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Experimental Analysis and Behaviour Modelling of the Deformation Mechanisms of a Ti-6242S Alloy under Hot and Superplastic Forming Conditions
No full textInternational audienceIn this work, the hot deformation characteristics of a near-α Ti-Al-2SnZr-2Mo alloy (Ti6242 alloy) with a Fine-Grained (FG) microstructure (d α = 2.86 µm) were investigated at two levels of temperature, T = 730 • C and T = 840 • C. The initial microstructure consists of equiaxed nodules of the α phase as well as some α lamellae sparsely distributed and separated by thin layers of the BCC β phase. For both temperatures, three strain rates (10 −4 , 10 −3 , 10 −2 s −1) were analysed during loading. Moreover, the microstructural evolution (α size and morphology) was also evaluated by conducting interrupted tensile tests. The different tensile testing conditions greatly influence the stress-strain response of the material as well as the microstructure evolution. Indeed, various phenomena can take place such as elongation of the grain structure, globularization, dynamic recrystallization and grain growth of the equiaxed areas depending on the temperature, the strain rate and the strain level. The FG Ti6242 alloy exhibits interesting superplastic ductility at T = 840 • C. At this temperature either a very gradual flow softening (at higher strain rate) or flow hardening (at lower strain rate) can be observed and are related respectively to one or more of the following mechanisms: lamellae globularization, DRX and grain growth. At the intermediate strain rate, both mechanisms, strain hardening and softening, coexist. At T = 730 • C, the onset of the α lamellae globularization was only promoted at low strain rate. A mechanical behavior model was developed in the temperature range of 730-840 • C, which was able to take into account all the observed phenomena: viscosity, softened behavior and strain hardening. Constitutive equations were calibrated from the stress-strain responses and microstructural observations, and the computed results were in good agreement with the experiments
New model for the prediction of the machining depth during milling of 3D woven composite using abrasive waterjet process
No full textInternational audienceThe goal is to study the influence of abrasive water jet (AWJ) machining parameters (jet pressure, traverse speed and scan step) on the cutting depth of 3D woven Carbon Fibres Reinforced Polymer (CFRP) composite. The original material linked to this non-conventional milling process has not been treated yet. The depths of cut were measured and characterized as a function of the machining parameters. Finally, two prediction models for the cutting depth are proposed and validated experimentally.An increase in cutting depth with the pressure and a decrease as the traverse speed and/or the scan step increase were observed. The first prediction model, based on the pocket depth measurements, has a mean error of 5%. However, the error increases (up to 23%) when the pocket becomes shallow (lesser than 1 mm). The second prediction model, based on the algebraic sum of elementary passes modelled with Gaussian bells, shows at first a mean error of 12%. A correction was performed depending on the erosion regime piloted by the depth of the elementary trench constitutive of the pocket. This enhancement, performed thanks to the primary jet diameters measurements with high speed camera, has improved the second model with a mean error of 5% (error<16%)
Fretting fatigue crack initiation and propagation in Ti6Al4V sheets under tribocorrosive conditions of artificial seawater and physiological solutions
No full textInternational audienceThe interaction of mechanical components experiencing relative movements and cyclic loads in a corrosive environment is known as fretting corrosion or tribocorrosion. In the current work, the mechanism of crack initiation and propagation in dovetail slots of Ti6Al4V samples (in contact with carbide rods) under fretting corrosion conditions was investigated. A newly developed test rig installed on a universal testing machine was used to conduct tests at 20 Hz frequency under 5 and 7.5 kN fretting loads. Tests were conducted at room temperature in 3.5% NaCl and phosphate-buffered saline solutions. Crack propagation in all samples was examined by a metallurgical microscope, and the detailed analysis of fractured samples was carried out by a scanning electron microscope. In comparison to dry conditions, early crack initiation and faster crack propagation were observed in salt and physiological solution environments. Colored spots and large amounts of chlorine, sodium, and oxygen were found around cracks, and plastically deformed regions in the 3.5% NaCl environment provided the evidence of a corrosive attack. Large amounts of oxygen, phosphorous, chlorine, potassium, and sodium were detected in the phosphate-buffered saline environment
A Humanitarian Supply Chain Maturity Model
No full textInternational audienceOver the past decades, humanitarian organizations have largely been criticized for their lack of effectiveness regarding their mission of assisting vulnerable population. However, few researches have investigated what ideal should humanitarian organizations tend toward and the path to undertake in such transformation. In this perspective, this paper intends to overcome this situation by proposing a supply chain maturity model specifically addressed to the humanitarian sector. In the form of a two-dimension matrix, the table aims at: 1) Objectify one organization’s position regarding its transformation journey 2) Depending on the organization, identify the specific improvement areas and suggest their sequence. An instantiation of the maturity model is also proposed through the case of the Indonesian red cros
Dry reforming of methane over calcium-deficient hydroxyapatite supported cobalt and nickel catalysts
No full textNational audienceCalcium-deficient hydroxyapatite (HAP) non-porous supports were synthesized from Ca(NO3)2 and NH4H2PO4 (conventional synthesis route, HAP_N support) and from CaCO3 and H3PO4 (new synthesis route, HAP_C support) by wet chemical precipitation method. Monometallic and bimetallic supported catalysts were prepared by incipient wetness impregnation using Co(NO3)2 and Ni(NO3)2 salts. The size of Co- and Ni-based particles varied from some nm to dozens nm. Dry reforming of methane (DRM) tests at 700 °C, and 1.6 bar shows that HAP-supported cobalt catalysts were systematically more active than HAP-supported nickel catalysts, which is a new finding in DRM. Increasing the molar ratio of Ca/P from 1.43 (HAP_N) to 1.60 (HAP_C) support led to a slight decrease of the catalytic activity but an improvement of the catalytic stability. Thus, HAP_C support synthesized from CaCO3 and H3PO4 is a good candidate for designing an efficient catalyst for the DRM reaction
Gasification of biofuels blended from olive mill solid wastes and pine sawdust under different carbon dioxide/nitrogen atmospheres
No full textInternational audienceIn this paper, we investigated the gasification of charcoals using a macro TG under CO2 mixed with nitrogen at different percentages (40%, 70% and 100% CO2) and at different fixed temperatures (750 °C, 800 °C, and 900 °C). For this purpose, two raw residues were selected; the exhausted olive mill solid wastes (EOMSW) and the pine sawdust (PS). Then, four different samples, which have not been previously studied with a gasification process, were prepared from these residues when investigating the impregnated and the non-impregnated ones using the olive mill waste water (OMWW) as by-product for the impregnation process. Moreover, a comparison between results obtained during this study and those obtained during a previous study based on steam gasification was carried out. It was found that the mass loss profiles are consistent with the usual lignocellulosic gasification behaviors. Also, the increase of temperatures or CO2 percentages affects positively the conversion, the gasification rate and the char reactivity. It is worth noting that CO2 acts differently from steam. With steam, gasification is found to be faster and more reactive
Extraction and Characterization of Nanomaterials from Agrowaste
No full textInternational audienceGreen chemistry started for the search of benign methods for the development of nanoparticles from nature and their use in the field of antibacterial, antioxidant, and antitumor applications. Biowastes are eco-friendly starting materials to produce typical nanoparticles with well-defined chemical composition, size, and morphology. Cellulose, starch, chitin and chitosan are the most abundant biopolymers around the world. All are under the polysaccharides family in which cellulose is one of the important structural components of the primary cell wall of green plants. Cellulose nanoparticles can be extracted from agrowaste resources such as jute, coir, bamboo, and pineapple leaves, coir etc. Chitin is the second most abundant biopolymer after cellulose, it is a characteristic component of the cell walls of fungi, the exoskeletons of arthropods and nanoparticles of chitin can be extracted from shrimp and crab shells. Chitosan is the derivative of chitin, prepared by the removal of acetyl group from chitin. Starch nanoparticles can be extracted from tapioca and potato wastes. The preparation of these nanoparticles includes both series of chemical as well as mechanical treatments; crushing, grinding, alkali, bleaching and acid treatments. Transmission electron microscopy, scanning electron microscopy and atomic force microscopy are used to investigate the morphology of nanoscale biopolymers. Fourier transform infrared spectroscopy and X-ray diffraction are being used to study the functional group changes, crystallographic texture of nanoscale biopolymers respectively. Since large quantities of bio wastes are produced annually, further utilization of cellulose, starch and chitins as functionalized materials is very much desired. These nanoparticles can be converted into smart and functional materials by functionalization through chemical modifications due to the presence of large amount of hydroxyl group on the surface. The cellulose, starch and chitin nanoparticles are currently obtained as aqueous suspensions which are used as reinforcing additives for high performance environment-friendly bio-degradable polymer materials. These nanocomposites are being used as biomedical composites for drug/gene delivery, nano-scaffolds in tissue engineering and cosmetic orthodontics. The reinforcing effect of these nanoparticles results from the formation of a percolating network based on hydrogen bonding forces
Role of grain size and crystallographic texture on tensile behavior induced by sliding mechanism in Ti-6Al-4V alloy
No full textInternational audienceThe aim of this paper is to investigate the coupled effects of grain size and crystallographic texture on the mechanical behavior induced by gliding mechanism in Ti-6Al-4V alloy. Thus, four microstructures of Ti-6Al-4V alloy whose grain size and crystallographic texture are different were examined by tensile tests along the rolling direction at room temperature. In this study, the contribution of gliding on basal , prismatic and pyramidal plans in the accommodation of plastic strain was estimated by means of a slip trace analysis. The role of the individual grain size and the crystallographic texture was then statistically evaluated. Based on the results of slip trace analysis, numerical optimizations of the Critical Resolved Shear Stress (CRSS) of basal , prismatic and pyramidal in the four microstructures were then carried out, using transition scale rules and a local behavior model. The results suggest that the mechanical behavior of Ti-6Al-4V is controlled by the activation of slip systems that depend not only on their CRSS but also the initial orientation and size of each individual grain. The low CRSS of prismatic slip systems can lead to early activation of these systems in favorably oriented coarse grains. Therefore, a local plastic deformation can be shown. At high levels of loading, increasing the grain size can minimize the crystallographic texture effects by deforming the unfavorably oriented coarse grains. Moreover, based on the results of the numerical optimization, it can be also suggested that the CRSS can decrease with the increase in grain size according to the local Hall-Petch relationship
Is-it possible to measure accurate strains field by 2D DIC methods at high temperature ?
No full textInternational audienceAlthough Digital Image Correlation method is relatively mastered at room temperature, it is more complex to implement this method at temperatures from 500 • C to 800 • C. The aim of this study is to develop new tools and algorithms to improve DIC accuracy and precision in this temperature range. New methods to implement speckles and new algorithm to decrease errors due to mirage effect are developed. Thes improvements will be used for aeronautical applications
Understanding ultrasonic piezoelectric transducers: Internal report N◦ IMTA-USTR-001
No full textPiezo-electric transducers are the most common electro-mechanical converters used in acoustic cavitation experiments. We found that after almost a century, there was a mutual unfamiliarity between transducer designers and cavitation users. This document aims at filling the gap. The necessary theoretical background in electricity and elastic solids mechanics is proposed, up to the full representation of a transducer by the matrix method