Abstract: The use of natural fibers as reinforcement in composites is emerging. Several studies are underway to improve the mechanical characteristics of these fibers and its matrix interface properties for better load transfer. However, the treatments generally used are relatively expensive and complicated to apply. This work deals with the effect of new Fibroline process on tensile and interfacial properties of hemp fiber reinforced in polypropylene. Fibroline is a dry powder impregnation method which consists of submitting fibers and polymer powder under strong alternating electric field. Morphology and tensile properties of hemp fibers after different surface treatments (raw, dried, raw and Fibroline-treated, dried and Fibroline-treated) are evaluated. Interface properties of treated hemp fibers on polypropylene matrix are then characterized by fragmentation test of monofilament composites. Results showed the Fibroline treatment reduces the fiber mechanical properties but improves the load transfer efficiency due to random generation of surface cracks and better fiber/matrix adherence, respectively. For the case of dried and Fibroline-treated hemp fibers, large decrease in mechanical and interfacial properties was observed.
Abstract: Because of its special properties and commercial significance, Poly(phenylene sulfide) (PPS) has been the subject of many research efforts since its commercial introduction in 1967. Intensive work has been done on its crystalline structure and morphology and its thermal behaviour. But fewer investigations have been carried out to understand long term behaviour in high temperature environments.
Always anticipating industrial needs linked to power integration, we have launched an extensive study on thermal aging in air of PPS at 250°C.
This study has shown that PPS thermal degradation in air happens by intermolecular branching reaction, similar to crosslinking. This phenomenon was already known for temperature above 300°C.
This crosslinking is evidenced by rheometry where the relative position of G’ and G’’ above melting temperature changes with aging. IR spectroscopy confirms that para substituted benzene in PPS molecule is transformed in 1,2,4 trisubstituted benzene. DSC measurements evidence both an elevation of melting temperature and a change in melting endotherm showing significant changes in crystalline morphology along aging, which tends to indicate that crosslinking occurs in crystalline phase.
Then degradation implies drastic loss of mechanical properties leading to destruction of the sample.
Abstract: The density increase of components used in power converters involves cooling problems and results to the use of new materials and innovative assembly processes (such as adhesive bonding). Searching for an adhesive family leads to the silicones. This paper focuses on the silicone adhesive formulation as regards the use requirements (temperature range, chemical agents). This first article is dealing with the influence of chemical structure on vulcanization process parameters and physical properties before ageing. The first adhesive family is the oxime terminated polydimethylsiloxane (PDMS) adhesive’s one which cures as soon as it enters in contact with air moisture. This cure process seems too sensitive to humidity and temperature and involves a too long time process for industrial applications. On the contrary the process for silicone cured by polyaddition is easy. Besides, the introduction of a few phenyl groups in dimethylsiloxane backbone makes the PDMS free from crystallisation ensuring the silicone flexibility in a higher use temperature range.
Abstract: Interface between Carbon NanoTubes (CNT) and epoxy matrix is admitted to play an important role in the dispersion quality and in the mechanical stress transfer. To improve the interfacial adhesion, we propose to chemically graft molecules onto CNT surface. To achieve this chemical modification, a controlled radical polymerization, named Nitroxide Mediated Polymerization NMP, is used to synthesize a diblock copolymer based on Acrylic Acid (PAA block) and Methyl MethAcrylate (PMMA block). In the present paper, this polymerization is performed “in situ”. The PAA block presents a good affinity with the CNT which enable grafting. The PMMA miscibility with epoxy is expected to give a good adhesion - between the CNT and the matrix - and to bring a better dispersion. In order to compare the chemical modification and the physical adsorption of the copolymers onto CNT dispersion, the same block copolymer was synthesized with and without CNT. The copolymer synthesis was controlled and characterized by different methods as NMR 1H (conversion and composition), SEC (molecular weight) and TGA (grafting density). We show that the better dispersion quality and better physical properties have been obtained with grafted CNT.
Abstract: The industrial application (under a confidential clause) concerns a ceramic/steel assembly. This PhD research is concerned with providing a reliable industrial bonding between ceramic and steel using structural adhesive. This industrial joint shall withstand a wide range of temperatures with brutal thermal changes. This paper focuses on the adhesive formulation. Indeed, we needed to compromise the adhesive flexibility: soft enough to resist the thermal stresses and compensate the coefficient of thermal expansion mismatch between the two substrates. We find this good compromise with DGEBA and Novolac epoxy based adhesive separated or blended. The influence of the different formulation parameters on the initial properties was studied by chemical, rheological and thermal tests on adhesive samples. Depending on those formulations, we observed different behaviours in term of glass transition temperature, cross-linking time and flexibility.
Abstract: The purpose of this work is to study the Iosipescu shear test and more precisely its ability to characterize the shear modulus of a carbone/epoxy composite material. The parameters influencing this identification are the fibre orientation, the geometry of the notch and the boundary conditions. Initially these parameters were studied through the finite element analysis of the shear test. Then, the measurement of the shear strains was carried out by traditional methods of measurement (strain gauges) but also by optical methods. These optical methods: the digital image correlation and the electronic speckle pattern interferometry (ESPI); allow for various levels of loading, to reach a full-field measurement of the shear strain. This enabled us to study the strain distribution on the section between the two notches. The finite element model enabled us to study the parameters influencing the calculation of the shear modulus in comparison with strain gauges, image correlation and ESPI. This work makes it possible to conclude on optimal parameters for the Iosipescu test.
Abstract: Biorefinery of sunflower whole plant can be realized using a twin-screw extruder. Thermo-mechanical fractionation and aqueous extraction are conducted simultaneously. A filter section is outfitted along the barrel to collect continuously an extract and a raffinate (cake meal). Oil yield obtained is 53%. Proteins are partly extracted at the same time, just as pectins and hemicelluloses. Protein yield is 46%. Cake meal is relatively moist (66% for the moisture content). It is first dried to make easier its conservation. It is largely composed of lignocellulosic fibres (59% of the dry matter) from depithed stalk. Lipid content is 13% of the dry matter or 35% of the oil in whole plant. Protein content is 7% of the dry matter or 45% of the proteins in whole plant. DSC measurements indicate that denaturation of proteins is almost complete in the cake meal. DMTA spectrum of its milled powder reveals a significant peak at high temperature (between 175 and 200°C). As already observed with industrial sunflower cake meal, it can be associated with the glass transition of proteins. As a mixture of fibres and proteins, the cake meal can be considered as a natural composite. It is successfully processed into biodegradable and value-added agromaterials by thermo-pressing. As for DMTA analysis, the glass transition of proteins in the cake meal is also observed with PVT analysis at around 180°C. It makes easier the choice of the best thermo-pressing conditions to produce panels with higher mechanical properties in bending. These properties increase simultaneously with temperature, pressure and time chosen for molding operation. The highest flexural strength at break (11.5 MPa) and the highest elastic modulus (2.22 GPa) are obtained for the next molding conditions: 200°C and 320 kgf/cm2 during 60 s. Drop angle measurements show that the corresponding panel is also the most resistant to water. No significant transition is observed inside this panel above 0°C and until 200°C with DMTA analysis. Proteins ensure the agromaterial cohesion without any phase change in this temperature range, and fibres entanglement also acts like reinforcement. This panel could be used as inter-layer sheets for pallets or for the manufacturing of biodegradable containers (composters, crates for vegetable gardening) by assembly of panels.
Abstract: Thin films produced by sputtering methods are used in many practical applications. This work aims at studying the failure mechanisms of thin hard coatings submitted to repeated impact conditions, with a special focus on the influence of the film thickness. Impact tests were carried out using a cyclic loading system developed in our laboratory. Steel balls (100Cr6) were used as impacting bodies. After tests, samples were observed by optical and scanning electron microscopy in order to investigate coating failure. The experimental work has permitted to identify blistering phenomena that has been related to the film mechanical properties and residual stress level. The influence of the film thickness is discussed and measurements of the residual stress in the buckled coatings have been performed using Raman spectroscopy to check the validity of our mechanical approach.
Abstract: This work studies the reliability of power electronic component in aeronautical environment to
the ageing eect of the thermal cycling. The structure fatigue is sensitive to the process assembly
conditions especially of the soldering process. To correclty evaluate the reliability of the power module,
the identication of the solder behavior is one of the rst steps. Anand Model is here identied.
Experimental test have to be established to evaluate the parameters of the law. A srt study is made
to evaluate the indetiability of the law according to the dierent experimantal test. Then, the scatter
of the parameters is evaluated in a context of time series. In the end, the scatter of the parameters is
included in a nite element model to understand the inuence of this scatter on the evaluation of the
number of cycle before failure.