密集颗粒体系的颗粒运动及结构测量技术

颗粒材料由大量粗颗粒堆积形成, 是复杂的多体相互作用体系, 呈现出颗粒尺度的结构不均匀和动力学不均匀性的基本特征, 这决定了颗粒材料具有很多独特的宏观性质. 借鉴学科历史的发展途径, 基于统计力学, 从颗粒结构和动力学开始建立颗粒材料体系的宏观连续介质力学理论框架是必然途径.但是, 颗粒材料的基本特征决定了从基本理论到实验手段上, 表征与建立颗粒材料结构与性能的相关性都极其困难.这是由于现有测试分析手段所描述的颗粒系统组织结构过于简单化, 缺乏对颗粒结构和动力学的真正认识, 从而制约了颗粒物质研究的发展.因此, 开展颗粒体系结构和动力学性质的测量, 是理解和认识颗粒材料重要物理和力学问题的基础和依据.笔者来自不同的科研院所, 近十年来开展了颗粒体系结构和动力学性质的测量研究, 主要集中于以下两个方向: (1)数字图像测速法、散斑能见度光谱法和X射线- CT等非侵入式测量技术在颗粒运动方面的应用; (2)体积响应谱、力学谱(有效质量和内耗等)和声速测量技术等直接或间接测量颗粒接触力和颗粒结构技术.本文综述了这些实验手段的基本原理及其特点、取得的主要成果, 以及国际最新进展和困难. 最后是对全文的总结, 结合笔者开展测量的经验和教训, 提出了自己的看法, 并试图展望颗粒材料测量技术研究的前景. 

Measurement techniques of grain motion and inter-grain structures in dense granular materials

A dense granular material is composed of a large number of densely packed coarse grains, which is widely encountered in nature, engineering and industries, such as granular debris flows, rock-filled dams, and pebble beds in nuclear reactors. It is a typical many-body system with intrinsic characteristics of structural and dynamic heterogeneities which leads to the complicated macroscopic behavior. By developing the statistic mechanics of structure and dynamics of grains, continuum theories of granular materials can be established and such macroscopic behaviors are then expected to be described. However, it is very difficult to fulfil these objectives because of the limitations of present measurement techniques and theoretical basis. The present measurement techniques are only applicable to study ideal and simple structure problems, and do not work well for granular structures. The shortage of granular structures properties makes the connections between structure and macroscopic properties impossible. Therefore, measurements of heterogeneity in structural and dynamics are the foundation for understanding the complex bulk properties of granular materials. The authors of this paper, from different research institutions, have conducted thorough experimental studies in the past decade, which can be classified into two categories: (1) non-invasive methods, mainly including digital image velocimetry, speckle visibility spectroscopy and Xray-CT, are developed to measure the moment of single grains; (2)response spectrum methods, mainly including volume response spectrum, mechanic response spectrum and acoustic measurements, are developed to directly or indirectly detect the inter-particle contact force. In this paper, we review the fundamental principles of these methods and their limitations, and state-of-the-art studies. Finally, we conclude the paper with our experience sand lessons learned, and provide suggestions for the future work.