The Experts below are selected from a list of 240 Experts worldwide ranked by ideXlab platform
Giang D Nguyen - One of the best experts on this subject based on the ideXlab platform.
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Localised Failure of geomaterials how to extract localisation band behaviour from macro test data
Geotechnique, 2021Co-Authors: Giang D Nguyen, Ha H Bui, Jose E AndradeAbstract:The formulation and calibration of constitutive models for geomaterials require material behaviour from experiments under a wide range of triaxial loading conditions. However, Failure of geomateria...
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a thermodynamics and mechanism based framework for constitutive models with evolving thickness of localisation band
International Journal of Solids and Structures, 2020Co-Authors: Giang D Nguyen, Ha H BuiAbstract:Abstract Localised Failure in geomaterials invalidates the assumption of homogeneous deformation that constitutive models based on continuum mechanics rest on. In such cases, the deformation and nonlinear processes inside the localisation zone dominate the inelastic response of the material, while the material outside this zone usually undergoes negligible inelastic or even elastic deformation. As a consequence, internal variables representing micromechanical Failure processes should better be defined inside the localisation zone, not averaged over the whole volume element containing it. In this study, we propose a thermodynamics-based framework for constitutive models that take into account the transition from homogenous to Localised deformation. Two spatial scales involved in the mechanisms of Localised Failure, macro scale of the considered volume element and smaller scale of the localisation zone, are included in the formulation and derived constitutive models. This separation of spatial scales is combined with enrichments of the constitutive kinematics for the integration of three constitutive relationships describing the behaviour of the materials inside and outside the localisation zone, and the evolving size of this zone. As a result, the internal variables are associated with their own spatial zones, instead of being averaged over the whole volume element like in classical continuum approaches. The gradual transition from homogenous to Localised deformation is represented by the onset and evolution of the thickness of the localisation band, both of which appear naturally in the proposed formulation. The obtained model therefore consists of both size and orientation of the localisation band, and three constitutive relationships connected through the equilibrium condition across the boundary of the localisation zone. They help provide a smooth transition from homogeneous to Localised Failure. Numerical examples show promising features of the proposed approach in connecting the macro behaviour with the underlying evolution of the localisation zone.
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Localisation and lode-angle dependence of geomaterial behaviour
Lecture Notes in Civil Engineering, 2019Co-Authors: Giang D Nguyen, Ha H Bui, Abdul Hamid Sheikh, Andrei KotousovAbstract:True triaxial test results of geomaterials have shown a strong dependence of the material responses on the third invariant of the deviatoric stress (or alternatively Lode angle). In constitutive modelling, this dependence is usually captured by incorporating different forms of the Lode angle (i.e. Lode angle parameter, third invariant of deviatoric stress) into the macroscopic yield function phenomenologically. In this paper, the mechanism of Localised Failure is analysed and identified as the underlying cause of the Lode angle dependence, from which a constitutive model is developed. The model includes an additional kinematic field with its own set of governing relationships to account for the high deformation gradient across the boundary of the localisation band. Since the mechanism of Localised Failure and its initiation, governed by true triaxial stress states, are included, the Lode-angle dependent behaviour is naturally captured without requiring any phenomenological relationships. In this short correspondence, key characteristics of the proposed approach are outlined together with its preliminary results validated against experimental data.
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A new SPH-based continuum framework with an embedded fracture process zone for modelling rock fracture
International Journal of Solids and Structures, 2019Co-Authors: Yingnan Wang, Giang D Nguyen, Ha H Bui, Pathegama Gamage RanjithAbstract:Abstract A new computational approach combining the Smooth Particle Hydrodynamics and a constitutive model that possesses an intrinsic length scale is proposed in this study for modelling rock fracture. In particular, a continuum-based size-dependent constitutive model with an embedded fracture process zone described by a cohesive model is adopted for modelling strain localisation in geomaterials. A length scale is introduced into the constitutive equations to describe the scale effect commonly observed in Localised Failure of geomaterials. The constitutive model is then employed in a mesh-free Taylor Smooth Particle Hydrodynamics (Taylor-SPH) framework to produce a new computational tool for rock fracture modelling. The key feature of the proposed numerical framework is that it describes the fracture geometry by a set of Lagrangian particles, which carry fracture information such as damage evolution and fracture orientation, thus bypassing the need to represent the fracture's topology and fracture orientation. To test the capability of this computational framework in simulating rock fracture behaviour, three mode-I numerical fracture tests including three-point bending test, Brazilian-disc test and semi-circular bending test are carried out and results validated with experimental data in the literature. The good agreement between numerical and experimental results suggests that the proposed method is a promising numerical approach to modelling rock fracture.
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Localised Failure mechanism as the basis for constitutive modelling of geomaterials
International Journal of Engineering Science, 2018Co-Authors: Giang D Nguyen, Ha H Bui, Abdul Hamid Sheikh, Andrei KotousovAbstract:Abstract Localised Failure of geomaterials in the form of cracks or shear bands always requires special attention in constitutive modelling of solids and structures. This is because the validity of classical constitutive models based on continuum mechanics is questionable once Localised inelastic deformation has occurred. In such cases, due to the fact that the macro inelastic responses are mainly governed by the deformation and microstructural changes inside the localisation zone, internal variables, representing these microstructural changes, should be defined inside this zone. In this paper, the Localised Failure mechanism is identified and employed as an intrinsic characteristic upon which a constitutive model is based on at the first place, instead of being dealt with after developing the model using various regularisation techniques. As a result, inelastic responses of the model are correctly associated with the localisation bands, and not smeared out over the whole volume element as in classical continuum constitutive models. It is shown that this inbuilt localisation mechanism in a constitutive model can naturally capture important features of the material and possess intrinsic regularisation effects while minimising the use of additional phenomenological treatments, and also possessing intrinsic regularisation effects. The development of the proposed model is based on an additional kinematic enhancement to account for high gradient of deformation across the localisation band. This enrichment allows the introduction of an additional constitutive relationship for the localisation band, which is represented in the form of a cohesive-frictional model describing traction-displacement jump relationship across two sides of the localisation band. The model, formulated within a thermodynamically consistent approach, possesses constitutive responses of the bulk material and two localisation bands connected through internal equilibrium conditions. Its key characteristics are demonstrated and validated against experimental data from different types of geomaterials under different loading conditions at both material and structural levels.
Ha H Bui - One of the best experts on this subject based on the ideXlab platform.
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Localised Failure of geomaterials how to extract localisation band behaviour from macro test data
Geotechnique, 2021Co-Authors: Giang D Nguyen, Ha H Bui, Jose E AndradeAbstract:The formulation and calibration of constitutive models for geomaterials require material behaviour from experiments under a wide range of triaxial loading conditions. However, Failure of geomateria...
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a thermodynamics and mechanism based framework for constitutive models with evolving thickness of localisation band
International Journal of Solids and Structures, 2020Co-Authors: Giang D Nguyen, Ha H BuiAbstract:Abstract Localised Failure in geomaterials invalidates the assumption of homogeneous deformation that constitutive models based on continuum mechanics rest on. In such cases, the deformation and nonlinear processes inside the localisation zone dominate the inelastic response of the material, while the material outside this zone usually undergoes negligible inelastic or even elastic deformation. As a consequence, internal variables representing micromechanical Failure processes should better be defined inside the localisation zone, not averaged over the whole volume element containing it. In this study, we propose a thermodynamics-based framework for constitutive models that take into account the transition from homogenous to Localised deformation. Two spatial scales involved in the mechanisms of Localised Failure, macro scale of the considered volume element and smaller scale of the localisation zone, are included in the formulation and derived constitutive models. This separation of spatial scales is combined with enrichments of the constitutive kinematics for the integration of three constitutive relationships describing the behaviour of the materials inside and outside the localisation zone, and the evolving size of this zone. As a result, the internal variables are associated with their own spatial zones, instead of being averaged over the whole volume element like in classical continuum approaches. The gradual transition from homogenous to Localised deformation is represented by the onset and evolution of the thickness of the localisation band, both of which appear naturally in the proposed formulation. The obtained model therefore consists of both size and orientation of the localisation band, and three constitutive relationships connected through the equilibrium condition across the boundary of the localisation zone. They help provide a smooth transition from homogeneous to Localised Failure. Numerical examples show promising features of the proposed approach in connecting the macro behaviour with the underlying evolution of the localisation zone.
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Localisation and lode-angle dependence of geomaterial behaviour
Lecture Notes in Civil Engineering, 2019Co-Authors: Giang D Nguyen, Ha H Bui, Abdul Hamid Sheikh, Andrei KotousovAbstract:True triaxial test results of geomaterials have shown a strong dependence of the material responses on the third invariant of the deviatoric stress (or alternatively Lode angle). In constitutive modelling, this dependence is usually captured by incorporating different forms of the Lode angle (i.e. Lode angle parameter, third invariant of deviatoric stress) into the macroscopic yield function phenomenologically. In this paper, the mechanism of Localised Failure is analysed and identified as the underlying cause of the Lode angle dependence, from which a constitutive model is developed. The model includes an additional kinematic field with its own set of governing relationships to account for the high deformation gradient across the boundary of the localisation band. Since the mechanism of Localised Failure and its initiation, governed by true triaxial stress states, are included, the Lode-angle dependent behaviour is naturally captured without requiring any phenomenological relationships. In this short correspondence, key characteristics of the proposed approach are outlined together with its preliminary results validated against experimental data.
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A new SPH-based continuum framework with an embedded fracture process zone for modelling rock fracture
International Journal of Solids and Structures, 2019Co-Authors: Yingnan Wang, Giang D Nguyen, Ha H Bui, Pathegama Gamage RanjithAbstract:Abstract A new computational approach combining the Smooth Particle Hydrodynamics and a constitutive model that possesses an intrinsic length scale is proposed in this study for modelling rock fracture. In particular, a continuum-based size-dependent constitutive model with an embedded fracture process zone described by a cohesive model is adopted for modelling strain localisation in geomaterials. A length scale is introduced into the constitutive equations to describe the scale effect commonly observed in Localised Failure of geomaterials. The constitutive model is then employed in a mesh-free Taylor Smooth Particle Hydrodynamics (Taylor-SPH) framework to produce a new computational tool for rock fracture modelling. The key feature of the proposed numerical framework is that it describes the fracture geometry by a set of Lagrangian particles, which carry fracture information such as damage evolution and fracture orientation, thus bypassing the need to represent the fracture's topology and fracture orientation. To test the capability of this computational framework in simulating rock fracture behaviour, three mode-I numerical fracture tests including three-point bending test, Brazilian-disc test and semi-circular bending test are carried out and results validated with experimental data in the literature. The good agreement between numerical and experimental results suggests that the proposed method is a promising numerical approach to modelling rock fracture.
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Localised Failure mechanism as the basis for constitutive modelling of geomaterials
International Journal of Engineering Science, 2018Co-Authors: Giang D Nguyen, Ha H Bui, Abdul Hamid Sheikh, Andrei KotousovAbstract:Abstract Localised Failure of geomaterials in the form of cracks or shear bands always requires special attention in constitutive modelling of solids and structures. This is because the validity of classical constitutive models based on continuum mechanics is questionable once Localised inelastic deformation has occurred. In such cases, due to the fact that the macro inelastic responses are mainly governed by the deformation and microstructural changes inside the localisation zone, internal variables, representing these microstructural changes, should be defined inside this zone. In this paper, the Localised Failure mechanism is identified and employed as an intrinsic characteristic upon which a constitutive model is based on at the first place, instead of being dealt with after developing the model using various regularisation techniques. As a result, inelastic responses of the model are correctly associated with the localisation bands, and not smeared out over the whole volume element as in classical continuum constitutive models. It is shown that this inbuilt localisation mechanism in a constitutive model can naturally capture important features of the material and possess intrinsic regularisation effects while minimising the use of additional phenomenological treatments, and also possessing intrinsic regularisation effects. The development of the proposed model is based on an additional kinematic enhancement to account for high gradient of deformation across the localisation band. This enrichment allows the introduction of an additional constitutive relationship for the localisation band, which is represented in the form of a cohesive-frictional model describing traction-displacement jump relationship across two sides of the localisation band. The model, formulated within a thermodynamically consistent approach, possesses constitutive responses of the bulk material and two localisation bands connected through internal equilibrium conditions. Its key characteristics are demonstrated and validated against experimental data from different types of geomaterials under different loading conditions at both material and structural levels.
Cedric Jorand - One of the best experts on this subject based on the ideXlab platform.
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Continental subduction and exhumation of high-pressure rocks: insights from thermo-mechanical laboratory modelling
Earth and Planetary Science Letters, 2004Co-Authors: David Boutelier, A. Chemenda, Cedric JorandAbstract:Abstract Thermo-mechanical physical modelling of continental subduction is performed to investigate the exhumation of deeply subducted continental crust. The model consists of two lithospheric plates made of new temperature sensitive analogue materials. The lithosphere is underlain by liquid asthenosphere. The continental lithosphere contains three layers: the weak sedimentary layer, the crust made of a stronger material, and of a still stronger lithospheric mantle. The whole model is subjected to a constant vertical thermal gradient, causing the strength reduction with depth in each lithospheric layer. Subduction is driven by both push force and pull force. During subduction, the subducting lithosphere is heating and the strength of its layers reduces. The weakening continental crust reaches maximal depth of about 120 km and cannot subduct deeper because its frontal part starts to flow up. The subducted crust undergoes complex deformation, including indicated upward ductile flow of the most deeply subducted portions and Localised Failure of the subducted upper crust at about 50-km depth. This Failure results in the formation of the first crustal slice which rises up between the plates under the buoyancy force. This process is accompanied by the delamination of the crustal and mantle layers of the subducting lithosphere. The delamination front propagates upwards into the interplate zone resulting in the formation of two other crustal slices that also rise up between the plates. Average equivalent exhumation rate of the crustal material during delamination is about 1 cm/year. The crust-asthenosphere boundary near the interplate zone is uplifted. The subducted mantle layer then breaks off, removing the pull force and thereby stopping the delamination and increasing horizontal compression of the lithosphere. The latter produces shortening of the formed orogen and the growth of relief. The modelling reveals an interesting burial/exhumation evolution of the sedimentary cover. During initial stages of continental subduction the sediments of the continental margin are dragged to the overriding plate base and are partially accreted at the deep part of the interplate zone (at 60–70 km-depth). These sediments remain there until the beginning of delamination during which the pressure between the subducted crust and the overriding plate increases. This results in squeezing the underplated sediments out. Part of them is extruded upwards along the interplate zone to about 30-km depth at an equivalent rate of 5–10 cm/year.
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Thermo-mechanical laboratory modelling of continental subduction: first experiments
2002Co-Authors: David Boutelier, A. Chemenda, Cedric JorandAbstract:Thermo-mechanical physical modelling of continental subduction is performed using new temperature sensitive analogue materials to model the lithospheric layers. The initially horizontal lithosphere model is underlain by the liquid asthenosphere and subjected to a constant vertical thermal gradient. The lithosphere contains three layers: the very weak sedimentary layer, the crust made of a stronger material in which strength reduces with depth due to the temperature increase, and the lithospheric mantle, made of a still stronger material with strength also droping with depth. During subduction, the temperature of all layers increases, causing reduction of their strength and limiting the depth of crustal subduction. The crust subducts to more than 100 km-depth and then undergoes large and complex deformation, including the upward ductile flow of the deeply subducted portions and a Localised Failure of the upper crust at depth of a few tens of kilometres. This deformation is accompanied by (is part of) the delamination of the crustal and mantle layers which can be stopped by the break off of the subducted continental mantle and the previously subducted oceanic lithosphere. The modelling reveals an interesting burial/exhumation evolution of the sedimentary cover. During initial stages of continental subduction the sediments of the continental margin are dragged to the overriding plate base and are partially accreted at the lower part of the interplate zone (at 60-70 km-depth). These sediments remain there until the beginning of delamination which results in the reduction of the coupling between the crust and the dense mantle, and in the growth of the interplate pressure between the subducted crust and the overriding plate. This pressure squeezes the underplated sediments out. A small amount of these sediments is rapidly extruded along the interplate zone to about 20 km-depth.
C M Martin - One of the best experts on this subject based on the ideXlab platform.
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An algorithm for adaptive introduction and arrangement of velocity discontinuities within 3D finite-element-based upper bound limit analysis approaches
Computer Methods in Applied Mechanics and Engineering, 2019Co-Authors: Josef Fussl, Markus Lukacevic, Josef Eberhardsteiner, C M MartinAbstract:Abstract This paper presents a new adaptive strategy to efficiently exploit velocity discontinuities in 3D finite-element-based upper bound limit analysis formulations. Based on an initial upper bound result, obtained with a conventional approach without velocity discontinuities, possible planes of plastic flow localisation are determined at each strain-rate evaluation node and, subsequently, this information is used to sequentially introduce discontinuities into the considered discretised structure. During a few iterations, by means of introducing new and adjusting existing discontinuities, an optimal velocity discontinuity layout is obtained. For the general 3D case, the geometry of this layout is defined by the well-known level set method , standardly used to define the geometry of cracks in the extended finite element method . To make this method also applicable for orthotropic strength behaviours, traction-based yield functions defining the plastic flow across discontinuities are derived from their stress-based counterparts. This procedure is outlined in detail and the obtained traction-based yield functions are verified numerically, to guarantee a consistent strength behaviour throughout the whole discretised structure. By means of three different examples, including isotropic as well as orthotropic yield functions, the performance of the proposed strategy is investigated and upper bound results as well as Failure modes are compared to reference solutions. The proposed approach delivers reliable upper bounds for each example and the majority of plastic flow takes place across the sensibly-arranged discontinuities. For this reason, very good upper bounds can be obtained with a quite coarse finite element mesh and only few introduced velocity discontinuities. This represents an attractive alternative to commonly-used adaptive mesh refinement strategies, where often a huge number of degrees of freedom need to be added to capture Localised Failure .
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a numerical upper bound formulation with sensibly arranged velocity discontinuities and orthotropic material strength behaviour
Journal of Theoretical and Applied Mechanics, 2018Co-Authors: Josef Fussl, Markus Lukacevic, Josef Eberhardsteiner, C M MartinAbstract:Numerical limit analysis allows for fast estimates of the collapse load of structures exhibiting ideal plastic material behaviour. In numerical upper bound formulations, the description of the unknown velocity field can be extended by introducing velocity discontinuities between finite elements. Through these additional degrees of freedom, Localised Failure modes may be approximated more accurately and better upper bounds can be obtained. In the existing formulations, such discontinuities are typically introduced between all elements and the description is restricted to isotropic Failure behaviour. In this work, a general 3D upper bound formulation is briefly proposed, allowing the consideration of both isotropic and orthotropic yield functions within finite elements as well as at velocity discontinuities. The concept of “projecting” a stress-based orthotropic yield function onto a certain discontinuity is presented, giving a traction-based yield function which allows for a consistent description of the material strength behaviour across the interface. The formulation is verified by means of two classical examples, the rigid strip footing and the block with asymmetric holes. Furthermore, based on the computation of potential orientations of plastic flow localisation, a simple concept for a sensible arrangement of velocity discontinuities is proposed. It is shown that this concept performs very well for isotropic as well as anisotropic material strength behaviour. A feature of the present work is that, velocity jumps are allowed only across the prescribed finite element interfaces determined from the sensible discontinuity arrangement. Good upper bounds similar to those in the existing works are obtained with far fewer degrees of freedom.
Abdul Hamid Sheikh - One of the best experts on this subject based on the ideXlab platform.
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Localisation and lode-angle dependence of geomaterial behaviour
Lecture Notes in Civil Engineering, 2019Co-Authors: Giang D Nguyen, Ha H Bui, Abdul Hamid Sheikh, Andrei KotousovAbstract:True triaxial test results of geomaterials have shown a strong dependence of the material responses on the third invariant of the deviatoric stress (or alternatively Lode angle). In constitutive modelling, this dependence is usually captured by incorporating different forms of the Lode angle (i.e. Lode angle parameter, third invariant of deviatoric stress) into the macroscopic yield function phenomenologically. In this paper, the mechanism of Localised Failure is analysed and identified as the underlying cause of the Lode angle dependence, from which a constitutive model is developed. The model includes an additional kinematic field with its own set of governing relationships to account for the high deformation gradient across the boundary of the localisation band. Since the mechanism of Localised Failure and its initiation, governed by true triaxial stress states, are included, the Lode-angle dependent behaviour is naturally captured without requiring any phenomenological relationships. In this short correspondence, key characteristics of the proposed approach are outlined together with its preliminary results validated against experimental data.
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Localised Failure mechanism as the basis for constitutive modelling of geomaterials
International Journal of Engineering Science, 2018Co-Authors: Giang D Nguyen, Ha H Bui, Abdul Hamid Sheikh, Andrei KotousovAbstract:Abstract Localised Failure of geomaterials in the form of cracks or shear bands always requires special attention in constitutive modelling of solids and structures. This is because the validity of classical constitutive models based on continuum mechanics is questionable once Localised inelastic deformation has occurred. In such cases, due to the fact that the macro inelastic responses are mainly governed by the deformation and microstructural changes inside the localisation zone, internal variables, representing these microstructural changes, should be defined inside this zone. In this paper, the Localised Failure mechanism is identified and employed as an intrinsic characteristic upon which a constitutive model is based on at the first place, instead of being dealt with after developing the model using various regularisation techniques. As a result, inelastic responses of the model are correctly associated with the localisation bands, and not smeared out over the whole volume element as in classical continuum constitutive models. It is shown that this inbuilt localisation mechanism in a constitutive model can naturally capture important features of the material and possess intrinsic regularisation effects while minimising the use of additional phenomenological treatments, and also possessing intrinsic regularisation effects. The development of the proposed model is based on an additional kinematic enhancement to account for high gradient of deformation across the localisation band. This enrichment allows the introduction of an additional constitutive relationship for the localisation band, which is represented in the form of a cohesive-frictional model describing traction-displacement jump relationship across two sides of the localisation band. The model, formulated within a thermodynamically consistent approach, possesses constitutive responses of the bulk material and two localisation bands connected through internal equilibrium conditions. Its key characteristics are demonstrated and validated against experimental data from different types of geomaterials under different loading conditions at both material and structural levels.
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A thermodynamics-based model for brittle to ductile behaviour and Localised Failure of porous rocks
International Journal of Solids and Structures, 2018Co-Authors: Arash Mir, Giang D Nguyen, Abdul Hamid SheikhAbstract:Abstract This study presents the development of a thermodynamics-based constitutive model for describing the macro-mechanical behaviour of porous rocks. The model formulation is developed within the well-established framework of generalised thermodynamics, or thermodynamics with internal variables (TIV), in order to guarantee the thermodynamics admissibility of the results. A new way for coupling different mechanisms of energy dissipation is proposed within the framework of TIV, so that the essential aspects of macro-mechanical behaviour of porous rocks ranging from brittle to ductile, dilation and compaction, as well as various modes of Localised Failure, under shearing at a wide range of confining pressures are correctly captured. The pressure sensitive behaviour of porous rocks is captured by introducing a shift stress in the formulation of the yield function, through the concept of ‘frozen elastic energy’ or non-dissipative part of the plastic work, which is stored within the material during loading. The proposed model allows the investigation of several important aspects of macro-mechanical behaviour of porous rocks at both material and specimen levels.
