Multiscale modeling of grain crushing in granular media

Abstract

Crushable granular materials are frequently handled in multiple fields across geotechnical engineering, mining industry, food and pharmaceutical industry, and chemical industry. Grain crushing in granular media presents an important micro-mechanical origin to many macro-mechanical behaviors of granular materials, including packing, strength, dilatancy, compressibility, and permeability. However, understanding on this important physical phenomenon has long been handicapped owing to the complexity of the phenomenon and limited capability in both experimental measurements and numerical modeling. In this thesis, focuses are placed upon developing novel computational approaches for rigorous and efficient modeling of crushable granular materials, and providing micro-mechanical insights into the grain crushing phenomenon of granular media pertaining to key relevant engineering applications. Key to the new computational framework is the employment and development of peridynamics, an emerging numerical approach, for modeling breakage of individual grains. The capability of the method is first demonstrated through simulations of the breakage of single sand particle under two or multiple contacts as it is in a granular assembly. Simulation results show reasonable agreement with experimental records with respect to particle strengths and crushing patterns. In examining the most appropriate criterion for predicting single particle crushing condition under multi-directional loadings, the study confirms that a maximum contact force based criterion performs better for gauging particle crushing in simplified models than many other criteria, such as those based on octahedral shear stress, mean principal stress, maximum tensile stress, or crushing energy. With an idealized material which is assumed to be elastic isotropic and homogeneous, a particle is found often to break into two to five pieces of major fragments while fine fragments can be numerous. For a given number of fragments, their size distribution may be reasonably described by a Gamma distribution function. The study is further extended to simulate continuous grain crushing in a granular assembly. A novel hybrid computational framework is established by combining peridynamics with non-smooth contact dynamics. The framework is of multiscale nature where particle level phenomena including initiation and propagation of cracks are handled by peridynamics while the physics on a representative volume element level, including interactions and motions of grains, are handled by the non-smooth contact dynamics. The computational framework also features novel implementations of statistical distribution and size effect on particle strength. The thesis presents simulations with two sets of codes developed with different implementations of non-smooth contact dynamics, both validated through simulation of one-dimensional compression of sand. The simulation results exhibit reasonable agreements with past experimental data in terms of particle size distribution, fractal dimension, normal compression, and particle shape. With the proposed hybrid computational framework, a thorough micro-mechanical study has been carried out to gain insights into particle shape evolution during continuous grain crushing process and particle shape effect on crushing characteristics, through simulation of a typical one-dimensional compression on sand. The study shows that distribution of the shape of fragments tends to approach a steady state profile after substantial grain crushing. The shape of grains exerts strong influences on crushing characteristics including strength, crushing energy, and crushing pattern. Sphericity is found to be a factor that well describes such shape effect. Particles with higher sphericity generally exhibit higher strength and crushing energy, and produces relatively more fragments under the mode of major splitting. The shape of fragments is found to be affected by the shape of their parent particle. Sphere-like particles tend to produce fragments that also possess relatively high sphericity and vice versa. The results from the simulations are further linked with continuum breakage mechanics to discuss energy consumption on particle crushing. It is found that with progress of breakage, the energy required to further advance breakage increases. The total energy consumption on particle breakage can be approximated by a quadratic function. A parametric study is performed to learn the influence of material properties such as inter-particle friction and elastic parameters on particle crushing behaviors. The computational framework outlined in this thesis helps to provide novel multiscale modeling techniques for crushable granular materials and open gateways to understanding the complicated physics and mechanics governing the crushing of granular media related to important engineering and industrial processes.

Date of Award	2019
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology