Xia Li

University of Nottingham | Notts · Faculty of Engineering

Wet granular materials are three-dimensionally simulated by the Discrete Element Method (DEM) with water bridges incorporated between particles. The water bridges are simplified as toroidal shapes, and the matric suction is constantly maintained in the material. A comparison with experimental tests in the literature indicates that the toroidal shap...

Formation of fluidised bubbles in a fluidised bed has significant impact on the design and scale-up of the bubbling fluidised bed. To fully understand bubble motions in the fluidised bed, fluidised bubble dynamics in a gas-solid fluidised bed is investigated using CFD-DEM numerical simulation. To reveal the mechanism of bubble formation in the bubb...

In this paper, we explore the particle-scale origin of the additional shear strength of unsaturated granular materials in pendular states induced by the capillary effect by applying the Stress-Force-Fabric (SFF) relationship theory into unsaturated granular material stress analysis. The work is based on Discrete Element simulations with the particl...

Bubble fluidisation in the fuel reactor adopted in chemical looping combustion (CLC) has a siganificant impact on the operation efficiency. Although a variety of numerical modellings of fluid dynamic process in the fuel reactors have been conducted, studies on predicting the fluidised bubble behaviours in the cylindrical fuel reactor where the effe...

Chemical looping combustion (CLC) process involves the combustion of gaseous fuel through heterogeneous chemical reaction with the oxygen carriers, usually granular metal oxides, in the fluidized bed fuel reactor. It has been recognized that the reaction kinetics of granular metal oxide in the fuel reactor affects the gas-solid flow behaviour signi...

The importance of internal structure on the stress-strain behavior of granular materials has been widely recognized. How to define the fabric tensor and use it in constitutive modeling, however, remains an open question. The definition of fabric tensor requires (1) identifying the key aspects of structure information, and (2) quantifying their impa...

This paper presents an experimental investigation revisiting the anisotropic stress–strain–strength behaviour of geomaterials in drained monotonic shear using hollow cylinder apparatus. The test programme has been designed to cover the effect of material anisotropy, preshearing, material density and intermediate principal stress on the behaviour of...

This paper presents a numerical investigation on the behavior of three dimensional granular materials during continuous rotation of principal stress axes using the discrete element method. A dense specimen has been prepared as a representative element using the deposition method and subjected to stress rotation at different deviatoric stress levels...

This study explored the features of particle motion and, in particular, the associated movements at contacts in response to direct shearing. A tailor-made direct shear box equipped with an image system was set up. The particle image velocimetry (PIV) technique and close-range photogrammetry were used together to help with the investigation. The tes...

This study presented the details of a specially designed biaxial testing system with a flexible boundary to examine the features of particle motion and associated contact movement during shearing. The whole testing system was modified from a CKC triaxial testing system so that the production cost was low, and this approach was applicable to other t...

This paper presents an experimental investigation revisiting the anisotropic behavior of geomaterials in drained monotonic shear using Hollow Cylinder Apparatus. The test program has been designed to cover the effect of material anisotropy, pre-shearing and material density on the behavior of Leighton Buzzard sand. Experiments have also been perfor...

This article presents the objective of current research to conduct experimental testing on the chosen set of granular materials composed of sand, silt and clay as the reference test materials used in the project, and to develop the advance constitutive model to predict the stress-strain behaviour of materials with diversified properties.

Rotational shear is the type of loading path where samples are subjected to cyclic rotation of principal stress directions while the magnitudes of principal stresses are maintained constant. This paper presents results from an experimental investigation on the drained deformation behaviour of saturated sand in rotational shear conducted in a hollow...

This paper presents an experimental investigation revisiting the anisotropic stress–strain–strength behaviour of geomaterials in drained monotonic shear using hollow cylinder apparatus. The test programme has been designed to cover the effect of material anisotropy, preshearing, material density and intermediate principal stress on the behaviour of...

Chemical looping combustion (CLC) as a potential CO2 capture technology has been considered as a promising and likely alternative to traditional combustion technology to mitigate the CO2 emission due to its prosecution of CO2 sequestration at a low cost. Although a number of studies on the hydrodynamic behaviours of the CLC process in fuel reactor...

The micro-mechanism of the capillary strengthening effect is studied in this paper with the aid of Discrete Element Methods (DEM) and the Stress-Force-Fabric (SFF) relationship. For an unsaturated granular material in pendular regime, pore water is discontinuously distributed at contact points in the form of capillary bridges, leading to the capill...

In micromechanics, the stress-force-fabric (SFF) relationship is referred to as an analytical expression linking the stress state of a granular material with microparameters on contact forces and material fabric. This paper employs the SFF relationship and discrete element modelling to investigate the micromechanics of fabric, force and strength an...

This paper explores the mechanism of deformation noncoaxiality from the particle scale.Amultiscale investigation has been carried out with the particle-scale information obtained from discrete element simulation on granular materials. The specimens were prepared anisotropically and sheared in various loading directions. The deformation noncoaxialit...

This paper presents an experimental investigation into the non-coaxial behaviour of saturated sand in rotational shear. A series of drained rotational shear tests has been performed on dense Leighton Buzzard sand in a hollow cylinder apparatus. The degrees of non-coaxiality defined as the deviation between the major principal stress direction and t...

In this paper, the results of three series of drained tests carried out on sands using hollow cylinder apparatus are presented. The noncoaxiality, defined as the difference between the major principal stress direction and the corresponding principal strain increment direction, is investigated. In the first series of tests, the sand was isotropicall...

This paper presents results of multi-scale investigations on granular materials behavior. The statistics of material fabric and particle interactions are analyzed using the directional statistical theory. The findings are used to establish a simplified form of the Stress-Force-Fabric (SFF) relationship, and lead to the micromechanical explanation o...

This paper employed the theory of directional statistics to study the stress state of granular materials from the particle scale. The work was inspired by the stress-force-fabric relationship proposed by Rothenburg and Bathurst (1989), which represents a fundamental effort to establish analytical macro-micro relationship in granular mechanics. The...

SUMMARY Multi-scale investigations aided by the discrete element method (DEM) play a vital role for current state-of-the-art research on the elementary behaviour of granular materials. Similar to laboratory tests, there are three important aspects to be considered carefully, which are the proper stress/strain definition and measurement, the applica...

How to incorporate the impact of the internal structure into the constitutive modeling has attracted much research interest. Towards the grand challenge of developing a constitutive models embedded with particle-scale mechanism, this paper mainly concerns about the internal structure and its role in constitutive modeling. The investigations to be r...

This paper presents an experimental investigation on the drained behavior of saturated sand in rotational shear conducted in a hollow cylinder apparatus. In the tests, samples were subjected to cyclic rotation of principal stress axes while the magnitudes of principal stresses were maintained constant. Special attention was placed on the stress-str...

This paper presents an experimental investigation on the drained behavior of saturated sand in rotational shear conducted in a hollow cylinder apparatus. In the tests, samples were subjected to cyclic rotation of principal stress axes while the magnitudes of principal stresses were maintained constant. Special attention was placed on the stress-str...

Information being dealt with in micro-mechanics is massive. Most of them are directional data. Macro-scale physical laws embedded with micro-scale fundamentals need to be developed in terms of the statistics of the micro-scale variable in a frame-indifferent form. Mathematical techniques and theories for characterizing the statistics of directional...

Starting from the micro-structural definition of the stress tensor, Rothenburg and Bathurst (1989) [1] derived a stress–force–fabric relationship for granular materials by approximating the directional distributions of the fabric, more specifically, the contact normal density distribution in this paper and the contact forces with Fourier functions...

Loading paths involving large principal stress rotation are frequently encountered in engineering practice. Examples include pavement and railway foundations under traffic loading, offshore foundations, foundations of wind turbine, and earthquake geotechnics. This paper is concerned with sand deformation induced by principal stress rotation. The in...

The paper presents results of multi-scale investigations on the non-coaxial behavior of granular materials to monotonic loading aided by the discrete element method. Specimens of granular materials have been prepared and subjected to monotonic loadings. Statistical characterization of the direction dependent particle-scale variables, including cont...

This paper presents numerical experimental results with respect to the responses of two-dimensional granular materials under principal stress rotations. The tests were carried out using the discrete element method. A numerical procedure has been developed to apply arbitrary stress or strain paths and to measure the induced strain or stress changes....

In granular mechanics, macroscopic approaches treat a granular material as an equivalent continuum at macro-scale, and study its constitutive relationship between macro-quantities, such as stresses and strains. On the other hand, microscopic approaches consider a granular material as an assembly of individual particles interacting with each other a...

Due to material anisotropy, the behavior of a granular material is generally directional-dependent. The directions of stress and strain increment have been observed non-coincident even for the case of loading in fixed directions. This paper uses the discrete element method to explore the underlying mechanisms of this non-coaxiality. Numerical exper...

We have attempted a multiscale quantification of the internal structure of granular materials. The internal structure of granular materials, i.e., the geometrical information on granular particles and their spatial arrangement, was described mathematically on the particle scale using Voronoi-Delaunay tessellations. These tessellations were further...

In granular mechanics, macroscopic approaches treat a granular material on macro-scale as an equivalent continuum, and study its constitutive relationship between macro-quantities (stress and strain) while microscopic approaches consider it on micro-scale as an assembly of individual particles interacting with each other, and the physical quantitie...

A granular material is most likely to possess an initial anisotropic structure due to the depositional process or the previous stress history. There is also induced fabric anisotropy during shearing. Concerning the significant influence of fabric anisotropy on the stress-strain behaviour of a granular material, in-depth investigation on the two kin...

Granular material exhibits complex responses when subjected to various external loading. Fundamental mechanisms have not been well established so far, including that about the critical state, one of the most important concepts in the modern soil mechanics. With the recognition that granular material is discrete in nature, the basic understanding ca...