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Expertise Summary
Dr. Xia Li is a lecturer in the department of chemical and environmental engineering at the University of Nottingham. Her research focuses on micromechanics and engineering application of granular materials. It is a multi-disciplinary subject and her research activities are jointly supported by the mechanics, materials and structure research division as well as the process and environmental research division within the Faculty of Engineering.
After completing her degree in Hydraulic and Hydro-power engineering at Tsinghua University, Dr Li studied for a PhD in granular mechanics with the title "Micro-scale investigation on the quasi-static behavior of granular material" in the Department of Civil Engineering at the Hong Kong University of Science and Technology. She has been awarded miscellaneous scholarship during her study at Tsinghua University and the 2003 Association of Geotechnical & Geoenvironmental Specialists (Hong Kong). After graduation in 2006, she worked as a research fellow in the Hong Kong University of Science and Technology and then joined the University of Nottingham in July, 2007. She received the Nottingham Advance Research Fellowship in 2010, and commenced a lectureship in the Department of Chemical and Environmental Engineering since January 2011.
Research Summary
Constitutive modelling of geo-materials
Multi-scale investigation and homogenisation processes in granular mechanics
Statistics analyses of directional data
Discrete element modelling of… read more
Selected Publications
LI, X. and YU, H.-S., 2011. Tensorial characterisation of direction data in micromechanics International Journal of Solids and Structures. 48(14-15), 2167-2176 LI, X., YU, H.-S. and LI, X.S., 2009. Macro-micro relations in granular mechanics International Journal of Solids and Structures. 46(25-26), 4331-4341 LI, X. and LI, X.S., 2009. Micro–macro quantification of the internal structure of granular materials Journal of Engineering Mechanics, ASCE. 135(7), 641-656
Current Research
- Constitutive modelling of geo-materials
- Multi-scale investigation and homogenisation processes in granular mechanics
- Statistics analyses of directional data
- Discrete element modelling of multi-phase granular materials, including unsaturated soil mechanics, fluidised bed, and contaminant transfer
- Study of granular dynamics involved in the process of granular materials storage, transport, separation, mixing and segregation using the techniques of discrete element simulations as well as photo-elastic measurement
- Trigger mechanism from solid-like behaviour to liquid-like behaviour, with particular interest in liquefaction, collapse, initiation and development of landslide
Past Research
My research has been devoted to granular mechanics, in particular, the particle scale investigations and the bridging-up between the macro and micro scales. It has been driven by the curiosity in the contrast between the simplicity in particle-scale mechanisms and the complexity in macroscopic material behaviors, and in the pursuit of the harmonic development of human well being and the natural environment by improving understandings of geomaterials covering earth surface.
- Quantitative description of the packing of granular particles
The key to the mystery in granular material behaviours is the pattern formed by granular particles. A topological representation has been proposed to mathematically describe the spatial arrangement of granular particles. The proposed geometrical systems are constructed based on the Voronoi-Delaunay tessellations and are purposeful for identifying the micro elements for the static and kinematic analyses.
- The macro-micro relations in granular mechanics
On the creative topological systems, the macro-micro relations have been set-up by deriving expressions of the continuum mechanical concepts, stress and strain, in terms of the internal structure and their particle-scale counterparts, contact forces and particle displacements. The newly derived macro-micro relations together with the topological systems form the foundations for the upscaling from the micro-scale fundamentals to the macro-scale phenomena.
- Virtual experiments to reproduce granular material behaviours using discrete element methods (DEM)
The research has been benefited from the usage of a vibrant computational tool, the discrete element method (DEM). A novel scheme to numerically reproduce the elementary behaviour of granular materials to general loading conditions has been proposed and leads to a number of interesting findings. The results from numerical modeling provide detailed micro-information, and serve as the database for multi-scale investigations.
- The fundamentals and constitutive modeling of granular material behaviors
Multi-scale investigations have been conducted to look into various features of granular material, which are important to engineering practice but yet poorly understood. Up to present, we have successfully explained the influence of material anisotropy, state-dependent dilatancy, non-coaxial deformation and distinct volume contraction induced by stress-rotation by looking into the homogenization process from the micro-scale to the macro-scale. These are the main concerns in the development of modern constitutive theories.
- Tensorial characterisation of directional data for micromechanics
Realizing that the micromechanics-based constitutive models are necessarily established on the statistical measures of particle-scale information, which are often direction-dependent, mathematical theories are proposed to characterise directional data in the frame indifferent form with tensors. The direction tensors could be used as new state variables characterizing material anisotropy. Applying the theory to particle-scale information on contact normal density, contact force and contact vector leads to a general expression of the stress-force-fabric relationship. It enhanced our understanding in the stress state of granular materials, and hence the strength of geomaterials under various conditions.
Future Research
Granular material is a multi-disciplinary and multi-physics subject boosted in the last few decades. Research based on multi-scale investigations and the homogenization process between the micro scale and the macro scale is expected to result in more fruitful discoveries in the near future. My current objective is to seek for a stable platform to support my next stage research. Besides continuing research on the development of the micromechanics-based constitutive models, my research interests and plans also include:
- Particle dynamics in process engineering
Pattern formation plays an important role not only for static problems, but also dynamic problems. In process engineering, traditional design method is mainly experiential. Aided by studies on the particle movements and interactions using the discrete element method, the process engineering could be benefited with more desirable product gradation, better estimation of impact force, more reasonable machinery design and longer service life.
- Fluidization and granular flow problems
Human lives are constantly threatened by natural disasters, such as, earthquake, avalanche, landslide, flood and so on. With the development of science and technology in the 20th century, disaster mitigation technology is an area demanding more research effort so as to meet higher expectation of life quality. In the new century, we are now facing bigger challenge due to global warming and climate change. Multi-scale investigation to understand the triggering mechanisms of liquefaction and landslides, the influential factors on their evolutions and impacts are important to take measure to prevent or mitigate the induced destructions.
- Virtual simulation of multi-phase granular medium
Geo-materials often come in the form of a multi-phase medium, with the existence of solid, liquid and air. Current particle-scale simulations are concentrated on dry granular materials. There are ongoing researches to couple DEM with computational fluid dynamics (CFD) to study particle-fluid flow problems in engineering and physical field, while consideration of the impact of gas phase is still limited. A project funded by the University of Nottingham is undertaken for this challenging topic by improving current DEM simulations with new elements representing the phase interfaces. A granular material is hence numerically modeled as a solid-liquid-gas coexisting system. The objective is to develop a computational platform to reproduce the behaviours of multiphase granular material originated from particle scale interactions. The ongoing work has a great potential for applications in a wide range of important engineering practices, and will certainly motivate further research. Examples of potential applications include:
² seepage problem encountered in riverbank design and high dam construction;
² contaminant transfer through soils in environmental engineering;
² leakage of CO2 in carbon capture and storage techniques;
² co-existence of natural gas and hydrate in ocean sediments in energy sectors.