References of "Noels, Ludovic"
     in
Bookmark and Share    
Full Text
See detailA shell fracture framework based on a full discontinuous Galerkin formulation combined with an extrinsic cohesive law
Becker, Gauthier ULg; Noels, Ludovic ULg

Conference (2011, June 06)

The cohesive method can be combined easily with Finite Element method to simulate a fracture problem which can contains fracture initiation and propagation. Nevertheless, the insertion of cohesive ... [more ▼]

The cohesive method can be combined easily with Finite Element method to simulate a fracture problem which can contains fracture initiation and propagation. Nevertheless, the insertion of cohesive elements is not straightforward. Indeed, the two classical approaches suffer from severe limitations. On one hand, in the intrinsic approach, as the cohesive element is inserted at the beginning, this element has to model the continuum stage of deformation before fracture. This is ensured by an initial slope in the cohesive law which leads to a stiffness modification and to an alteration of propagation of wave. On the other hand, the introduction of the cohesive element during the simulation in extrinsic approach requests a dynamic modification of mesh. This operation is very difficult to implement especially in the case of a parallel implementation which is almost mandatory due to the very important number of degrees of freedom inherent to a fine mesh used to track the crack path. A solution to these limitations, pioneered by J. Mergheim and R. Radovitzky is to recourse to a discontinuous Galerkin formulation. Indeed this one used discontinuous test functions and integration at the interface of elements to discretize a structure with discontinuous elements. The integration on the boundary of elements allows ensuring weakly the continuity of displacements in a stable and consistent manner. As interface elements are present they can be easily substituted by cohesive elements when a fracture criterion is reached. The interest of the method has been recently proved by R. Radovitzky etal. for 3D elements and by the authors for Euler-Bernoulli beams. An extension of the formulation to Kirchhoff-Love shell is presented here. A novel extrinsic cohesive law is developed to model a through the thickness fracture. In fact, as in thin bodies formulation the thickness is not “discretized” this operation is not straightforward. Indeed, as the fracture occurs only in tension, in a pure bending case the position of neutral axis has to be move to propagate the fracture. To avoid this complicated step, it is suggested to integrate on the thickness the cohesive law which is then applies on resultant efforts. The coupling between the openings in displacement and rotation is performed in a way which guarantees a proper release of energy for any loading. Furthermore, the combination between fracture modes I and II is realized as suggested by M. Ortiz etal. Some numerical quasi-static and dynamic benchmarks are simulated to show the interest and the good performance of the presented framework. [less ▲]

Detailed reference viewed: 15 (4 ULg)
Full Text
Peer Reviewed
See detailA fracture framework for Euler Bernoulli beams based on a full discontinuous Galerkin formulation/extrinsic cohesive law combination
Becker, Gauthier ULg; Noels, Ludovic ULg

in International Journal for Numerical Methods in Engineering (2011), 85(10), 12271251

A new full Discontinuous Galerkin discretization of Euler Bernoulli beam is presented. The main interest of this framework is its ability to simulate fracture problems by inserting a cohesive zone model ... [more ▼]

A new full Discontinuous Galerkin discretization of Euler Bernoulli beam is presented. The main interest of this framework is its ability to simulate fracture problems by inserting a cohesive zone model in the formulation. With a classical Continuous Galerkin method the use of the cohesive zone model is di cult because as insert a cohesive element between bulk elements is not straightforward. On one hand if the cohesive element is inserted at the beginning of the simulation there is a modification of the structure stiffness and on the other hand inserting the cohesive element during the simulation requires modification of the mesh during computation. These drawbacks are avoided with the presented formulation as the structure is discretized in a stable and consistent way with full discontinuous elements and inserting cohesive elements during the simulation becomes straightforward. A new cohesive law based on the resultant stresses (bending moment and membrane) of the thin structure discretization is also presented. This model allows propagating fracture while avoiding through-the-thickness integration of the cohesive law. Tests are performed to show that the proposed model releases, during the fracture process, an energy quantity equal to the fracture energy for any combination of tension-bending loadings. [less ▲]

Detailed reference viewed: 169 (76 ULg)
Full Text
See detailFinite Element Computation of Nonlinear Normal Modes
Renson, Ludovic ULg; Deliège, Geoffrey ULg; Noels, Ludovic ULg et al

in Fifth International Conference on Advanced COmputational Methods in ENgineering (2011)

Detailed reference viewed: 39 (7 ULg)
Full Text
See detailDesign of Microswitch Systems Avoiding Stiction due to Surface Contact
Wu, Ling ULg; Noels, Ludovic ULg; Rochus, Véronique et al

in Proulx, Tom (Ed.) MEMS and Nanotechnology, Volume 2 (2011)

Stiction which results from contact between surfaces is a major failure mode in micro electro-mechanical systems (MEMS). Increasing restoring forces using high spring constant allows avoiding stiction but ... [more ▼]

Stiction which results from contact between surfaces is a major failure mode in micro electro-mechanical systems (MEMS). Increasing restoring forces using high spring constant allows avoiding stiction but leads to an increase of the actuation voltage so that the switch’s efficiency is threatened. A statistical rough surfaces interaction model, based on Maugis’ and Kim’s formulations is applied to estimate the adhesive forces in MEMS switches. Based on the knowledge of these forces, the proper design range of the equivalent spring constant, which is the main factor of restoring force in MEMS switches, can be determined. The upper limit of equivalent spring constant depends mainly on the expected actuator voltage and on the geometric parameters, such as initial gap size and thickness of dielectric layer. The lower limit is assessed on the value of adhesive forces between the two contacting rough surfaces. It mainly depends on the adhesive work of contact surfaces and on the surfaces’ roughness. In order to study more complicated structures, this framework will be used in a multiscale model: resulting unloading micro adhesive contact-distance curves of two rough surfaces will be used as contact forces in a finite-element model. In this paper the extraction of these curves for the particular case of gold to gold micro-switches is pursued. [less ▲]

Detailed reference viewed: 54 (22 ULg)
Full Text
Peer Reviewed
See detailA scalable 3D fracture and fragmentation algorithm based on a hybrid, discontinuous Galerkin, Cohesive Element Method
Radovitzky, Raúl; Seagraves, Andrew; Tupek, Mike et al

in Computer Methods in Applied Mechanics & Engineering (2011), 200(1-4), 326-344

A scalable algorithm for modeling dynamic fracture and fragmentation of solids in three dimensions is presented. The method is based on a combination of a discon- tinuous Galerkin (DG) formulation of the ... [more ▼]

A scalable algorithm for modeling dynamic fracture and fragmentation of solids in three dimensions is presented. The method is based on a combination of a discon- tinuous Galerkin (DG) formulation of the continuum problem and Cohesive Zone Models (CZM) of fracture. Prior to fracture, the flux and stabilization terms aris- ing from the DG formulation at interelement boundaries are enforced via interface elements, much like in the conventional intrinsic cohesive element approach, albeit in a way that guarantees consistency and stability. Upon the onset of fracture, the traction-separation law (TSL) governing the fracture process becomes operative without the need to insert a new cohesive element. Upon crack closure, the rein- statement of the DG terms guarantee the proper description of compressive waves across closed crack surfaces. The main advantage of the method is that it avoids the need to propagate topo- logical changes in the mesh as cracks and fragments develop, which enables the indistinctive treatment of crack propagation across processor boundaries and, thus, the scalability in parallel computations. Another advantage of the method is that it preserves consistency and stability in the uncracked interfaces, thus avoiding issues with wave propagation typical of intrinsic cohesive element approaches. A simple problem of wave propagation in a bar leading to spall at its center is used to show that the method does not affect wave characteristics and as a consequence properly captures the spall process. We also demonstrate the ability of the method to capture intricate patterns of radial and conical cracks arising in the impact of ceramic plates which propagate in the mesh impassive to the presence of processor boundaries. [less ▲]

Detailed reference viewed: 217 (64 ULg)
Full Text
Peer Reviewed
See detailProceedings of the Fourth International Conference on Advanced Computational Methods in Engineering (ACOMEN 2008)
Hogge, Michel ULg; Van Keer, Roger; Dick, Erik et al

in Journal of Computational & Applied Mathematics (2010), 234(7),

Detailed reference viewed: 73 (30 ULg)
Full Text
See detailA Full Discontinuous Galerkin Formulation Of Euler Bernoulli Beams In Linear Elasticity With Fractured Mechanic Applications
Becker, Gauthier ULg; Noels, Ludovic ULg

Conference (2010, July 21)

A full discontinuous Galerkin method is used to predict the fracture of beams thanks to insertion of an extrinsic cohesive element. In fact, The formulation developed originally by G. Wells etal. to ... [more ▼]

A full discontinuous Galerkin method is used to predict the fracture of beams thanks to insertion of an extrinsic cohesive element. In fact, The formulation developed originally by G. Wells etal. to guarantee weakly the high order derivatives of plates with only displacement field unknown and extended by L. Noels etal. for shells is derived for beam with full discontinuous elements. This new formulation can be advantageously combined, as shown first by J. Mergheim etal. , with an extrinsic cohesive approach as there is no need to modify dynamically the mesh, which is the major drawback of this approach. The pre-fractured stage is modeled by full discontinuous elements in a manner which is proved stable and consistent and the fracture is modeled by a cohesive law applied on stress resultant an stress couple defined by J.C. Simo etal. The suggested study produces two type of results. On one hand, it is shown analytically and verified by numerical examples that the presented framework has got the properties of consistency and convergence expected for a numerical scheme. On the other hand, it is proved by some test cases that the energy released during fracture process is equal to the fracture energy except in the case where the difference of internal energy between not fractured and fractured configurations is bigger than the fracture energy. In this case, the fracture occurs in one time step. The presented work proposed a novel interesting manner to take into account fracture in thin bodies. The verification made on the particularized case of beams suggested great perspectives for plates and shells which allow to simulate more complex problems. [less ▲]

Detailed reference viewed: 44 (8 ULg)
Full Text
See detailApplication of discontinuous Galerkin methods to shells and fracture of thin structures
Noels, Ludovic ULg

Scientific conference (2010, January)

Detailed reference viewed: 1 (0 ULg)
Full Text
See detailPrediction of Stiction in Microswitch Systems
Wu, Ling ULg; Rochus, Véronique ULg; Noels, Ludovic ULg et al

in EUROSIME 2010 (2010)

Stiction is a major failure mode of MEMS as microscopic structures tend to adhere to each other when their surfaces enter into contact. Although increasing the restoring forces of switch devices could ... [more ▼]

Stiction is a major failure mode of MEMS as microscopic structures tend to adhere to each other when their surfaces enter into contact. Although increasing the restoring forces of switch devices could overcome the stiction effect, this is not practical, as in turn, it also increases the actuation voltage. Therefore stiction prediction is important to be considered when designing micro- and nano- devices. In this paper, the numerical prediction of stiction for capacitive MEMS switches is considered. Toward this end, a micro-adhesive-contact law is derived from previous work and combined with a finite-element model. [less ▲]

Detailed reference viewed: 77 (26 ULg)
Full Text
See detailEvaluation of Tribo-Mechanical Properties of Thin Films Using Atomic Force Microscope
Pustan, Marius ULg; Rochus, Véronique ULg; Wu, Ling ULg et al

in First European Conference on Nanofilm ECNF2010 (2010)

Experimental investigations of mechanical and tribological properties of thin films using an atomic force microscope and its combination with nanoidentation are presented in this paper. The normal mode of ... [more ▼]

Experimental investigations of mechanical and tribological properties of thin films using an atomic force microscope and its combination with nanoidentation are presented in this paper. The normal mode of an atomic force microscope is used to measure the stiffness and hardness of thin films which are tribologically characterized by roughness, nano-scale adhesion forces and friction forces. The friction forces are measured using the lateral force mode of atomic force microscope. In order to measure the adhesion forces of thin films, spectroscopy in point with atomic force microscope was performed. Direct measurement of tribological and mechanical behaviour of thin films is important to increase the lifetime of microstructures which use thin films for friction and stiction reduction of microsystems. [less ▲]

Detailed reference viewed: 81 (23 ULg)
Full Text
See detailDesign of Microswitch Systems Avoiding Stiction due to Surface Contact
Wu, Ling ULg; Noels, Ludovic ULg; Rochus, Véronique ULg et al

in SEM 2010 Annual Conference & Exposition on Experimental and Applied Mechanics (2010)

Stiction which results from contact between surfaces is a major failure mode in micro electro-mechanical systems (MEMS). Increasing restoring forces using high spring constant allows avoiding stiction but ... [more ▼]

Stiction which results from contact between surfaces is a major failure mode in micro electro-mechanical systems (MEMS). Increasing restoring forces using high spring constant allows avoiding stiction but leads to an increase of the actuation voltage so that the switch’s efficiency is threatened. A statistical rough surfaces interaction model, based on Maugis’ and Kim’s formulations is applied to estimate the adhesive forces in MEMS switches. Based on the knowledge of these forces, the proper design range of the equivalent spring constant, which is the main factor of restoring force in MEMS switches, can be determined. The upper limit of equivalent spring constant depends mainly on the expected actuator voltage and on the geometric parameters, such as initial gap size and ,thickness of dielectric layer. The lower limit is assessed on the value of adhesive forces between the two contacting rough surfaces. It mainly depends on the adhesive work of contact surfaces and on the surfaces’ roughness. In order to study more complicated structures, this framework will be used in a multiscale model: resulting unloading micro adhesive contact-distance curves of two rough surfaces will be used as contact forces in a finite-element model. In this paper the extraction of these curves for the particular case of gold to gold micro-switches is pursued. [less ▲]

Detailed reference viewed: 127 (35 ULg)
Full Text
See detailMulti-scale computational homogenization for structured shells
Kouznetsova, Varvara; Coenen, Erica; Fioole, Joost et al

Conference (2010)

Substructured and layered thin sheets can be found in a variety of structural, e.g. structured panels, as well as high-tech applications. A typical example is flexible electronics, e.g. flexible displays ... [more ▼]

Substructured and layered thin sheets can be found in a variety of structural, e.g. structured panels, as well as high-tech applications. A typical example is flexible electronics, e.g. flexible displays, where layers of different materials and interconnects are stacked to provide the necessary functionality. The resulting complex three dimensional geometry of the structured thin sheets in this type of applications prohibits the use of classical layer-wise composite shell theory. For these problems, a computational homogenization technique for thin structured sheets is proposed in this work, based on the computational homogenization concepts previously developed for the first- and second-order continua. [less ▲]

Detailed reference viewed: 98 (8 ULg)
Full Text
See detailDisplacement Extrapolation Method : An alternative to J-integral for stress intensity factors computation using X-FEM
Minnebo, Hans; Majérus, Julien; Noels, Ludovic ULg

Conference (2010)

Stress intensity factors (SIFs) computation techniques based on J-integral methods [1] are widely used in finite element codes. However, the Displacement Extrapolation Method (DEM) [2] is an alternative ... [more ▼]

Stress intensity factors (SIFs) computation techniques based on J-integral methods [1] are widely used in finite element codes. However, the Displacement Extrapolation Method (DEM) [2] is an alternative way to compute them. The idea is to process an extrapolation based on the relative displacement of the crack face nodes. This method is generally less accurate than integral methods, but it requires much less computational resources. The method is well documented and is implemented in some finite element software. The X-FEM introduced in [3] is suited for crack problems. However, the literature on DEM in the X-FEM case seems to be non-existent in the present time. This paper tries to fill this lack in the literature by describing a new method using X-FEM. The approach followed here is only developed for Linear Elastic Fracture Mechanics. Note that the finite element software Code_Aster [4] already computes SIFs with DEM in X-FEM, but in a slightly different way. [less ▲]

Detailed reference viewed: 185 (9 ULg)
Full Text
Peer Reviewed
See detailInfluence of Adhesive Rough Surface Contact on Micro-Switches
Wu, Ling ULg; Rochus, Véronique ULg; Noels, Ludovic ULg et al

in Journal of Applied Physics (2009), 106(11), 113502

Stiction is a major failure mode in micro electro-mechanical systems (MEMS). Undesirable stiction, which results from contact between surfaces, threatens the reliability of MEMS severely as it breaks the ... [more ▼]

Stiction is a major failure mode in micro electro-mechanical systems (MEMS). Undesirable stiction, which results from contact between surfaces, threatens the reliability of MEMS severely as it breaks the actuation function of MEMS switches for example. Although it may be possible to avoid stiction by increasing restoring forces using high spring constants, it follows that the actuation voltage has also to be increased significantly, which reduces the efficiency. In our research, an electrostatic-structural analysis is performed to estimate the proper design range of the equivalent spring constant which is the main factor of restoring force in MEMS switches. The upper limit of equivalent spring constant is evaluated based on the initial gap width, the dielectric thickness, and the expected actuation voltage. The lower limit is assessed on the value of adhesive forces between the two contacting rough surfaces. The MEMS devices studied here are assumed to work in a dry environment. In these operating conditions only the Van der Waals forces have to be considered for adhesion. A statistical model is used to simulate the rough surface, and the Maugis’s model is combined with Kim’s expansion to calculate adhesive forces. In the resulting model, the critical value of the spring stiffness depends on the material and surface properties, such as the elastic modulus, surface energy and surface roughness. The aim of this research is to propose simple rules for design purposes. [less ▲]

Detailed reference viewed: 115 (50 ULg)
Full Text
See detailA one-field discontinuous Galerkin formulation of non-linear Kirchhoff-Love shells
Noels, Ludovic ULg

in International Journal of Material Forming (2009), 2(Suppl. 1), 877-880

Spatially-discontinuous Galerkin methods constitute a generalization of weak formulations, which allow for discontinuities of the problem unknowns in its domain interior. This is particularly appealing ... [more ▼]

Spatially-discontinuous Galerkin methods constitute a generalization of weak formulations, which allow for discontinuities of the problem unknowns in its domain interior. This is particularly appealing for problems involving high-order derivatives, since discontinuous Galerkin (DG) methods can also be seen as a means of enforcing higher-order continuity requirements. Recently, DG formulations of linear and non-linear Kirchhoff-Love shell theories have been proposed. This new one-field formulations take advantage of the weak enforcement in such a way that the displacements are the only discrete unknowns, while the C1 continuity is enforced weakly. The Resulting one field formulation is a simple and efficient method to model thin structures and can be applied to various computational methods. [less ▲]

Detailed reference viewed: 32 (10 ULg)
Full Text
See detailA One-Field Discontinuous Galerkin Formulation of Non-Linear Kirchhoff-Love Shells
Noels, Ludovic ULg

Conference (2009)

Spatially-discontinuous Galerkin methods constitute a generalization of weak formulations, which allow for discontinuities of the problem unknowns in its domain interior. This is particularly appealing ... [more ▼]

Spatially-discontinuous Galerkin methods constitute a generalization of weak formulations, which allow for discontinuities of the problem unknowns in its domain interior. This is particularly appealing for problems involving high-order derivatives, since discontinuous Galerkin (DG) methods can also be seen as a means of enforcing higher-order continuity requirements. Recently, DG formulations of linear and non-linear Kirchhoff-Love shell theories have been proposed. This new one-field formulations take advantage of the weak enforcement in such a way that the displacements are the only discrete unknowns, while the C1 continuity is enforced weakly. The resulting one field formulation is a simple and efficient method to model thin structures and can be applied to various computational methods. [less ▲]

Detailed reference viewed: 52 (9 ULg)
Full Text
See detailInfluence of Adhesive Rough Surface Contact on Micro-Switches
Wu, Ling ULg; Rochus, Véronique ULg; Noels, Ludovic ULg et al

Scientific conference (2009)

Detailed reference viewed: 47 (19 ULg)
Full Text
Peer Reviewed
See detailA discontinuous Galerkin formulation of non-linear Kirchhoff–Love shells
Noels, Ludovic ULg

in International Journal for Numerical Methods in Engineering (2009), 78(3), 296-323

Discontinuous Galerkin (DG) methods provide a means of weakly enforcing the continuity of the unknown-field derivatives and have particular appeal in problems involving high-order derivatives. This ... [more ▼]

Discontinuous Galerkin (DG) methods provide a means of weakly enforcing the continuity of the unknown-field derivatives and have particular appeal in problems involving high-order derivatives. This feature has previously been successfully exploited (Comput. Methods Appl. Mech. Eng. 2008; 197:2901-2929) to develop a formulation of linear Kirchhoff-Love shells considering only the membrane and bending responses. In this proposed one-field method - the displacements are the only unknowns, while the displacement field is continuous, the continuity in the displacement derivative between two elements is weakly enforced by recourse to a DG formulation. It is the purpose of the present paper to extend this formulation to finite deformations and non-linear elastic behaviors. While the initial linear formulation was relying on the direct linear computation of the effective membrane stress and effective bending couple-stress from the displacement field at the mid-surface of the shell, the non-linear formulation considered implies the evaluation of the general stress tensor across the shell thickness, leading to a reformulation of the internal forces of the shell. Nevertheless, since the interface terms resulting from the discontinuous Galerkin method involve only the resultant couple-stress at the edges of the shells, the extension to non-linear deformations is straightforward. [less ▲]

Detailed reference viewed: 98 (21 ULg)