Browse ORBi by ORBi project

- Background
- Content
- Benefits and challenges
- Legal aspects
- Functions and services
- Team
- Help and tutorials

Application of discontinuous Galerkin methods to shells and fracture of thin structures Noels, Ludovic Scientific conference (2010, January) Detailed reference viewed: 1 (0 ULg)Prediction of Stiction in Microswitch Systems Wu, Ling ; Rochus, Véronique ; Noels, Ludovic 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: 95 (32 ULg)Evaluation of Tribo-Mechanical Properties of Thin Films Using Atomic Force Microscope Pustan, Marius ; Rochus, Véronique ; Wu, Ling 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: 89 (24 ULg)Design of Microswitch Systems Avoiding Stiction due to Surface Contact Wu, Ling ; Noels, Ludovic ; Rochus, Véronique 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: 158 (41 ULg)Multi-scale computational homogenization for structured shells ; ; 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: 154 (9 ULg)Displacement Extrapolation Method : An alternative to J-integral for stress intensity factors computation using X-FEM ; ; Noels, Ludovic 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: 246 (9 ULg)Influence of Adhesive Rough Surface Contact on Micro-Switches Wu, Ling ; Rochus, Véronique ; Noels, Ludovic 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: 138 (53 ULg)Discontinuous Galerkin methods for solid mechanics: Application to fracture, shells & strain gradient elasticity Noels, Ludovic Scientific conference (2009, October) Detailed reference viewed: 4 (0 ULg)A one-field discontinuous Galerkin formulation of non-linear Kirchhoff-Love shells Noels, Ludovic 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: 37 (10 ULg)Discontinuous Galerkin methods for solid mecha nics: Application to fracture, shells & strain gradient elasticity Noels, Ludovic Scientific conference (2009, August) Detailed reference viewed: 3 (0 ULg)A One-Field Discontinuous Galerkin Formulation of Non-Linear Kirchhoff-Love Shells Noels, Ludovic 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: 58 (9 ULg)Influence of Adhesive Rough Surface Contact on Micro-Switches Wu, Ling ; Rochus, Véronique ; Noels, Ludovic et al Scientific conference (2009) Detailed reference viewed: 52 (21 ULg)A discontinuous Galerkin formulation of non-linear Kirchhoff–Love shells Noels, Ludovic 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: 105 (25 ULg)Ça plane pour toi …: Manuel d’aérodynamisme de base à l’attention des élèves du secondaire supérieur Noels, Ludovic Learning material (2009) Detailed reference viewed: 78 (11 ULg)Computational biology — Modeling of primary blast effects on the central nervous system ; ; et al in NeuroImage (2009), 47(Sup. 2), 10-20 Objectives Recent military conflicts in Iraq and Afghanistan have highlighted the wartime effect of traumatic brain injury. The reason for the prominence of TBI in these particular conflicts as opposed to ... [more ▼] Objectives Recent military conflicts in Iraq and Afghanistan have highlighted the wartime effect of traumatic brain injury. The reason for the prominence of TBI in these particular conflicts as opposed to others is unclear but may result from the increased survivability of blast due to improvements in body armor. In the military context blunt, ballistic and blast effects may all contribute to CNS injury, however blast in particular, has been suggested as a primary cause of military TBI. While blast effects on some biological tissues, such as the lung, are documented in term of injury thresholds, this is not the case for the CNS. We hypothesized that using bio-fidelic models, allowing for fluid-solid interaction and basic material properties available in the literature, that a blast wave would interact with CNS tissue and cause a possible concussive effect. Methods The blast shockwave on CNS tissue was modeled using a coupled computational fluid-solid dynamic simulation. The model included a complex finite element mesh of the head and intra-cranial contents. The effects of threshold and 50% lethal blast lung injury were compared with concussive impact injury using the full head model allowing know upper and lower bounds of tissue injury to be applied using pulmonary injury as the reference tissue. Results The effects of a 50% lethal dose blast lung injury (LD50) were comparable with concussive impact injury using the DVBIC – MIT full head model. Interpretation CNS blast concussive effects were found to be similar between impact mild TBI and the blast field associated with LD50 lung blast injury sustained without personal protective equipment. With the ubiquitous use of personal protective equipment this suggests that blast concussive effects may more readily occur in personnel due to enhanced survivability in the current conflicts. [less ▲] Detailed reference viewed: 107 (16 ULg)A Discontinuous Galerkin Formulation of Kirchhoff-Love Shells: From Linear Elasticity to Finite Deformations Noels, Ludovic ; Conference (2008, June) Spatially-discontinuous Galerkin methods constitute a generalization of weak formulations, which allow for discontinuities of the problem unknowns in its domain interior [1]. When considering problems ... [more ▼] Spatially-discontinuous Galerkin methods constitute a generalization of weak formulations, which allow for discontinuities of the problem unknowns in its domain interior [1]. When considering problems involving high-order derivatives, discontinuous Galerkin methods can also be seen as a means of enforcing higher-order continuity requirements in a weak manner [2,3]. Recently, the authors [4] have proposed a DG formulation for Kirchhoff-Love shell theory for which both the membrane and the bending response of the shell are considered. The proposed one-field formulation takes 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 consistency, stability and rate of convergence of the numerical method are demonstrated for the case of a linear elastic material. In this work, this method is extended to shell problems involving finite displacements and finite deformations. [less ▲] Detailed reference viewed: 17 (1 ULg)Actes du Congrès ACOMEN 2008, Second international Conference on Advanced Computational Methods in Engineering, Liège, Belgium, May 26-38, 2008. Hogge, Michel ; ; Noels, Ludovic et al Book (2008) Detailed reference viewed: 63 (6 ULg)A first-order energy-dissipative momentum-conserving scheme for elasto-plasticity using the variational updates formulation Noels, Ludovic ; Stainier, Laurent ; Ponthot, Jean-Philippe in Computer Methods in Applied Mechanics & Engineering (2008), 197(6-8), 706726 In a previous paper [L. Noels, L. Stainier, J.-P. Ponthot, An energy momentum conserving algorithm using the variational formulation of visco-plastic updates, Int. J. Numer. Methods Engrg. 65 (2006) 904 ... [more ▼] In a previous paper [L. Noels, L. Stainier, J.-P. Ponthot, An energy momentum conserving algorithm using the variational formulation of visco-plastic updates, Int. J. Numer. Methods Engrg. 65 (2006) 904-942] the authors demonstrated the efficiency of the variational formulation of elasto-plastic updates to develop energy-momentum conserving time integration algorithms. Indeed, within such a framework, the stress tensor always derives from an incremental potential, even when plastic behavior is considered. Therefore the verification of the conservation of energy in the non-linear range can easily be demonstrated: the sum of the reversible stored energy and irreversible dissipated energy exactly corresponds to the work of the external forces applied to the structure. Although this formulation was shown to be accurate and robust, the introduction of numerical dissipation for high-frequency numerical modes can be necessary to simulate complex phenomena. In this work, we propose a modification of the variational updates framework to introduce this numerical property, leading to a new energy-dissipative momentum-conserving time-integration algorithm for elasto-plasticity. (c) 2007 Elsevier B.V. All rights reserved. [less ▲] Detailed reference viewed: 66 (18 ULg)Comparative study of numerical explicit schemes for impact problems ; Ponthot, Jean-Philippe ; Noels, Ludovic in International Journal of Impact Engineering (2008), 35(12), 1688-1694 Explicit numerical schemes are used to integrate in time finite element discretization methods. Unfortunately, these numerical approaches can induce high-frequency numerical oscillations into the solution ... [more ▼] Explicit numerical schemes are used to integrate in time finite element discretization methods. Unfortunately, these numerical approaches can induce high-frequency numerical oscillations into the solution. To eliminate or to reduce these oscillations, numerical dissipation can be introduced. The paper deals with the comparison of three different explicit schemes: the central difference scheme which is a nondissipative method, the Hulbert Chung dissipative explicit scheme and the Tchamwa-Wielgosz dissipative scheme. Particular attention is paid to the study of these algorithms’ behavior in problems involving high-velocity impacts like Taylor anvil impact and bullet-target interactions. It has been shown that Tchamwa-Wielgosz scheme is efficient in filtering the high-frequency oscillations and is more dissipative than Hulbert Chung explicit scheme. Although its convergence rate is only first order, the loss of accuracy remains limited to acceptable values. [less ▲] Detailed reference viewed: 118 (15 ULg)An explicit discontinuous Galerkin method for non-linear solid dynamics. Formulation, parallel implementation and scalability properties. Noels, Ludovic ; in International Journal for Numerical Methods in Engineering (2008), 74 An explicit-dynamics spatially-discontinuous Galerkin (DG) formulation for non-linear solid dynamics is proposed and implemented for parallel computation. Discontinuous Galerkin methods have particular ... [more ▼] An explicit-dynamics spatially-discontinuous Galerkin (DG) formulation for non-linear solid dynamics is proposed and implemented for parallel computation. Discontinuous Galerkin methods have particular appeal in problems involving complex material response, e.g. non-local behavior and failure, as, even in the presence of discontinuities, they provide a rigorous means of ensuring both consistency and stability. In the proposed method, these are guaranteed: the former by the use of average numerical fluxes, and the latter by the introduction of appropriate quadratic terms in the weak formulation. The semi-discrete system of ordinary differential equations is integrated in time using a conventional second-order central-difference explicit scheme. A stability criterion for the time integration algorithm, accounting for the influence of the DG discretization stability, is derived for the equivalent linearized system. This approach naturally lends itself to efficient parallel implementation. The resulting DG computational framework is implemented in three dimensions via specialized interface elements. The versatility, robustness and scalability of the overall computational approach are all demonstrated in problems involving stress-wave propagation and large plastic deformations. [less ▲] Detailed reference viewed: 91 (10 ULg) |
||