关键词:颗粒材料;微形态模型;波传播;频散;频率带隙 Abstract The design of metamaterials is paid more attention to control the behaviors of the wave propagation based on response characteristics of shock and wave in granular materials, and it requires in-depth understanding of the propagation mechanism and control mechanism of waves for granular materials. The dispersion behavior and frequency band gap of granular materials are closely related to the heterogeneity. Generally, the dispersion behavior and frequency band gap are based on the elastic theory framework to establish a generalized continuum model including the microstructural continuum or the high order gradient continuum. This study proposes a micromorphic continuum model based on micromechanics for granular materials. In this model, the translation and the rotation of particles are taken into consideration, and the relative motion between particles is decomposed into two parts: the macroscopic mean motion and the microscopic actual motion. Based on this decomposition, a complete pattern of deformation is obtained. The macroscopic deformation energy is defined by a summation of the microscopic deformation energy at each contact. As a result, the micromorphic constitutive relation can be derived, and the corresponding constitutive modulus can be derived by microscopic parameters in terms of contact stiffness parameters and microscopic geometric parameters. The proposed model investigates the propagation of waves in an elastic granular medium, give dispersion curves for different waves such as longitudinal, transverse and rotational waves and predict the frequency band gap. It proves that the proposed model has the ability to describe dispersion behaviors and predict the frequency band gap in granular materials.
显示原图|下载原图ZIP|生成PPT 图4所建议模型与Misra模型[28]的横向剪切波、横向旋转波、横波与纵波的频散曲线对比图 -->Fig. 4Dispersion curves of the proposed model vs. Misra model [28] for transverse shear wave, transverse rotational wave, transverse wave and longitudinal wave -->
图4所示为所建议模型与Misra模型 [28]的横向剪切波、横向旋转波、横波与纵波的频散曲线对比图. 对于横向剪切波,频 散曲线基本重合. 对于横向旋转波,本文所建议模型多具有1条低频的频散曲线,而另1条基本重合. 对于横波,本文所建议模型多具有1条较低频的直线型频散曲线,而另外3条与Misra模型 [28]的频散曲线的变化规律相近,其中1条频散曲线基本重合,所建议模型低频的频率带隙比Misra模型[28]更窄,高频的频率带隙基本一致. 对于纵波,两个模型均具有3条频散曲线,且相互重合,频率带隙基本一致. 横向旋转波与横波同Misra模型所预测存在差异,而其他形式的波基本一致,主要因为本文所建议模型多考虑了一个独立的转动自由度,因而具备了一个考虑转动的变形模式,通过式(52)~式(54)可以反映出来,转动自由度主要影响横向剪切波与横波的频散方程. 图5所示为所建议模型预测频率带隙的宽度. 从图中可以看到,综合考虑横向剪切波、横向旋转波、横波与纵波的频散曲线,存在两个狭长带(图中红色与蓝色阴影区域),无论波数如何变化,期间均无任何频率的波能够在此介质传播,即此介质的频率带隙为5.13 rad/s 与6.34 ×10 rad/s. 对比于Misra模型[28]预测的频率带隙,总体上,频率带隙的宽度相一致,但横波的频率带隙的宽度,本文建议模型所预测要窄于Misra模型,这还是由于所建议模型具有多考虑转动的变形模式的存在所引起的. 显示原图|下载原图ZIP|生成PPT 图5所建议模型预测频率带隙 -->Fig. 5The frequency band gaps predicted by the proposed model -->
7 结论
颗粒材料的细观力学方法已经被应用于发展颗粒材料的微形态模型. 基于此方法发展的颗粒材料宏观连续体模型可以反映细观结构信息. 本文针对颗粒材料提出了一个基于细观力学的微形态模型. 首先采用Mindlin-Eringen的微结构理论 [29-30]进行运动分析,对颗粒的平动与转动均进行分解,分解为宏观平均运动(平动与转动)和细观真实运动(平动与转动)两部分,并认为颗粒的细观真实运动(平动与转动)是由宏观平均运动(平动与转动)与相对运动(平动与转动)组成. 基于此分解,求得相应的细观本构关系和细观变形能函数,并且宏观变形能由细观变形能求和得到. 由此可获得基于细观量表示的宏观本构关系与相应的本构模量. 基于所建议的微形态模型预测和探讨波在弹性颗粒介质中传播的现象,考察了纵波、横波、横向剪切波、横向旋转波的频散关系,并对其频率带隙进行预测,主要结论如下: (1)所建议模型能够模拟频散现象. 由于纵波、横波、横向剪切波、横向旋转波的频散关系分别受宏-细观尺度上的与平动项与转动项相关的参数的影响,其频散曲线各不相同. 横向剪切波具有1条平直的频散曲线;横向旋转波具有1条平直的频散曲线与1条呈双曲线衰减型频散曲线;纵波具有3条频散曲线,且与Misra [28]所预测的频散曲线基本重合;由于所建议模型具有考虑了转动自由度的变形模式,横波具有四条频散曲线,比Misra [28]所预测的多出一条低频的呈线性增长的频散曲线,另外3条变化规律相似且其中一条频散曲线基本重合. (2)所建议模型预测了弹性颗粒介质的频率带隙. 无论波数如何变化,频率范围为5.13 ×10 rad/s 与6.34 ×10 rad/s 的波均不能在此介质中传播,即为所建议模型预测的频率带隙. The authors have declared that no competing interests exist.
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