Investigating sand inelasticity and breakage alongside its role in enhancing rail adhesion

Abstract

Understanding the strength of particles is essential for enhancing the efficiency, sustainability, and performance of applications involving their breakage. Despite valuable insights gained from extensive laboratory testing, a shared agreement on crushing strength estimation has yet to be achieved, as the geometric and material anisotropy, combined with inherent heterogeneity, pose significant challenges to the standardisation of testing methods. This thesis aims to advance state-of-the-art research by developing a micro-scale computational framework to investigate the inelasticity and breakage behaviour of sand particles. The framework begins with image acquisition, where the morphology of natural sand particles is virtualised using micro-computed tomography. To simulate their mechanical response, a combined discrete-finite element method is employed to represent them as continuum bodies, while capturing fragment-to-fragment interactions during breakage. This integrated framework is referred to as the micro finite element (µFE) analysis. Validation of the µFE model yields results that compare well with experimental data from the Brazilian tests and the single particle compression tests. The breakage behaviour of single particles is further examined through a point-loading test using spherical indenters with different radii. The actual contact area between particle and indenter normalised by its corresponding Hertzian area is found to be independent of indenter size, providing new insights into harmonising variations in crushing strength estimated using different indenters. One application directly controlled by sand breakage is the adjustment of adhesion level at the wheel-rail interface. By reproducing the train operation environment, sand breakage under extreme load is studied, and its effect on adhesion enhancement, considering surface roughness and plastic deformation of the rail, is explored. It reveals the fact that adhesion enhancement is primarily influenced by the number of fragments at the wheel-rail interface, which is directly related to the newly generated surface area.