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You searched for +publisher:"ETH Zürich" +contributor:("Jia, Xiaoping"). Showing records 1 – 2 of 2 total matches.

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ETH Zürich

1. Lemrich, Laure. Discrete Element Modeling of Acoustic Wave Propagation in Granular Media: Nonlinearity and Material Softening.

Degree: 2019, ETH Zürich

Granular media is one of the most common form of material which exists in many geophysical phenomena such as landslides and earthquakes. It is also present in the industry for transport, storage, and for mixing processes. Constitutive equations are essential to predict and control granular media behavior, but they are not fully developed yet. In particular, the history of a granular packing has a determinant influence on the arrangement of its particles and the organization and nature of inter-particle contacts. The arrangement of particles and contacts in turn affects the material response to external perturbations such as compression, shear and wave propagation. This thesis studies the propagation of acoustic waves through the contact network. The Effective Medium Theory and continuum descriptions partially solve the constitutive equations of dense packings of granular media but cannot fully describe the phenomenon as the Effective Medium Theory models ensembles and continuum descriptions neglect particle rearrangements. Dynamic testing has shown a dependency of resonance frequency on strain at low strains in confined granular packings (Johnson and Jia 2005; Inserra et al. 2008) but this study has not been extended to high strains. We present a detailed study of dynamic testing in confined random packings of weakly polydisperse glass beads using the Discrete Element Method to measure the resonance frequency over a range of strains. We find a decrease in resonance frequency at high strain, called softening, that only depends on the normal component of the contact force, the coordination number and the average inter-particle overlap. The mean-field assumption of the Effective Medium Theory successfully predicts the dependency of softening on confining stress. The Effective Contact Theory is a model developed in this thesis to relate the breaking of contacts and decrease in coordination number to softening by defining a granular temperature based on kinetic energy. For the first time, granular temperature is used to model contact evolution rather than particle motion. The Effective Contact Theory successfully relate the probability of a contact breaking to softening in granular media. Simulations of wave propagation in static packings show an excellent agreement between wave velocity measured from direct wave propagation, dynamic testing and bulk and shear compression tests. Wave damping and distortion increases with amplitude and frequency and, at high frequency, waves are propagated through scattering and rapidly damped. Advisors/Committee Members: Herrmann, Hans J., Jia, Xiaoping.

Subjects/Keywords: Granular media; Condensed Matter Physics; wave propagation; Nonlinear phenomena; Effective field theories; effective contact theory; granular temperature; softening; Weakening; discrete element method; granular material; granular matter; Condensed matter; info:eu-repo/classification/ddc/530; Physics

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Lemrich, L. (2019). Discrete Element Modeling of Acoustic Wave Propagation in Granular Media: Nonlinearity and Material Softening. (Doctoral Dissertation). ETH Zürich. Retrieved from http://hdl.handle.net/20.500.11850/342965

Chicago Manual of Style (16th Edition):

Lemrich, Laure. “Discrete Element Modeling of Acoustic Wave Propagation in Granular Media: Nonlinearity and Material Softening.” 2019. Doctoral Dissertation, ETH Zürich. Accessed April 17, 2021. http://hdl.handle.net/20.500.11850/342965.

MLA Handbook (7th Edition):

Lemrich, Laure. “Discrete Element Modeling of Acoustic Wave Propagation in Granular Media: Nonlinearity and Material Softening.” 2019. Web. 17 Apr 2021.

Vancouver:

Lemrich L. Discrete Element Modeling of Acoustic Wave Propagation in Granular Media: Nonlinearity and Material Softening. [Internet] [Doctoral dissertation]. ETH Zürich; 2019. [cited 2021 Apr 17]. Available from: http://hdl.handle.net/20.500.11850/342965.

Council of Science Editors:

Lemrich L. Discrete Element Modeling of Acoustic Wave Propagation in Granular Media: Nonlinearity and Material Softening. [Doctoral Dissertation]. ETH Zürich; 2019. Available from: http://hdl.handle.net/20.500.11850/342965


ETH Zürich

2. Dorostkar, Omid. Stick-slip dynamics in dry and fluid saturated granular fault gouge investigated by numerical simulations.

Degree: 2018, ETH Zürich

Mature faults in the earth comprise a granular fault gouge created due to communition and fragmentation of host rock at the core of the damage zone. It is well understood that granular interactions in this fault gouge govern the dynamics of the fault. The stick-slip dynamics in a sheared granular layer in experiments and simulations is understood to simulate similar physical mechanisms that are involved in earthquakes. The dynamics of a sheared granular fault gouge can be strongly affected by its particle properties, shear driving velocity, confining stress, temperature and pore fluid. Fluids are found to play a significant role in fault mechanics, where they can not only alter the physicochemical properties of gouge materials, but also affect the stick-slip dynamics of the fault. Laboratory experiments have been widely used to investigate the underlying physics of granular interactions in fault mechanics, showing the important role of fluids in slip nucleation and failure process. However, experiments lack detailed information at grain scale to understand the role of fluids in stick-slip dynamics. Furthermore, in experiments the different effects of fluids, such as physicochemical and hydro-mechanical are not distinguishable. Numerical simulations at grain scale however allow unraveling such questions. Here, a 3D coupled Computational Fluid Dynamics-Discrete Element Method is used to model stick-slip dynamics in a granular fault system with fluids. First, a dry granular fault gouge is modeled and its dynamics for different particle properties and under different loading configurations is studied. Next, the effect of capillary cohesion at low saturation degrees, where the capillary bridges are in pendular regime, is studied. The results show an increase of recurrence time between slips and an increase in stress drop due to a rearrangement of the particles owing to the presence of rather low cohesive forces between the wet granular particles. This particle rearrangement results in more stable configurations of the granular layer as indicated by an increase in particle coordination number and shear stiffness of the layer during the stick phase. The results for a fluid saturated fault gouge show that slip events are characterized by a higher drop in friction coefficient, in potential energy and thickness of the gouge layer compared to the dry conditions. The drained fluid saturated granular fault gouge shows higher values for pre-seismic potential energy, describing that the fluid leads to more stable configurations of the granular layer illuminated by a higher particle coordination number. The higher potential energy drop is accompanied by a higher release of kinetic energy in fluid saturated granular fault gouge. The spatial correlation of regions with high fluid velocity, particle-fluid interaction and particle kinetic energy during slip show that the mechanisms of particle rearrangement, increase of fluid pressure and particle-fluid interaction forces are strongly coupled phenomena explaining the macroscopic… Advisors/Committee Members: Carmeliet, Jan E., Bonn, Daniel, Jia, Xiaoping.

Subjects/Keywords: fault mechanics; granular physics; fault gouge; stick-slip dynamics; fluid flow in faults; info:eu-repo/classification/ddc/550; info:eu-repo/classification/ddc/620; Earth sciences; Engineering & allied operations

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Dorostkar, O. (2018). Stick-slip dynamics in dry and fluid saturated granular fault gouge investigated by numerical simulations. (Doctoral Dissertation). ETH Zürich. Retrieved from http://hdl.handle.net/20.500.11850/283977

Chicago Manual of Style (16th Edition):

Dorostkar, Omid. “Stick-slip dynamics in dry and fluid saturated granular fault gouge investigated by numerical simulations.” 2018. Doctoral Dissertation, ETH Zürich. Accessed April 17, 2021. http://hdl.handle.net/20.500.11850/283977.

MLA Handbook (7th Edition):

Dorostkar, Omid. “Stick-slip dynamics in dry and fluid saturated granular fault gouge investigated by numerical simulations.” 2018. Web. 17 Apr 2021.

Vancouver:

Dorostkar O. Stick-slip dynamics in dry and fluid saturated granular fault gouge investigated by numerical simulations. [Internet] [Doctoral dissertation]. ETH Zürich; 2018. [cited 2021 Apr 17]. Available from: http://hdl.handle.net/20.500.11850/283977.

Council of Science Editors:

Dorostkar O. Stick-slip dynamics in dry and fluid saturated granular fault gouge investigated by numerical simulations. [Doctoral Dissertation]. ETH Zürich; 2018. Available from: http://hdl.handle.net/20.500.11850/283977

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