232 research outputs found
New Internal Stress Driven on-Chip Micromachines for Extracting Mechanical Properties of Thin Films
A new concept of micromachines has been developed for measuring the
mechanical properties of thin metallic films. The actuator is a beam undergoing
large internal stresses built up during the deposition process. Al thin films
are deposited partly on the actuator beam and on the substrate. By etching the
structure, the actuator contracts and pulls the Al film. Full stress strain
curves can be generated by designing a set of micromachines with various
actuator lengths. In the present study, the displacements have been measured by
scanning electronic microscopy. The stress is derived from simple continuum
mechanics relationships. The tensile properties of Al films of various
thicknesses have been tested. A marked increase of the strength with decreasing
film thickness is observed.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
A New Model for Void Coalescence by Internal Necking
A micromechanical model for predicting the strain increment required to bring a damaged material element from the onset of void coalescence up to final fracture is developed based on simple kinematics arguments. This strain increment controls the unloading slope and the energy dissipated during the final step of material failure. Proper prediction of the final drop of the load carrying capacity is an important ingredient of any ductile fracture model, especially at high stress triaxiality. The model has been motivated and verified by comparison to a large set of finite element void cell calculations.
FOUCAULT, Michel, Histoire de la sexualité. Tome I. La volonté du savoir
The interest of using polyimide as a sacrificial and anchoring layer is demonstrated for post-processing surface micromachining and for the incorporation of metallic nanowires into microsystems. In addition to properties like a high planarization factor, a good resistance to most non-oxidizing acids and bases, and CMOS compatibility, polyimide can also be used as a mold for nanostructures after ion track-etching. Moreover, specific polyimide grades, such as PI-2611 from HD Microsystemsâą, involve a thermal expansion coefficient similar to silicon and low internal stress. The process developed in this study permits higher gaps compared to the state-of-the-art, limits stiction problems with the substrate and is adapted to various top-layer materials. Most metals, semiconductors or ceramics will not be affected by the oxygen plasma required for polyimide etching. Released structures with vertical gaps from one to several tens of ÎŒm have been obtained, possibly using multiple layers of polyimide. Furthermore, patterned freestanding nanowires have been synthesized with diameters from 20 to 60ânm and up to 3âÎŒm in length. These results have been applied to the fabrication of two specific devices: a generic nanomechanical testing lab-on-chip platform and a miniaturized ionization sensor
Integrated modeling of friction stir welding of 6xxx series Al alloys: Process, microstructure and properties
International audienceCompared to most thermomechanical processing methods, friction stir welding (FSW) is a recent technique which has not yet reached full maturity. Nevertheless, owing to multiple intrinsic advantages, FSW has already replaced conventional welding methods in a variety of industrial applications especially for Al alloys. This provides the impetus for developing a methodology towards optimization, from process to performances, using the most advanced approach available in materials science and thermomechanics. The aim is to obtain a guidance both for process fine tuning and for alloy design. Integrated modeling constitutes a way to accelerate the insertion of the process, especially regarding difficult applications where for instance ductility, fracture toughness, fatigue and/or stress corrosion cracking are key issues. Hence, an integrated modeling framework devoted to the FSW of 6xxx series Al alloys has been established and applied to the 6005A and 6056 alloys. The suite of models involves an in-process temperature evolution model, a microstructure evolution model with an extension to heterogeneous precipitation, a microstructure based strength and strain hardening model, and a micro-mechanics based damage model. The presentation of each model is supplemented by the coverage of relevant recent literature. The "model chain" is assessed towards a wide range of experimental data. The final objective is to present routes for the optimization of the FSW process using both experiments and models. Now, this strategy goes well beyond the case of FSW, illustrating the potential of chain models to support a "material by design approach" from process to performances
Dislocation structures and the role of grain boundaries in cyclically deformed Ni micropillars
Transmission electron microscopy and finite element-based dislocation simulations were combined to study the development of dislocation microstructures after cyclic deformation of single crystal and bicrystal Ni micropillars oriented for multi-slip. A direct correlation between large accumulation of plastic strain and the presence of dislocation cell walls in the single crystal micropillars was observed, while the presence of the grain boundary hampered the formation of wall-like structures in agreement with a smaller accumulated plastic strain. Automated crystallographic orientation and nanostrain mapping using transmission electron microscopy revealed the presence of lattice heterogeneities associated to the cell walls including long range elastic strain fields. By combining the nanostrain mapping with an inverse modelling approach, information about dislocation density, line orientation and Burgers vector direction was derived, which is not accessible otherwise in such dense dislocation structures. Simulations showed that the image forces associated with the grain boundary in this specific bicrystal configuration have only a minor influence on dislocation behavior. Thus, the reduced occurrence of âmatureâ cell walls in the bicrystal can be attributed to the available volume, which is too small to accommodate cell structures
Analyse par géomatique des bilans et des flux d'azote et de phosphore dans un bassin versant agricole: le cas de la riviÚre Boyer.
L'essor et la concentration géographique des productions animales, dans certains bassins versant
à vocation agricole, génÚrent d'imposants volumes de fumiers et de lisiers qui dépassent souvent
la superficie de sol disponible pour les valoriser. On assiste alors Ă une surfertilisation des cultures,
qui provoque l'accroissement de la teneur en phosphore du sol, le risque de transfert vers
l'environnement ainsi que le lessivage des nitrates. Ces nutriments, lorsqu'ils dépassent un certain
niveau, sont responsables de l'eutrophisation des cours d'eau. Cette problématique de pollution
diffuse est actuellement largement responsable de la détérioration de la qualité des eaux dans
plusieurs régions du Québec.
Ce projet de maĂźtrise a pour objectif d'analyser les bilans et les flux d'azote et de phosphore dans
le bassin versant de la riviĂšre Boyer, un bassin oĂč la pollution diffuse a Ă©tĂ© constatĂ©e. Ceci permet
d'estimer les stocks d'Ă©lĂ©ments nutritifs prĂ©sents et de dĂ©finir les zones critiques oĂč les pertes vers
l'environnement sont les plus susceptibles de se produire. Pour ce faire, nous devons considérer
plusieurs données spatiales des différents facteurs qui interviennent, telles que l'occupation du sol,
la topographie, la pédologie, les conditions climatiques, les différentes pratiques culturales, etc.
Ces informations sont intégrées en exploitant les propriétés des SystÚmes d'Information
Géographique (SIG) qui tiennent compte de la localisation spatiale des données considérées. Dans
le cadre du présent travail, nous exploitons le SIG IDRISI.
Afin d'obtenir une représentation réaliste du mode de fonctionnement du bassin versant à l'étude,
un bilan des apports et des exportations d'azote et de phosphore et des modĂšles de pertes
annuelles d'azote et de phosphore sont Ă©tablis au sein du SIG. Les pertes annuelles de phosphore
particulaire et soluble peuvent ĂȘtre estimĂ©es en intĂ©grant un modĂšle de perte de sol au SIG. Les
pertes annuelles d'azote, par lessivage et ruissellement superficiel, sont estimées à l'aide d'un
modÚle intégré au SIG, qui est construit à partir de coefficients de pertes tirés de la littérature.
Les charges annuelles de phosphore et d'azote sont cumulées sur l'ensemble du bassin versant.
Le cumul se fait suivant les directions d'écoulement, qui sont déterminées grùce à un algorithme
de drainage interfacé au logiciel IDRISI. Cet algorithme utilise un modÚle numérique d'altitude
(MNA) pour générer les directions d'écoulement suivant la plus forte pente. Une comparaison
entre les valeurs des concentrations d'azote et de phosphore, issues des simulations et des valeurs
mesurées en 28 points d'échantillonnage du bassin, permet de discuter des modÚles développés
et d'établir des relations avec certaines caractéristiques du territoire
Measurement of the creep behavior of thin ZrNi metallic glass films â a comparison between nanoindentation relaxation, nanoindentation creep and lab-on-chips experiments
The characterization of the time-dependent behavior of thin metallic glass films is one of the key-issue for surface engineering. Such a measurement requires loading a constant material volume located in the thin film. Unfortunately, this condition is not fulfilled in the commonly used creep nanoindentation testing, contrary to micro tensile lab-on-chip experiments or micropillar compression testing. In this paper, we show that nanoindentation relaxation is an efficient alternative to nanoindentation creep. For that purpose, an extensive study of ZrNi metallic glasses viscoplastic behavior is performed using several experimental set-up (lab on chips, nanoindentation relaxation, nanoindentation creep, constant strain rate, ...). An innovative nanoindentation methodology is used to perform long-term relaxation tests up to 10 h with excellent reproducibility. It consists in maintaining a constant contact area during the test by controlling the contact stiffness between the tip and the material. Nanoindentation relaxation, constant strain rate loading and lab-on-chips data lead to similar values of apparent activation volume and strain rate sensitivity, whereas nanoindentation creep clearly overestimates the activation volume (Fig 1). Finite element modelling of nanoindentation creep and nanoindentation relaxation also confirms this trend. We evidence, thanks to the long-term indentation relaxation test that the underlying deformation mechanisms remain unchanged on the entire investigated strain rate range.
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Visco-Plastic Behaviour of a Polymer Matrix at the Fibre Diameter Length Scale: a Finite Element Mesoscale Model Relying on Shear Transformation Zone (STZ) Dynamics
Effect of annealing on mechanical properties and thermal stability of ZrCu/O nanocomposite amorphous films synthetized by pulsed laser deposition
Binary ZrCu nanocomposite amorphous films are synthetized by pulsed laser deposition (PLD) under vacuum (2 Ă 10â3 Pa) and 10 Pa He pressure, leading to fully amorphous compact and nanogranular morphologies, respectively. Then, post-thermal annealing treatments are carried out to explore thermal stability and crystallization phenomena together with the evolution of mechanical properties. Compact films exhibit larger thermal stability with partial crystallization phenomena starting at 420 °C, still to be completed at 550 °C, while nanogranular films exhibit early-stage crystallization at 300 °C and completed at 485 °C. The microstructural differences are related to a distinct evolution of mechanical properties and residual stress, with compact TFMGs showing the highest values of Youngâs modulus (157 GPa), hardness (12 GPa), strain rate sensitivity (0.096), and local residual stress (+691 MPa) upon annealing at 550 °C, while nanogranular films reach the maximum values of mechanical properties at 485 °C followed by relaxation at higher temperatures due to complete crystallization. We show that PLD in combination with post-thermal annealing can generate different families of amorphous films with varying nanoscale morphologies, resulting in tunable mechanical properties and thermal stability, which can thus be used for designing novel film configurations for different fields of application
Strain gradient plasticity analysis of the strength and ductility of thin metallic films using an enriched interface model
The mechanical response of thin metallic films is simulated using a two-dimensional strain gradient plasticity finite-element model involving grain boundaries in order to investigate the effect of the thickness, grain shape and surface constraint on the strength, ductility and back-stress. The grain boundaries and surface layers are modeled as initially impenetrable to dislocations while allowing for relaxation at a critical stress level. The model captures the variation of the strength with grain size, film thickness, and with the presence or not of constraining surface layers, in agreement with experimental results on Al and Cu films. A decrease in the uniform elongation is predicted with decreasing film thickness due to a loss of strain-hardening capacity and the possible presence of imperfections. These two effects dominate over the stabilizing contribution of the plastic strain gradients. Accounting for the relaxation of the interface constraint affects the magnitude of the back-stress as well as the drop in ductility.Institute of Mechanics, Materials and Civil Engineering, UniversiteÂŽ catholique de Louvain, 1348 Louvain-la-Neuve, Belgium b UniversiteÂŽ Libre de Bruxelles, Building, Architecture & Town Planning Dept. (BATir) CP 194/02, Avenue F.D. Roosevelt 50, 1050 Bruxelles, Belgiu
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