Estudo experimental e numérico de juntas adesivas em T com aderentes de alumínio

Nos dias de hoje, a ligação adesiva de estruturas complexas que não poderiam ou não seriam tão fáceis de ser fabricadas numa só peça é cada vez mais usual. As juntas adesivas têm vindo a substituir muitos outros métodos de ligação, como por exemplo ligações aparafusadas, rebitas ou soldadas, devido às vantagens de facilidade na sua fabricação, resistência superior e capacidade de unir materiais diferentes. Por esta razão as juntas adesivas têm vindo a ser aplicadas cada vez mais em várias industrias como aeroespacial, aeronáutica, automóvel, naval e calçado. O tipo de adesivo a usar em determinada aplicação é principalmente escolhido consoante as suas características mecânicas e o tipo de resposta pretendida às solicitações impostas. Como exemplo de adesivo resistente e frágil existe o Araldite® AV138. Por outro lado, o adesivo Araldite® 2015 é menos resistente, mas apresenta maior ductilidade e flexibilidade. Além dos adesivos Araldite® comerciais, existem adesivos de poliuretano que combinam características de elevada resistência com características de grande ductilidade e flexibilidade, como por exemplo o Sikaforce® 7752. Esta dissertação tem como objetivo estudar experimentalmente e numericamente, através de modelos de dano coesivo (MDC), o comportamento de diferentes configurações de junta em T quando sujeitas a solicitações de arrancamento. Consideram-se os adesivos anteriormente mencionados para testar as juntas sob diferentes tipos de adesivos. A junta em T é constituída por 2 aderentes em L de alumínio e um aderente base também em alumínio, unidos por uma camada de adesivo. Experimentalmente é feito um estudo da resistência da junta com a variação da espessura dos aderentes em L (tP2). Com a análise numérica são estudadas as distribuições de tensões, evolução do dano, modos de rotura e resistência. Além disso, realizou-se um estudo numérico da existência ou não de adesivo de preenchimento na zona da curvatura dos aderentes em L nas tensões e na resistência da junta. Mostrouse que a variação da geometria nos aderentes em L, a presença de adesivo de preenchimento e o tipo de adesivo têm uma influência direta na resistência de junta. Os ensaios experimentais validaram os resultados numéricos e permitiram concluir que os MDC são uma técnica precisa para o estudo das geometrias das juntas em T.

Caracterização termo-mecânica de interconectores metálicos recobertos com filmes de LaCrO3

The cells unitaria of the solid oxide fuel cell are separated by means of interconnects, which serve as electrical contact between the cells. Lanthanum Chromite (LaCrO3) has been the most common material used as interconnect in solid oxide fuel cells. Reducing the operating temperature around 800 º C of cells to solid oxide fuel make possibilite the use of metallic interconnects as an alternative to ceramic LaCrO3. Metallic interconnects have advantages over ceramic interconnects such as high thermal conductivity, electricity, good ductility, low cost, good physical and mechanical properties. In this work evaluate the thermo-mechanical properties of the metallic substrate and coated metallic substrate with the ceramic LaCrO3 film via spray-pyrolysis, in order to demonstrate the feasibility of using this material as a component of a fuel cell solid oxide. The materials were characterized by X-ray diffraction, oxidation behavior, mechanical strength, optical microscopy (OM) and scanning electron microscopy (SEM). The X-ray diffraction proved the formation phase of the LaCrO3 on the metallic substrate and the identification of the phases formed after the oxidative test and mechanical strength at high temperature. The oxidation behavior showed the increased oxidation resistance of the coated metallic substrate. It was noted that the mechanical resistance to bending of the coated metallic substrate only increases at room temperature. The optical microscopy (OM) has provided an assessment of both the metallic substrate and the LaCrO3 film deposited on the metal substrate that, in comparison with the micrographs obtained from SEM. The SEM one proved the formation of Cr2O3 layer on the metallic substrate and stability of LaCrO3 film after oxidative test, it can also observe the displacement of the ceramic LaCrO3 film after of mechanical testing and mapping of the main elements as chromium, manganese, oxygen, lanthanum in samples after the thermo-mechanical tests.

Adição de vermiculita expandida mícron em pastas de baixa densidade para cimentação de poços de petróleo

The northeastern region of Brazil has a large number of wells producing oil using a method of secondary recovery steam injection, since the oil produced in this region is essentially viscous. This recovery method puts the cement / coating on thermal cycling, due to the difference in coefficient of thermal expansion between cement and metal coating causes the appearance of cracks at this interface, allowing the passage of the annular fluid, which is associated with serious risk socioeconomic and environmental. In view of these cracks, a correction operation is required, resulting in more costs and temporary halt of production of the well. Alternatively, the oil industry has developed technology for adding new materials in cement pastes, oil well, providing high ductility and low density in order to withstand the thermo-mechanical loads generated by the injection of water vapor. In this context, vermiculite, a clay mineral found in abundance in Brazil has been applied in its expanded form in the construction industry for the manufacture of lightweight concrete with excellent insulation and noise due to its high melting point and the presence of air in their layers lamellar. Therefore, the vermiculite is used for the purpose of providing low-density cement paste and withstand high temperatures caused by steam injection. Thus, the present study compared the default folder containing cement and water with the folders with 6%, 8% and 10% vermiculite micron conducting tests of free water, rheology and compressive strength where it obtained the concentration of 8 % with the best results. Subsequently, the selected concentration, was compared with the results recommended by the API standard tests of filtered and stability. And finally, analyzed the results from tests of specific gravity and time of thickening. Before the study we were able to make a folder with a low density that can be used in cementing oil well in order to withstand the thermo-mechanical loads generated by steam injection

A alma-camaleão e sua plasticidade : dualismo platônicos no Fédon

Este paper tem como objetivo analisar o problema dos graus de separação do corpo e da alma no Fédon de Platão, em busca tanto de seus pressupostos ontológicos como de suas consequências epistemológicas. Apesar deste diálogo ser normalmente abordado como pedra miliar literária e filosófica para todos os dualismos psico‑físicos da história de nosso pensamento, entendo que é possível distinguir dois sentidos fundamentais, duas maneiras diferentes de pensar esta separação. O primeiro sentido indicaria uma separação intencional, isto é, fundamentalmente dependente do que o filósofo pensa ou com aquilo do qual o filósofo se procurar curar: o filósofo, como tal, se curaria da alma, mas não se curaria do corpo. Uma segunda maneira de pensar esta separação entre corpo e alma é aquela que privilegia a ideia de uma separação ontológica segundo a qual a alma seria, a tal ponto independente do corpo, que poderia sobreviver após a morte deste. Apesar do sucesso que ambas as abordagens tiveram ao longo da história do platonismo até nossos dias, a duplicidade dos sentidos expressos contém contudo, em si, uma irrevogável ambiguidade e tensão. O objetivo deste paper é o de propor uma solução diferente para a referida ambiguidade. A nossa proposta tem como ponto de partida, a consideração ontológica dos graus de plasticidade da alma, que Bostock (1986, p.119 @Phd. 79c), em seu comentário ao diálogo, chama ‘traços camaleônicos da alma’, isto é, como se a alma pudesse assumir feições corpóreas para conhecer a realidade sensível. A separação entre corpo e alma, antes do que pressuposto ontológico, parece precisar de um esforços permanente do indivíduo, tanto em sentido epistemológico como em sentido ético. _______________________________________________________________________________ ABSTRACT

Obtenção de pós de tântalo metálico a partir da redução aluminotérmica com ignição a plasma

Metallic tantalum has a high commercial value due to intrinsic properties like excellent ductility, corrosion resistance, high melt and boiling points and good electrical and thermal conductivities. Nowadays, it is mostly used in the manufacture of capacitors, due to excellent dielectric properties of its oxides. In the nature, tantalum occurs in the form of oxide and it is extracted mainly from tantalite-columbite ores. The tantalum is usually produced by the reduction of its oxide, using reductants like carbon, silicon, calcium, magnesium and aluminum. Among these techniques, the aluminothermic reduction has been used as the industrial method to produce niobium, tantalum and their alloys, due to the easy removal of the Al and Al2O3 of the system, easing further refining. In conventional aluminothermic reduction an electrical resistance is used to trigger the reaction. This reaction self-propagates for all the volume of material. In this work, we have developed a novel technique of aluminothermic reduction that uses the hydrogen plasma to trigger the reaction. The results obtained by XRD, SEM and EDS show that is possible to obtain a compound rich in tantalum through this technique of aluminothermic reduction in the plasma reactor

Atomic-level structure and deformation in metallic glasses

Metallic glasses (MGs) are a relatively new class of materials discovered in 1960 and lauded for its high strengths and superior elastic properties. Three major obstacles prevent their widespread use as engineering materials for nanotechnology and industry: 1) their lack of plasticity mechanisms for deformation beyond the elastic limit, 2) their disordered atomic structure, which prevents effective study of their structure-to-property relationships, and 3) their poor glass forming ability, which limits bulk metallic glasses to sizes on the order of centimeters. We focused on understanding the first two major challenges by observing the mechanical properties of nanoscale metallic glasses in order to gain insight into its atomic-level structure and deformation mechanisms. We found that anomalous stable plastic flow emerges in room-temperature MGs at the nanoscale in wires as little as ~100 nanometers wide regardless of fabrication route (ion-irradiated or not). To circumvent experimental challenges in characterizing the atomic-level structure, extensive molecular dynamics simulations were conducted using approximated (embedded atom method) potentials to probe the underlying processes that give rise to plasticity in nanowires. Simulated results showed that mechanisms of relaxation via the sample free surfaces contribute to tensile ductility in these nanowires. Continuing with characterizing nanoscale properties, we studied the fracture properties of nano-notched MGnanowires and the compressive response of MG nanolattices at cryogenic (~130 K) temperatures. We learned from these experiments that nanowires are sensitive to flaws when the (amorphous) microstructure does not contribute stress concentrations, and that nano-architected structures with MG nanoribbons are brittle at low temperatures except when elastic shell buckling mechanisms dominate at low ribbon thicknesses (~20 nm), which instead gives rise to fully recoverable nanostructures regardless of temperature. Finally, motivated by understanding structure-to-property relationships in MGs, we studied the disordered atomic structure using a combination of in-situ X-ray tomography and X-ray diffraction in a diamond anvil cell and molecular dynamics simulations. Synchrotron X-ray experiments showed the progression of the atomic-level structure (in momentum space) and macroscale volume under increasing hydrostatic pressures. Corresponding simulations provided information on the real space structure, and we found that the samples displayed fractal scaling (r^d ∝ V, d < 3) at short length scales (< ~8 Å), and exhibited a crossover to a homogeneous scaling (d = 3) at long length scales. We examined this underlying fractal structure of MGs with parallels to percolation clusters and discuss the implications of this structural analogy to MG properties and the glass transition phenomenon.

Mechanical properties and microstructure of powder metallurgy Ti-xNb-yMo alloys for implant materials

In this study, a series of Ti-xNb-yMo (x = 5-40 wt.% in 5 wt.% increments; and y = 3, 5, 10 wt%) alloys were fabricated by powder metallurgy and studied with respect to their microstructures, compressive mechanical properties and hardness. Increases in Nb and Mo content led to decreases in compressive and yield strengths, elastic modulus and hardness of the sintered alloys. Among the studied alloys, Ti-10Nb-3Mo alloy exhibited the optimum combination of strength and ductility. Alloys with a lower amount of Nb (≤ 25 wt.%) and Mo (≤ 5 wt.%) developed Widmanstätten structure, while further increase in Nb and Mo additions led to the microstructure predominantly consisting of β phase with varying regions of α + β phase. The effects of sintering temperature on elastic modulus and hardness were also investigated for Ti-xNb-3Mo alloys.

Transesterification induced mechanical properties enhancement of PLLA/PHBV bio-alloy

In order to improve the miscibility and mechanical properties of poly(l-lactic acid) (PLLA) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) bio-alloy, small amount of transesterification catalyst, zinc acetate was added in the melt blending process. We show that the PLLA-PHBV copolymer generated during the melt blending significantly improves the miscibility and therefore enhances the mechanical properties of the product. Dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), and tensile tests were performed to study the miscibility and mechanical properties of the blends. Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC) were used to reveal the molecular structural, and molecular weight changes of PLLA and PHBV after melt mixing with zinc acetate. SEM and FTIR results have clearly shown that the PLLA-PHBV copolymer generated from transesterification reaction acted as a compatibilizer and therefore resulted in an improved interfacial miscibility and ductility of PLLA/PHBV blend. In our mechanistic study, a competition between the PLLA/PHBV transesterification reaction and the thermal decomposition of PHBV was identified for the first time. On the basis of these observations, a new mechanism of transesterification reaction was proposed.

An investigation of major factor of influence of strength gain or loss of geopolymer when exposed to 800° C

When fly ash based geopolymer mortars were exposed to an elevated temperature of 800oC, it was found that the strength after the exposure sometimes decreased, but at other times increased compared to the original strength. The aim of this investigation is to find the reason for this contrasting behaviour. Fol-lowing exposure to high temperature, residual strengths of specimens prepared with two different fly ashes, with initial strengths ranging from 5MPa to 60MPa, were investigated. The parameter that was found to have a major influence on the contrasting behaviour was the ductility of the mortars. The results indicate that the higher the ductility the lower the strength loss. This correlation is attributed to the fact that mortars with high ductility could provide higher capacity to accommodate thermal incompatibility than mortars with low ductil-ity. Beyond the particular threshold of ductility, some mortars even increased strength after the exposure, pos-sibly due to sintering.

Enhanced superplasticity of magnesium alloy AZ31 obtained through equal-channel angular pressing with back-pressure

Excellent superplastic elongations (in excess of 1,200%) were achieved in a commercial cast AZ31 alloy processed by low temperature equal-channel angular pressing (ECAP) with a back-pressure to produce a bimodal grain structure. In contrast, AZ31 alloy processed by ECAP at temperatures higher than 200 °C showed a reasonably uniform grain structure and relatively low ductility. It is suggested that a bimodal grain structure is advantageous because the larger grains contribute to strain hardening thus delaying the onset of necking, while grain boundary sliding associated with small grains provides a stabilizing effect due to enhanced strain rate sensitivity. © 2008 Springer Science+Business Media, LLC.

Processing and properties of ultrafine-grain aluminium alloy 6111 sheet

The equal channel angular processing (ECAP) technique has been applied to an automotive aluminium alloy sheet (A6111). The technique utilizes a machine that was specially designed for this purpose at Monash University. It was determined that ECAP is able to refine the grain size of the sheet, diminish the detrimental as-rolled texture components in the sheet and retain an acceptable level of bi-axial ductility such as is required during the automotive forming process. Experiments were carried out on annealed, as-received sheets that were subjected to either one or two passes through the ECAP machine. For the second ECAP pass, the sheet could be processed in the same orientation as the first pass (route A) or it could be rotated 180° about the direction of feeding (route C). It was determined that route A produced marginally improved properties compared to sheet processed via route C, and that due to the frictional heating generated during the second pass, a significant amount of recovery occurred in the sheet such that an improved combination of texture and formability resulted after two passes compared to the same sheet exposed to only a single pass.

The effect of thermomechanical treatment on the microstructure and the mechanical behavior of a supersaturated Cu-Ag alloy

Supersaturated Cu-3at.% Ag alloy was processed by cold rolling and short-time annealing in order to achieve a combination of high strength and good tensile ductility. After annealing of the rolled samples a heterogeneous solute atom distribution was developed due to the dissolution of nanosized Ag particles in some volumes of the matrix. In regions with higher solute content, the high dislocation density formed due to rolling was stabilized, while in other volumes the dislocation density decreased. The heterogeneous microstructure obtained after annealing exhibited a much higher ductility and only a slightly lower strength than in the as-rolled state.

An analytical model for web-warping in variable width flexible roll forming

The development of ultra/advanced high strength steels (U/AHSS) has challenged traditional forming methods due to their higher strength and reduced formability. An alternative method is flexible roll forming, which allows the manufacture of sheet metal of high strength and limited ductility into complex and weight-optimized components. However, one major problem in flexible roll forming is the web-warping defect, which is the deviation in height of the web over the length of the profile. The authors’ previous work developed an analytical model to predict the magnitude of web-warping. That model was purely geometric and neglected the effect of material properties. This work develops an analytical solution for the prediction of web-warping that considers both geometric and material parameters. The model results were validated by comparison with numerical and experimental results. The impact of this new model will be the ability to provide a rapid initial design assessment before an intensive numerical analysis of flexible roll forming is conducted.

High performance PP/SEBS/CNF composites: evaluation of mechanical, thermal degradation, and crystallization properties

Nanocomposites comprising carbon nanofibers (CNF) were prepared and evaluated in terms of morphology, mechanical performance, thermal stability and crystallization properties. It was found that addition of CNF reinforced polypropylene (PP) matrix by marginally increasing the strength and modulus, but at the expense of toughness and ductility. To improve the toughness of the composites, polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) was used. Presence of SEBS remarkably improved the toughness and ductility of the composites. The optimum level of reinforcement was observed at 0.1 wt% of CNF in the composites. Phase morphology studies revealed that at this concentration, CNF were well dispersed in polymer phases and beyond it, agglomeration occurred. PP/SEBS/CNF (0.1 wt%) nanocomposites exhibited good strength, excellent toughness and decent modulus, which make them suitable for cost effective, light-weight, tough and stiff material for engineering applications. It was observed that thermal stability of composites is only marginally improved whereas crystallinity of PP drastically reduced by the addition of CNF.