Ifsttar PhD subject

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Title : Quasi-static behaviour of cohesive powders: experiment sand numerical simulations.

Main host Laboratory - Referent Advisor   -

Director of the main host Laboratory   -

PhD Speciality Mécanique des matériaux et des sols, rhéologie des matériaux granulaires.

Axis of the performance contract 2 - COP2017 - More efficient and resilient infrastructure

Main location Marne-la-Vallée

Doctoral affiliation UNIVERSITE PARIS-EST

PhD school SCIENCES, INGENIERIE ET ENVIRONNEMENT (SIE)

Planned PhD supervisor ROUX Jean-Noël  -  Université Gustave Eiffel  -  Navier

Planned PhD co-supervisor TANG Ahn-Minh  -  Université  -  Navier

Planned financing Contrat doctoral  - Ifsttar

Abstract

CONTEXT. Clays (provided a characteristic aggregate size might be identified), fresh cements (akin to suspension of adhesive particles), powders (the mesoscopically sized grains of which are sensitive to van der Waals attraction), colloids (with attractive short-ranged interparticle forces) are all granular materials with adhesion in the contacts. Model powders [1], made of assembled spherical grains capable of tensile resistance in the contacts, are an archetypal example of this large class of materials, simple enough for relations between contact network micromechanics and macroscopic behaviour to be accessible. Macroscopic solid grains connected by capillary bridges [2,3,5,6] exhibit analogous behaviours, but the resulting cohesion values remain quite moderate (a few kPa at most for 0.1mm diameter grains).
Two aspects of the mechanical behaviour of such systems were studied, both experimentally [1,2,3,4,5] and via discrete-element type numerical simulations (DEM) [5,3,6,7]: isotropic or oedometric compression [7,3,2], in which growing applied stress intensities, gradually dominating contact tensile strengths, entail the collapse of initially loose structures; and maintained quasistatic shear flow (the critical state of geomechanics) [3,6,5], the description and model thereof generalizes to inertial flows. Between those two extreme situations of the quasistatic quasistatic evolution of the material, as classically studied in soil mechanics under monotonically growing deviatoric loads, is well explored and related to internal changes of the contact network for cohesionless materials. Cohesive ones, which exhibit a much wider variety of microstructures, due the stabilization of loose configurations, remain however little known.

OBJECTIVES OF THE THESIS. WORK PROGRAM.
This thesis aims at investigating this regime of cohesive granular material behaviour, at identifying relevant state parameters, and at relating macroscopic features (plasticity and yield criteria, resistance to shear, dilatancy or contractant) to the microscopic ingredients of the model. Is it possible to delineate an elastic regime, to characterize an elastoplastic behaviour? To what extent is the Cam-clay behaviour retrieved? How may one efficiently characterize initial states through a small number of internal variables (density, coordination, fabric anisotropy)? What is the influence of contact characteristics such as rolling resistance? These are some of the issues expected to be dealt with, DEM studies enabling detailed investigations of the role of micromechanical parameters. DEM model materials are to be applied to generic, non-Brownian cohesive particles; by focussing on quasistatic, solid-like behaviour, one should avoid difficulties associated with the numerical treatment of interstitial fluid flow.
In parallel with the numerical simulations, exploiting the numerical tools developed in the Rheophysics group (J.-N. Roux), experiments will be carried out in the Géotechnique group laboratories, on well chosen model systems: bentonite powders, model clays, polystyrene microbeads. The moisture content of such materials will be controlled by oven drying, insuring removal of adsorbed water, and their assembling process will exploit various techniques such vacuum pluviation of air fluidization and sedimentation. Samples will be subjected to triaxial compression tests. Their initial homogeneity and internal evolution during compression might be tested or monitored by X-ray microtomography. Volumetric strain will be followed by the double cell device developed within the Geotechnique team [8].
By confronting DEM simulations an d experiments, one expects to identify a small number of relevant dimensionless parameters and a robust classification of initial states, which should enable a semiquantitative comparison of the main features of observed mechanical behaviours. Those should thus be better characterized and related to elementary material constituents and rheophysical mechanisms.

SCIENTIFIC ENVIRONMENT.
The work carried out in the framework of this PhD thesis will lend itself to fruitful exchanges with other research projects, as pursued in the ‘Multiscale’ team of Laboratoire Navier on molecular level modeling of clays (L. Brochard). Macroscopic behaviours of clays are routinely charcartrozed within the géotechnique group. One ongoing project within the Rheophysics group (X. Chateau, J. Goyon, A. Lemaître) dealing with colloidal gels formed by cohesive silica particles, studying its rupture and flow properties, should also lead to interesting comparisons of observations and of models.

PRODUCTION. Papers should be published, presenting the thesis results, both in general physics journals and in géotechnique and process engineering ones — in te transdisciplinary spirit of Laboratoire Navier.

PREREQUISITES. Applicants should have a master’s degree, with a solid background in solid state mechanics, physics of materials, géotechnique and/or geophysics; and be willing to carry out innovative work both in experimental and in numerical fields.

REFERENCES
[1] A. Castellanos. Advances in Physics, Vol. 54, No. 4, 263–376 (2005).
[2] V.-D. Than, Compression behavior of loose wet granular materials: experiment and discrete numerical simulation, thèse de doctorat, université Paris Est, 2017.
[3] V.-D. Than, S. Khamseh, A.-M. Tang, J.-M. Pereira, J.-N. Roux. Basic mechanical properties of wet granular materials : a DEM study, ASCE J. Engineering Mechanics, 143(1) Special Issue : SI, C4016001 (2017).
[4] J. K. Mitchell, K. Soga. Fundamentals of soil behavior. Wiley, 2005.
[5] M. Badetti, A. Fall, F. Chevoir, J.-N. Roux. Shear strength of wet granular materials : macroscopic cohesion and effective stress. Eur. Phys. J. E 41 : 68 (2018)
[6] S. Khamseh, J.-N. Roux, F. Chevoir, Flow of wet granular materials : a numerical study,.Physical Review E 92, 022201 (2015)
[7] F. Gilabert, J.-N. Roux, A. Castellanos, Computer simulation of model cohesive powders: Plastic consolidation, structural changes, and elasticity under isotropic loads, Physical Review E 78, 031305 (2008)
[8] Y. J. Cui, A. M., Tang, D. Marcial, J. Terpereau, G. Marchadier, X. Boulay. Use of a Differential Pressure Transducer for the Monitoring of Soil Volume Change in Cyclic Triaxial Test on Unsaturated Soils. Geotechnical Testing Journal, 30 (3), 227-233 (2007) 

Keywords : powders, clay, soils, cohesion, friction, discrete element simulation, elasticity , plasticity, colloidal aggregates, triaxial compression, soil mechanics, Cam-Clay, critical state

Main host Laboratory - Referent Advisor	-
Director of the main host Laboratory	-
PhD Speciality	Mécanique des matériaux et des sols, rhéologie des matériaux granulaires.
Axis of the performance contract	2 - COP2017 - More efficient and resilient infrastructure
Main location	Marne-la-Vallée
Doctoral affiliation	UNIVERSITE PARIS-EST
PhD school	SCIENCES, INGENIERIE ET ENVIRONNEMENT (SIE)
Planned PhD supervisor	ROUX Jean-Noël - Université Gustave Eiffel - Navier
Planned PhD co-supervisor	TANG Ahn-Minh - Université - Navier
Planned financing	Contrat doctoral - Ifsttar
Abstract CONTEXT. Clays (provided a characteristic aggregate size might be identified), fresh cements (akin to suspension of adhesive particles), powders (the mesoscopically sized grains of which are sensitive to van der Waals attraction), colloids (with attractive short-ranged interparticle forces) are all granular materials with adhesion in the contacts. Model powders [1], made of assembled spherical grains capable of tensile resistance in the contacts, are an archetypal example of this large class of materials, simple enough for relations between contact network micromechanics and macroscopic behaviour to be accessible. Macroscopic solid grains connected by capillary bridges [2,3,5,6] exhibit analogous behaviours, but the resulting cohesion values remain quite moderate (a few kPa at most for 0.1mm diameter grains). Two aspects of the mechanical behaviour of such systems were studied, both experimentally [1,2,3,4,5] and via discrete-element type numerical simulations (DEM) [5,3,6,7]: isotropic or oedometric compression [7,3,2], in which growing applied stress intensities, gradually dominating contact tensile strengths, entail the collapse of initially loose structures; and maintained quasistatic shear flow (the critical state of geomechanics) [3,6,5], the description and model thereof generalizes to inertial flows. Between those two extreme situations of the quasistatic quasistatic evolution of the material, as classically studied in soil mechanics under monotonically growing deviatoric loads, is well explored and related to internal changes of the contact network for cohesionless materials. Cohesive ones, which exhibit a much wider variety of microstructures, due the stabilization of loose configurations, remain however little known. OBJECTIVES OF THE THESIS. WORK PROGRAM. This thesis aims at investigating this regime of cohesive granular material behaviour, at identifying relevant state parameters, and at relating macroscopic features (plasticity and yield criteria, resistance to shear, dilatancy or contractant) to the microscopic ingredients of the model. Is it possible to delineate an elastic regime, to characterize an elastoplastic behaviour? To what extent is the Cam-clay behaviour retrieved? How may one efficiently characterize initial states through a small number of internal variables (density, coordination, fabric anisotropy)? What is the influence of contact characteristics such as rolling resistance? These are some of the issues expected to be dealt with, DEM studies enabling detailed investigations of the role of micromechanical parameters. DEM model materials are to be applied to generic, non-Brownian cohesive particles; by focussing on quasistatic, solid-like behaviour, one should avoid difficulties associated with the numerical treatment of interstitial fluid flow. In parallel with the numerical simulations, exploiting the numerical tools developed in the Rheophysics group (J.-N. Roux), experiments will be carried out in the Géotechnique group laboratories, on well chosen model systems: bentonite powders, model clays, polystyrene microbeads. The moisture content of such materials will be controlled by oven drying, insuring removal of adsorbed water, and their assembling process will exploit various techniques such vacuum pluviation of air fluidization and sedimentation. Samples will be subjected to triaxial compression tests. Their initial homogeneity and internal evolution during compression might be tested or monitored by X-ray microtomography. Volumetric strain will be followed by the double cell device developed within the Geotechnique team [8]. By confronting DEM simulations an d experiments, one expects to identify a small number of relevant dimensionless parameters and a robust classification of initial states, which should enable a semiquantitative comparison of the main features of observed mechanical behaviours. Those should thus be better characterized and related to elementary material constituents and rheophysical mechanisms. SCIENTIFIC ENVIRONMENT. The work carried out in the framework of this PhD thesis will lend itself to fruitful exchanges with other research projects, as pursued in the ‘Multiscale’ team of Laboratoire Navier on molecular level modeling of clays (L. Brochard). Macroscopic behaviours of clays are routinely charcartrozed within the géotechnique group. One ongoing project within the Rheophysics group (X. Chateau, J. Goyon, A. Lemaître) dealing with colloidal gels formed by cohesive silica particles, studying its rupture and flow properties, should also lead to interesting comparisons of observations and of models. PRODUCTION. Papers should be published, presenting the thesis results, both in general physics journals and in géotechnique and process engineering ones — in te transdisciplinary spirit of Laboratoire Navier. PREREQUISITES. Applicants should have a master’s degree, with a solid background in solid state mechanics, physics of materials, géotechnique and/or geophysics; and be willing to carry out innovative work both in experimental and in numerical fields. REFERENCES [1] A. Castellanos. Advances in Physics, Vol. 54, No. 4, 263–376 (2005). [2] V.-D. Than, Compression behavior of loose wet granular materials: experiment and discrete numerical simulation, thèse de doctorat, université Paris Est, 2017. [3] V.-D. Than, S. Khamseh, A.-M. Tang, J.-M. Pereira, J.-N. Roux. Basic mechanical properties of wet granular materials : a DEM study, ASCE J. Engineering Mechanics, 143(1) Special Issue : SI, C4016001 (2017). [4] J. K. Mitchell, K. Soga. Fundamentals of soil behavior. Wiley, 2005. [5] M. Badetti, A. Fall, F. Chevoir, J.-N. Roux. Shear strength of wet granular materials : macroscopic cohesion and effective stress. Eur. Phys. J. E 41 : 68 (2018) [6] S. Khamseh, J.-N. Roux, F. Chevoir, Flow of wet granular materials : a numerical study,.Physical Review E 92, 022201 (2015) [7] F. Gilabert, J.-N. Roux, A. Castellanos, Computer simulation of model cohesive powders: Plastic consolidation, structural changes, and elasticity under isotropic loads, Physical Review E 78, 031305 (2008) [8] Y. J. Cui, A. M., Tang, D. Marcial, J. Terpereau, G. Marchadier, X. Boulay. Use of a Differential Pressure Transducer for the Monitoring of Soil Volume Change in Cyclic Triaxial Test on Unsaturated Soils. Geotechnical Testing Journal, 30 (3), 227-233 (2007)
Keywords :	powders, clay, soils, cohesion, friction, discrete element simulation, elasticity , plasticity, colloidal aggregates, triaxial compression, soil mechanics, Cam-Clay, critical state

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