倍频振幅(μ_(2ω))与基频振幅(μ_(1ω))平方的比值(μ_(2ω)/μ_(1ω)~2)均随压强的增大而幂率减小,分别为α∝p^-(-0.1879),和μ_(2ω)/μ_(1ω)~2∝p^(-0.866).

Owing to their efficient penetration into elastic media, the measurement of sound waves can provide a sensitive probe of both the structural and mechanical properties of the materials through which they propagate. In this work, we first investigate the transversal and longitudinal wave velocities in granular assemblies composed of glass beads under uniaxial load by the time-of-flight method. Then the ratio G/B, （G is the shear modulus and B is the bulk modulus） as a function of pressure is analyzed, based on the theory of classical elasticity. Experimental results show that, with the pressure increasing from 10 to 100 kPa, i） the velocity of longitudinal wave （CL） is obviously faster than that of transversal one （CT） in the granular system（the ratio CL/CT is about 1.6）, and the CL and cw of the system show power law scaling, i.e. CL ∝ p0.3817, CT ∝ p0.2809; ii） the ratio G/B decreases in the low pressure range for glass beads packing, i.e. G/B ∝ p-0.4539. It is found that the power-law exponent of G/B with pressure is very close to -1/2 （the prediction in 2015 Phys. Rev. Lett. 114 035502）, suggesting that the granular system lies in glass L state within the pressure range in our experiment. Furthermore, the fast Fourier transform method is used to study the variation of acoustic attenuation and nonlinear characteristics in granular materials. Our results reveal that the acoustic attenuation coefficient （α） and the ratio of the second harmonic amplitude （μ2ω） to the square of fundamental amplitude （μ1ω） at the receiving end in the granular system, μ2ω/μ^2_1ω, both decrease in power law with the increase of pressure, i.e. α ∝ p-0.1879, μ2ω/μ^2_1ω ∝ p-0.866, respectively.