Micromechanical investigation of particle breakage behavior in confined compression tests

Abstract

This paper aims to give a micromechanical investigation into the progressive size reduction mechanism and its effects on macro responses of granular assemblies during confined compression tests. A series of numerical studies based on the cohesive zone model are presented, incorporating internal deformation and fragmentation behaviors, and the variations of feed profiles are considered. Simulation results reasonably reproduce experimental observations in terms of macro compression responses and typical particle breakage modes. A scaling method is proposed for considering the contributions of fine products. With this method, the fragment size distribution is refined and compare favorably with physical results. The breakage degree is found to be dependent on input energy, regardless of variations of feed profiles including material heterogeneity, gradations, or friction conditions. Furthermore, a quantitative micro-analysis of normalized and cumulative normal contact force distribution, as well as contact orientation distribution is presented in this study, demonstrating the different roles of fragmentation in micro-mechanical response of particle assemblies owing to different materials or boundary friction effects.

Introduction

Particle breakage is frequently encountered in engineering applications. The fragmentation behavior is influenced by feed particle size, gradation, mineralogical composition, as well as loading and boundary conditions, etc. Extensive experimental studies have been presented in literature for investigating breakage behaviors of particle assemblies, such as one-dimensional compression, triaxial compression, rolling compression or shearing tests (Lee and Farhoomand, 1967, Lade et al., 1996, Bagherzadeh-Khalkhali and Mirghasemi, 2009, Casini et al., 2013, Chi et al., 2015). Recently, some advanced monitoring techniques have been applied for investigating internal microstructures during particle fracturing, such as the high-speed microscope camera, X-ray micro-topography, etc. (Wang and Coop, 2016, Zhao et al., 2015). However, an in-depth view of the internal progressive size reduction process at the microscale is still hard to obtain by merely physical tests.

There has been growing interest in numerically modeling particle breakage and investigating the macroscopic response of particle assemblage from a microscopic viewpoint, which is difficult to achieve in physical experiments. DEM simulations have been frequently utilized in modelling particulate materials. In terms of modelling methods of breakage behavior, one solution is the bonded particle methods pioneered by Robertson (2000), where the granule is treated as agglomerates formed by bonding an assembly of elementary rigid balls. McDowell and Harireche (2002), as well as Cheng et al., 2003, Cheng et al., 2004 used this approach in modelling grain crushing individually or in isotropic compression tests and described statistical crushing strength by introducing the Weibull theory. On this basis, some other scholars (Jiang et al., 2005, Wang and Yan, 2013, de Bono and McDowell, 2014) went on investigating the effects of particle breakage on macroscopic responses of granular assemblies. Another strategy is replacing the original grain with an equivalent group of smaller particles once the pre-defined breakage criterion was fulfilled (Aström and Herrmann, 1998, Tsoungui et al., 1999, Lobo-Guerrero and Vallejo, 2005, Hosseininia and Mirghasemi, 2006). For instance, Tsoungui et al. (1999) focused on an individual grain subjected to an arbitrary set of contact forces and replaced the broken grain with a set of twelve fragments of four different sizes. Lobo-Guerrero and Vallejo (2005) further simplified the criterion of crushable materials loaded under uniaxial compression. DEM could reasonably characterize micro-mechanical response of granular materials. However, there is no consensus on whether particle breakage can be reliably simulated based on the particle-type DEM. Also, such type of methods have certain subjectivity in selecting appropriate bonding parameters and particle replacement criteria, and their limitations in reproducing realistic grain shapes were also reported in literature.

In past years, some coupled FEM-DEM approaches have been proposed in simulating particle breakage behaviors combining the advantages of both methods (Munjiza, 2004, Ma et al., 2014). Recently, some other numerical methods, such as Peridynamics, a nonlocal mesh-free method has been applied and offered new insights into the crushing behavior of a single sand particle (Zhu and Zhao, 2019). However, this method has some limitations in computational efficiency for elastic problems, particularly considering complex material models and particle morphology. Even though quite a few numerical work has been presented in literature for characterizing particle breakage behaviors, more in-depth investigation is still demanded in terms of the progressive size reduction mechanism from the microscopic viewpoint. In this paper, the particle breakage process and underlying mechanisms under one-dimensional (1D) compression is numerically investigated based on the cohesive zone model, incorporating the deformable material properties. Typical variations of factors such as particle size, material heterogeneity, and friction coefficient are considered in the study. A comparison is presented between numerical and physical responses in terms of load-displacement plots, normal compression line, global or local particle breakage modes, as well as particle size distributions. A scaling method is proposed for considering the contributions of fine products, which could overcome the limitation of traditional numerical simulation. Furthermore, the progressive evolutions of grain size distribution, particle contact force are discussed at a microscopic scale. The work presented herein forms part of an on-going micro-mechanical investigation into fundamental aspects of progressive particle fragmentation behaviors.