21 research outputs found
A new impact test for the identification of a dynamic crack propagation criterion using a gas-gun device
No full textThe modelling of damage and fracture behaviour under high rates of loadings for metallic structures presents the more and more interests for engineering design, especially for crash phenomena.In order to perform a numerical simulation of such phenomena a crack propagation criterion must be identified using adapted laboratory tests. The objective of this paper is to present a new impact test intended for the identification of a cohesive crack criterion implemented into a home-made FEM code based on Extended Finite ElementMethod. Therefore, a double-notched specimen is impacted using a gas-gun device in order to obtain different crack paths depending on projectile speed. A post-impact macro-photographic observation allows to measure the crack path, the angles and the advancing length. These experimental results are used as input responses in the identification procedure for determining the crack cohesive criterion parameters. Some experimental results, for an aluminium alloy crack criterion identification, are presented to illustrate the proposed approach
Non-Parametric Inference for Bivariate Extreme-Value Copulas
No full textExtreme-value copulas arise as the possible limits of copulas of componentwise maxima of independent, identically distributed samples.The use of bivariate extreme-value copulas is greatly facilitated by their representation in terms of Pickands dependence functions.The two main families of estimators of this dependence function are (variants of) the Pickands estimator and the Caperaa-Fougeres-Genest estimator.In this paper, a unified treatment is given of these two families of estimators, and within these classes those estimators with the minimal asymptotic variance are determined.Main result is the explicit construction of an adaptive, minimum-variance estimator within a class of estimators that encompasses the Caperaa-Fougeres-Genest estimator.estimator;nonparametric inference
A new dynamic test for the identification of high speed friction law using a gas-gun device
No full textIn 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
Étude des vibrations de pièce mince durant l'usinage par stéréo corrélation d'images
No full textLe 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
Integration of a Multi-Physics Model in a Reliability-Based Design Framework: Application to Power Converters
No full textDevelopment of an object-oriented finite element program: application to metal-forming and impact simulations
No full textDuring the last 50 years, the development of better numerical methods and more powerful computers has been a major enterprise for the scientific community. In the same time, the finite element method has become a widely used tool for researchers and engineers. Recent advances in computational software have made possible to solve more physical and complex problems such as coupled problems, nonlinearities, high strain and high-strain rate problems. In this field, an accurate analysis of large deformation inelastic problems occurring in metal-forming or impact simulations is extremely important as a consequence of high amount of plastic flow. In this presentation, the object-oriented implementation, using the C++ language, of an explicit finite element code called DynELA is presented. The object-oriented programming (OOP) leads to better-structured codes for the finite element method and facilitates the development, the maintainability and the expandability of such codes. The most significant advantage of OOP is in the modeling of complex physical systems such as deformation processing where the overall complex problem is partitioned in individual sub-problems based on physical, mathematical or geometric reasoning. We first focus on the advantages of OOP for the development of scientific programs. Specific aspects of OOP, such as the inheritance mechanism, the operators overload procedure or the use of template classes are detailed. Then we present the approach used for the development of our finite element code through the presentation of the kinematics, conservative and constitutive laws and their respective implementation in C++. Finally, the efficiency and accuracy of our finite element program are investigated using a number of benchmark tests relative to metal forming and impact simulations
An incremental formulation for the linear analysis of thin viscoelastic structures using generalized variables
No full textNumerical implementation of the eXtended Finite Element Method for dynamic crack analysis
No full textA 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
Bending, Buckling and Free Vibration of Sandwich Composite Beams with a Transverse Shear Stress Continuity Model
No full textRealization of a Platform for Multidisciplinary Optimization Applied to Steel Constructions
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