Numerical implementation of the eXtended Finite Element Method for dynamic crack analysis

A numerical implementation of the eXtended Finite Element Method (X-FEM) to analyze crack propagation in a structure under dynamic loading is presented in this paper. The arbitrary crack is treated by the X-FEM method without re-meshing but using an enrichment of the classical displacement-based finite element approximation in the framework of the partition of unity method. Several algorithms have been implemented, within an Oriented Object framework in C++, in the home made explicit FEM code. The new module, called DynaCrack, included in the dynamic FEM code DynELA, evaluates the crack geometry, the propagation of the crack and allow the post-processing of the numerical results. The module solves the system of discrete equations using an explicit integration scheme. Some numerical examples illustrating the main features and the computational efficiency of the DynaCrack module for dynamic crack propagation are presented in the last section of the paper

Méthodologies d'évaluation de la vulnérabilité sismique de bâtiments existants à partir d'une instrumentation in situ

La France Métropolitaine est composée de régions à sismicité modérée mais néanmoins vulnérables aux tremblements de terre. En effet, 85% des bâtiments existants ont été construits avant l apparition des règles de construction parasismique. Pour évaluer la vulnérabilité sismique de ces structures, il existe différentes méthodes à grande échelle telles que Hazus ou Risk-UE, non adaptées à l échelle d un bâtiment. Deux typologies de structures ont été étudiées dans ce travail : les structures récentes en béton armé représentatives des grands bâtiments stratégiques, et les structures en maçonnerie non renforcée, représentatives des centres villes historiques. Compte-tenu de la sismicité modérée, les structures récentes en béton armé ont un comportement linéaire élastique. Dans ce cadre, l instrumentation d un bâtiment, tel que la Tour de l Ophite, est essentielle car elle permet de déterminer les vibrations ambiantes d une structure et d en extraire les paramètres modaux (fréquences propres, amortissements et déformées modales) qui incluent naturellement des informations sur la qualité des matériaux utilisés, leur vieillissement, leur endommagement, etc. De plus, un outil, basé sur la méthode stochastique par sous-espaces à l aide des matrices de covariance (SSI-COV), a été développé afin de détecter au mieux les modes propres très proches (modes doubles), lors du traitement des données issues de l instrumentation de la Tour de l Ophite. Un modèle numérique par Eléments Finis est également proposé afin de prédire, dans le domaine linéaire, les déplacements de la Tour de l Ophite soumise à un séisme identique à celui des Abruzzes en Italie en 2009. Pour les structures en maçonnerie non renforcée, un modèle de comportement non-linéaire des matériaux, avec une approche de type endommagement fragile, a été développé et utilisé pour la simulation numérique du comportement ductile des panneaux, remplaçant ainsi la mise en œuvre d essais expérimentaux lourds et coûteux. A partir des travaux précédents, une méthodologie analytique d évaluation de la vulnérabilité sismique des bâtiments existants, validée par comparaison avec le code numérique TreMuRi, est proposée et appliquée à un bâtiment. Par exemple, la généricité de notre méthodologie a permis de mener une investigation sur un matériau local, les murs en galets.France is a country composed of moderate seismic hazard regions however vulnerable to earthquakes. 85% of existing buildings have been built before the application of paraseismic codes. Several current large-scale seismic vulnerability assessment methods are used, such as Hazus or Risk-UE, but they are inappropriate for specific building analysis. Two structure types have been evaluated in this thesis; the recent reinforced concrete structures for high strategic buildings, and the unreinforced masonry structures, for historical city centres. In view of the moderate seismic risk, recent reinforced concrete structures have an elastic behaviour. In this context, the instrumentation of a building, such as the Ophite Tower, is essential as it determines the ambient vibrations of a structure and extracts modal parameters (natural frequencies, modal shapes and damping), which naturally include information such as quality of materials, ageing, damage, etc. Moreover, as a tool based on the Covariance driven Stochastic Subspace Identification method (SSI-COV), it was developed in order to improve the detection of very close natural modes (double modes), during data processing from the instrumentation of Ophite Tower. A Finite Element numerical model (linear) was also proposed to predict the displacements of Ophite Tower under a seismic motion similar to Abruzzes earthquake (Italy, 2009). For unreinforced masonry structures, a model for nonlinear behaviour of materials with a brittle cracking approach has been developed. This is used for the numerical simulation of the ductile behaviour of panels and replaces the expensive experimental tests. From previous works, an analytical seismic vulnerability assessment method of existing buildings, validated par comparison with TreMuRi code, was proposed and applied to a building. For example, the genericity of this methodology has led to an investigation of a local material (pier walls).TOULOUSE-INP (315552154) / SudocSudocFranceF

A new dynamic test for the identification of high speed friction law using a gas-gun device

In the framework of metal forming, which involves high speed loads, the determination of the friction law is one of the most essential topics. Especially for Finite-element simulation the development of innovative local friction laws improves the quality of the numerical results. One of the most critical point reside in the accuracy of the identification of the governing friction law parameters. A new experimental test is based on conical extrusion. The idea is to launch a cylindrical projectile into a target provided with a conical bore prolonged with a cylindrical one. The projectile is stopped by friction forces occurring at the interface between those materials. After impact the length of extrusion is taken into account for friction law identification. The experimental set-up used is a ballistic gas-gun device capable to launch the projectile, in vacuum conditions, up to 300 m/s. The experiment is simulated by a numerical model using ABAQUS Explicit finite-element code. This code allows the implementation of various user friction laws through a FORTRAN subroutine. In order to obtain accurate results, the viscoplastic constitutive law used for both materials (projectile and target) was previously experimentally identified. The friction law identification uses a combined Monte-Carlo and Levenberg-Marquardt algorithm which provides a very precisely set of parameters law. The test presented in this paper involves two metallic materials: steel 42CrMo4 for the target and aluminum 2017 for the projectile. The friction law for the pair of materials used was validated using experimental test at different speeds of impact (149 up to 235 m/s) and the results are quite good proving a good identification of the friction law parameters

A coupled electro-thermo-mechanical FEM code for large scale problems including multi-domain and multiple time-step aspects

This work concerns the development of a virtual prototyping tool for large scale electro-thermo-mechanical simulation of power converters used in railway transport including multi-domain and multiple time-steps aspects. For this purpose, Domain Decomposition Method (DDM) is used to give on one hand the ability to treat large scale problems and on the other hand, for transient analysis, the ability to use different time-steps in different parts of the numerical model. An Object-Oriented programming in C++ is used for the development of the application, allowing integration in a single interface, high evolutivity and code maintenance. Finally, parallelization is introduced to improve the performances of the code and obtain computing times compatible with industrial developments. In a first part, we present the formulation of DDM based on a dual approach in steady-state analysis including the building of the interfacial problem. Then, as presented by Combescure et al. we extend this formulation to multiple time-step transient analysis. The Euler semi-implicit scheme used to integrate first-order systems and the Newmark scheme used to integrate second order systems are detailed for both single time-step and multiple time-step computations. Then, we present a discussion concerning the choice of the continuity at the interface for first-order systems in single time-step and multiple time-step transient analysis. In the second part, the numerical implementation of the proposed approach in the FEM code DynELA is presented with a focus on the treatment of multi-physic aspects. Parallelization of the code on a Shared Memory Processors (SMPs) computer is achieved using the OpenMP programming standard. The last part of this work concerns the study of an industrial benchmark concerning the power converters: the electro-thermal simulation of a semi-conductor chip in transient analysis. This example allows to compare different strategies of tearing into subdomains and to couple different time-steps on the same structure

2D and 3D numerical models of metal cutting with damage effects

In this paper a two-dimensional and a three-dimensional finite element models of unsteady-state metal cutting are presented. These models take into account dynamic effects, thermo-mechanical coupling, constitutive damage law and contact with friction. The simulations concern the study of the unsteady-state process of chip formation. The yield stress is taken as a function of the strain, the strain rate and the temperature in order to reflect realistic behavior in metal cutting. Unsteady-state process simulation needs a material separation criterion (chip criterion) and thus, many models in the literature use an arbitrary criterion based on the effective plastic strain, the strain energy density or the distance between nodes of parts and tool edge. The damage constitutive law adopted in models presented here allows defining advanced simulations of tool's penetration in workpiece and chip formation. The originality introduced here is that this damage law has been defined from tensile and torsion tests, and we applied it for machining process. Stresses and temperature distributions, chip formation and tool forces are shown at different stages of the cutting process. Finally, we present a three-dimensional oblique model to simulate the unsteady-state process of chip formation. This model, using the damage law defined before, allows an advanced simulation close to the real cutting process. The final part shows a milling application. An arbitrary Lagrangian Eulerian formulation (ALE) is used for these simulations; this formalism combines both the advantages of Eulerian and Lagrangian representations in a single description, it is exploited to reduce finite element mesh distortions

Identification de lois constitutives et de lois de frottement adaptées aux grandes vitesses de sollicitation.

Cet article présente une méthodologie d'identification de lois constitutives et de lois de contact adaptées aux matériaux métalliques sous chargement dynamique à grande vitesse de déformation. Les essais sont effectués à partir de montages expérimentaux adaptés à un lanceur à gaz permettant d'obtenir une vitesse de projectile de l'ordre de 350 m/s pour une masse totale de 30 gr. Le premier essai consiste en un impact de Taylor correspondant à un chargement mécanique de type compression. Le second essai de type "extrusion conique" permet la détermination des lois de frottement à grande vitesse. La procédure générale d'identification des lois de comportement à partir d'essais dynamiques se fait au moyen d'une analyse post-mortem des échantillons et de la corrélation entre ces résultats expérimentaux et un modèle numérique des essais. Pour les deux cas précédemment cités, nous présentons la configuration optimale d'essai ainsi que les résultats obtenus à partir d'un algorithme d'optimisation de type Levenberg-Marquard

Modélisation thermo-mécanique et fiabilité des échangeurs réacteurs en carbure de silicium

Les échangeurs réacteurs intensifiés (ERI) sont des échangeurs de chaleur utilisés comme réacteurs chimiques afin d'intensifier les procédés de synthèse. L'ERI étudié dans ce travail est composé de plaques en carbure de silicium, lui conférant ainsi des propriétés thermiques et chimiques importantes. En contrepartie, l'utilisation du SiC accentue les risques de rupture fragile, à ajouter aux risques principaux associés aux ERI qui sont les fuites de fluide et l'emballement thermique de la réaction. L'intensification des procédés représente un gain important en terme de productivité, de sécurité et d'impact environnemental, à condition de maîtriser la conception et la mise en œuvre de l'ERI. L'objectif de ce travail est de proposer une démarche d'étude adaptée à ce type de produit en général, et d'optimiser la conception de cet ERI en particulier. Pour cela, les modèles thermo-chimique et thermo-mécanique de l'ERI sont développés puis utilisés dans une étude mécano-fiabiliste, notamment au moyen à la méthode AK-MCS basée sur l'interpolation par krigeage de la fonction de performance.Intensified heat exchanger reactors (ERI) are heat exchangers used as chemical reactors for process intensification. The ERI studied in this work is composed of silicon carbide (SiC) based plates, giving it interesting thermal and chemical properties. In return, the use of SiC increases the brittle fracture risk, in addition of the main risks that are leaking fluid and thermal runaway reaction. The process intensification represents a significant gain in productivity, safety and environmental impact, provided that the design and the implementation procedure of the ERI are optimized. The objective of this work is to propose a process of study suited to this type of product in general, and to optimize the design of this ERI in particular. For this, the thermo-chemical and thermo-mechanical models are developed and used in a mechano-reliability study, notably using the AK-MCS method based on kriging interpolation of the performance function.TOULOUSE-INP (315552154) / SudocSudocFranceF

Étude des vibrations de pièce mince durant l'usinage par stéréo corrélation d'images

Le travail présenté dans cette thèse vise à comprendre les vibrations de pièce mince durant l usinage. De nombreux travaux proposent des modélisations de ce phénomène, mais des écarts persistent entre résultats de modélisation et réalité. Ce constat nous pousse à nous interroger sur l emploi dans les modèles des modes propres de la pièce, sans y intégrer le contact de l outil. Face à l incapacite de vérifier la validité de cette hypothèse par mesures ponctuelles, la mesure de champ s impose comme une alternative prometteuse. La deuxième partie du travail porte sur la mise au point d un protocole expérimental novateur. Il inclut le relevé des déformées vibratoires d une pièce mince en usinage par mesure de champs de déplacements. La stéréo corrélation d images numériques se confronte à de nombreuses limitations dans ce contexte. Nous avons développé une méthode de réglage des capteurs permettant de contourner rapidement certaines difficultés. Cette méthode présentée sous forme graphique souligne la nécessité d optimiser les paramètres de mesure dans un tel contexte. La troisième partie met en oeuvre le protocole de mesures. Le test des capteurs montre le fort intérêt de la mesure sans contact vis-à-vis de l objectif recherché. Des essais d usinage sont présentés en se basant sur une modélisation existante du broutement. Les déformées mesurées pendant l usinage livrent des informations d un type nouveau. Leur exploitation a impliqué la mise en place d une procédure spécifique de traitement. La dernière partie présente les analyses de deux usinages. L étude est effectuée au regard des états de surface obtenus, du comportement temporel, fréquentiel, et spatial. Cette approche souligne les subtilités de la génération d état de surface en la présence de vibrations. L examen des mesures de champs permet de relever des incohérences avec l emploi des modes propres, classiquement utilisés en modélisation.The work presented in this thesis aims at understanding thin part vibrations during machining. Many works propose modelings of this phenomenon but differencies still exist between modeling results and tests. This observation lead us to wonder about the employment of natural modes of the part in the models, without taking into account the tool presence. The fact that punctual measurements don t enable to verify the validity of this hypothesis, field measurement prove to be a hopeful alternative. The second part focuses on adjusting a novel experimental protocol. It includes the recording of the thin part vibrating shapes by displacement field measurement. Digital Image Stereo Correlation is confronted to many limitations in this context. We developed a method to set sensors enabling the quick avoidance of difficulties. This method is presented in a graphical form, and underlines the need of optimising measurement parameters in such an environment. In the third part of the work, the measurement protocol is used. The sensors testing shows the high interest of contactless measurement for the aimed goal. Machininng tests are presented in connection with an existing model of chatter. The measured shapes during machining give a new sort of informations. So, their analyse implied the building of a specific processing procedure. The last part presents analyses of two machining tests. The study is done by parallely looking at the machined surface, and the behavior in temporal and frequency space as so as the part displacement fields. This approach underlines subtleties of surface generation under vibration conditions. The fields inspection enables to mark inconsistencies if employing the natural modes that are classically used in models.TOULOUSE-INP (315552154) / SudocSudocFranceF

Développement d'un module de prototypage virtuel multiphysique, multidomaine et multitemps (application aux convertisseurs de puissance)

TOULOUSE-ENSEEIHT (315552331) / SudocTARBES-ENIT (654402301) / SudocSudocFranceF

Integration of a multi-physics model in a reliability-based design framework: application to power converters

Structures of power converters constitute very complex systems, the behaviour of which introduces several phenomenons (electric, thermal and mechanic). An accurate modelling of their behaviour, thus, requires a multi-physics model which makes it possible to take into account the physical phenomenon in consideration. To achieve this, a FEM code (e. g. MulPhyDo) adapted to power converters and dedicated to simulation of structure in a multi-physics context has been built in the laboratory. The aim of the present study is to include in the modelling the existence of randomness and uncertainties, particularly of interest in the case of power converters. In order to achieve this, the multi-physics FEM code is coupled with a program dedicated to structural reliability analysis, in an Oriented Object Programming context. In this way, it is possible to compute the failure reliability of the structure under consideration and the sensitivity of the reliability index to each random variable. An application example of a microelectronic package is presented