Etude de la microstructuration du verre par étincelage assisté par attaque chimique:une approche électrochimique

We are surrounded by glass! This material has been greatly used for thousand years. More than 2000 years ago in ancient Egypt glass was used to manufacture bottles of perfume. So its first uses were primarily related to decoration and jewellery. The researchers left its decorative use to introduce it to new technologies. Today miniaturisation is a major technological challenge. Material machining to micrometric or submicrometric scale becomes one of the key techniques for the future. Glass is often involved in these miniaturized devices. This interest can be explained by its properties: glass is transparent and has a good chemical durability. Many techniques of glass micromachining exist from laser to HF etching and sand powder blasting. In this work, we introduce a glass microstructuring technique, that we called Spark Assisted Chemical Engraving (SACE). It was presented for the first time in the Sixties to drill microholes in glass (diameter = 6 μ). Since the first use of SACE as machining technology in 1968, theoretical approaches were proposed in literature to understand the process. They are based on thermal models by finite elements method or on electrical characterization, with ohmic resistances calculations. They give a good quantitative approximation of the material removal rate. The goals of the thesis are to achieve a local understanding of the process and to highlight electrochemical and thermal phenomena involved around tool-electrode tip before and during SACE process. This thesis will try to answer two questions: How does SACE work? and how to apply it to microstructure glass? A new approach based on potential sweep is proposed to study the process. The originality consists in matching electrochemical measurements with high resolution photographs from the tool-electrode. This report is structured in three parts. A bibliographical part, which introduces not only glass as a material but also presents technologies for glass microstructuration (including SACE). A theoretical part, which focuses on describing current behaviour before spark formation, which allows material removal. It starts by a theoretical recall of variations of electric parameters (conductivity, resistance) as a function of gas hold-up. This theory is then applied to build a model, based on resistance calculations in the system. The goal of this model is to explain electrochemical measurements. An experimental part, which validates the theory developed to explain what happens before spark formation. Characteristics of sparks are then studied by a technique based on voltage pulses and used in a finite elements model. Finally, we will present concrete results, illustrating SACE possibilities to drill microholes. A mechanical prototype for SACE was developed during this thesis and is presented for microreactors fabrication and other glass microstructurations

Experimental investigation of impact on composite laminates with protective layers

This paper presents an experimental study of low energy impacts on composite plates covered with a protective layer. In service, composite materials are subjected to low energy impacts. Such impacts can generate damage in the material that results in significant reduction in material strength. In order to reduce the damage severity, one solution is to add a mechanical protection on composite structures. The protection layer is made up of a low density energy absorbent material (hollow spheres) of a certain thickness and a thin layer of composite laminate (Kevlar). Energy absorption ability of these protective layers can be deduced from the load/displacement impact curves. First, two configurations of protection are tested on an aluminium plate in order to identify their performance against impact, then the same are tested on composite plates. Test results from force–displacement curves and C-scan control are compared and discussed and finally a comparison of impact on composite plates with and without protection is made for different configurations

Mechanical protection for composite structures submitted to low energy impact

Composite materials are widely used in aeronautical structures. These materials can be submitted to low energy impacts like tool drop, routine operations… Such impacts can generate damages in the material that significantly reduce the structure strength. A solution to reduce the severity of damages due to impact is to add a mechanical protection on composite structures (patent n° 2 930 478). In this paper, an experimental study on different concepts of protective layers is presented. This protection is made of a certain thickness of low density energy absorbent material (foam, honeycomb or stacking of hollow spheres) and a thin layer of composite laminate (Kevlar). Experimental impact tests with a spherical impactor of 20 mm diameter at low velocity and low energy are made on aluminum plates, with different protections, and for different levels of energy. Analyses of Load/Displacement curves enable to study the capability of each mechanical protection to absorb energy. Resistance of these protections is then compared and discussed, taking into account the thickness and the surface density of the protections

Dissipation mechanisms identification of soft hollow particle-dampers in honeycomb structures for micro-vibrations environment

Particle dampers are enclosures partially filled with metallic or glass small spheres, attached to the vibrating structure. This paper deals with replacing hard classical particles by soft hollow ones to maximize damping and mass ratio. Hence, one aspect of this damping method is obtained by mixing the kinetic energy conversion of the structure into heat(frictional losses and collisions) and the elastic energy conversion into heat (visco-elastic deformation). This study is oriented toward experimental and theoretical investigations in order to distinguish the dissipation phenomena. The experimental approach first relies on identification and, then, on validation applied on composite aluminum honeycomb plates. Indeed, equivalent viscous damping is identified on small honeycomb samples; then cantilever honeycomb beams are filled with particles and studied. Theoretically, beyond the nonlinear dissipation by impact and friction, these particles add a visco-elastic behavior. The shapes of the hysteretic loops highlight that this behavior is predominant. Hence, oscillators are added in the FE model and permit to consider the effect of the particles. These kinds of particle dampers are highly nonlinear as a function of excitation frequency and amplitudes. The aim of this study is to provide a structural damping solution for space applications which require high pointing stability to enhance mission performances. In this perspective, damping of micro-vibrations was thought as a possible application; nevertheless it is shown that best efficiency is achieved in high frequency range

Mechanical protection for composite structures submitted to low energy impact

International audienceComposite materials are widely used in aeronautical structures. These materials can be submitted to low energy impacts like tool drop, routine operations… Such impacts can generate damages in the material that significantly reduce the structure strength. A solution to reduce the severity of damages due to impact is to add a mechanical protection on composite structures (patent n° 2 930 478). In this paper, an experimental study on different concepts of protective layers is presented. This protection is made of a certain thickness of low density energy absorbent material (foam, honeycomb or stacking of hollow spheres) and a thin layer of composite laminate (Kevlar). Experimental impact tests with a spherical impactor of 20 mm diameter at low velocity and low energy are made on aluminum plates, with different protections, and for different levels of energy. Analyses of Load/Displacement curves enable to study the capability of each mechanical protection to absorb energy. Resistance of these protections is then compared and discussed, taking into account the thickness and the surface density of the protections

Machining of non-conducting materials using electrochemical discharge phenomenon – An overview

Machining with electrochemical discharges is an unconventional technology able to machine several electrically non-conductive materials like glass or some ceramics. After almost 40 years of its first mention in literature, this technology remains an academic application and was never applied in industrial context. The knowledge about machining of non-conducting materials using electrochemical discharge phenomenon is reviewed up to this date with some particular attention to the electrochemical point of view. Some main limiting factors are highlighted and possible solutions are discussed

A stochastic model for electrode effects

The mechanism leading to the onset of the electrode effects is still under discussion in the literature. In this contribution it is proposed that the main mechanism responsible of the electrode effects is the formation of a gas film isolating the electrode. This gas film is formed because of a high population density of bubbles on the electrode surface. A simple model considering the bubble evolution as a stochastic renewal process is presented. By introducing some phenomenological relations, the model allows to evaluate the critical voltage and current density as well as the static current-voltage characteristics leading to the onset of the electrode effects