An Investigation of Breakage Behaviour and Micro-Mechanics of Sand Particles Using Digital Imaging Techniques

利用數字圖像技術進行的砂土顆粒破碎及微觀力學性質的研究

Student thesis: Doctoral Thesis

View graph of relations

Author(s)

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date24 Aug 2017

Abstract

In the past decades, the macro mechanics of geomaterials have been well studied by means of “element” laboratory tests as well as numerical analyses. Despite using macroscopic element testing, the role of single particle failure and the prevalent micro-mechanisms in the macroscopic mechanical behaviour and deformation properties of granular materials have been recognised and relevant research has been initiated. However, there is still little agreement on particle breakage criteria and the mechanisms of breakage still remain uncertain, partly because of the often rapid brittle-failure of sand particles and the complex stress field inside.
This study aims at a better understanding of the microscopic behaviour of natural sands by means of experimental methods utilizing digital imaging techniques. Three different aspects are explored though the thesis. In the first part, a series of uniaxial compression tests including platen loading tests and point loading tests were performed and discussed on single sand particles. Monitoring by a high speed camera combined with a microscope lens, the breakage behaviour corresponding to the mechanical response of a single particle can be obtained. Different breakage modes for single particles and the mechanical properties of each mode were proposed. Meanwhile, the crack initiation and propagation processes in the particle can be observed. Subsequently the crack initiation location related to different failure criteria was then studied based on statistical analysis. The effect of single particle morphology, including aspect ratio, sphericity and roundness on the mechanical response was instigated. A new shape descriptor “local roundness”, which focuses on the sharpness at the contact area of the single particle with the loading platen, was proposed and quantified to highlight its influence on the breakage mode and strength.
In the second part, the characteristics of single particle subjected to multiple contacts during compression were explored. Using the high speed microscope camera, the breakage modes for single particles in different loading configurations were investigated. The important role of the hardness of the particle mineral in the crushing behaviour and the resultant strength was revealed. The influences of particle morphology and coordination number were also emphasized.
At last, the breakage properties of artificially cemented particles in different loading paths were investigated. Different cement agents and morphologies of sand particles at the contact with the bond were tested. A novel preparation method for cemented particles was developed, which was challenging considering the very small scale. Finally, an innovative apparatus was designed and built, which enabled the cemented particles to be tested under complex loading paths: compression, compression combined with shearing as well as compression combined with shearing and bending. With the assistance of a digital microscope, the breakage behaviour of cemented particles under different loading patterns was investigated and quantified.

    Research areas

  • single sand particles, cemented particles, breakage behaviour, micro-mechanics of soils, experimental methods