Ciência & Tecnologia dos Materiais最新文献

This work reports a theoretical study aimed to identify the plasmonic resonance condition for a system formed by metallic nanoparticles embedded in an a-Si:H matrix. The study is based on a Tauc-Lorentz model for the electrical permittivity of a-Si:H and a Drude model for the metallic nanoparticles and the polarizability of an aluminium sphereshaped particle with radius of 10-20 nm. We also performed FDTD simulations of light propagation inside this structure reporting about the effects caused by a single nanosphere of aluminium, silver and, as a comparison, an ideally perfectly conductor. The simulation results show that it is possible to obtain a plasmonic resonance in the red part of the spectrum (600-650 nm) when 10-20 nm radius aluminium spheres are embedded into a-Si:H.

This paper concerns the study of residual stresses and distortions in fusion butt joint welding, using the computational modelling software ESI Sysweld. Thermal gradients across the part will introduce geometrical variations that cause residual stress and distortions; their study and prediction are critical to ensure a sound welding.

To foresee the welding behaviour is a complex task because there are many physical-chemical phenomena involved in the welding processes. Using Sysweld - a finite element method based software - it is possible to integrate all the physical-chemical phenomena and elaborate computational models for most welding cases.

A real case consisting of three sets of aluminium plates welded by laser will be studied in this paper. A finite element model is realized for each case and the results are analysed using Sysweld capabilities.

NiTi alloys used for orthodontic applications need to show superelastic characteristics at room and oral temperatures. The ideal scenario is that where the material has a final austenitic phase transformation value below the room temperature. This study aims at understanding the influence of the ageing treatments in the austenitic structure at room temperature on a wire of a Ni-rich NiTi alloy produced by rotary forging by the evaluation of the phase transformation temperatures and mechanical behaviour in order to promote the superelastic behaviour at room temperature. The investigation was conducted using DSC (Differential Scanning Calorimetry) analysis and instrumented ultramicrohardness. The solubilisation at 950 °C for 120 min with water quenching showed a satisfactory amount of B2 phase at room temperature when compared to the sample after forging. After solubilisation, ageing treatment at 350 °C for 30 min gave a relatively higher hardness value and an Af temperature below the room temperature, ensuring the presence of austenitic phase at room and oral temperatures.

The present study deals with the analysis of experimental results, regarding of load carrying capacity and strains, obtained from tests on reinforced concrete (RC) columns, strengthened with external carbon fibre reinforced polymer (CFRP) sheets. The experimental parameters include: number of wrap layers, slenderness of the columns (L/a or L/D) and section geometry (circular or square). A total of 48 specimens were subjected to axial compression. All test specimens were loaded to failure. Compressive stress, both axial and hoop strains have been recorded to evaluate the stress-strain relationship, ultimate stress, stiffness, and ductility. First, the effects of test parameters are analysed and compared. Results clearly demonstrate that composite wrapping can enhance the structural performance of RC columns in terms of both maximum strength and ductility.

Modern and competitive structures are sought to be strong, reliable and lightweight, which increased the industrial and research interest in adhesive bonding. With this joining technique, design can be oriented towards lighter structures. The large-scale application of a given joint technique supposes that reliable tools for design and failure prediction are available. Cohesive Zone Models (CZM) are a powerful tool, although the CZM laws of the adhesive bond in tension and shear are required as input in the models. This work evaluated the value of shear fracture toughness (G_IIC) and CZM laws of bonded joints. The experimental work consisted on the shear fracture characterization of the bond by a conventional and the J-integral techniques. Additionally, by the J-integral technique, the precise shape of the cohesive law is defined. For the J-integral, a digital image correlation method is used for the evaluation of the adhesive layer shear displacement at the crack tip (δ_s) during the test, coupled to a Matlab^® sub-routine for extraction of this parameter automatically. As output of this work, fracture data is provided in shear for the selected adhesive, allowing the subsequent strength prediction of bonded joints.

This work shows a simple way of producing fixed bed microreactors or packed microcolumns by using electrodynamic focusing. Based on previous results that showed fibre focalization on orifices is possible, we explored this technique to electrospun fibres inside microchannels. Different polymeric nanofibres - either neat or composite, i.e. fibres containing particles - can be deposited without meaningful difference on deposition parameters, resulting in packed structures. Moreover, distinct substrates can be used for mask production, which can be reused with the respective setup. Finally, the use of a dry process shortens the process time, increasing productivity.

Most of titanium aluminide (TiAl) castings used in the automotive and aeronautical industries, such as turbines, are high added- value parts with complex geometries. Due to high reactivity of TiAl cast into ceramic moulds, most of the castings require post- processing in order to remove a brittle surface layer named alpha case. Furthermore, the complex geometry and thin walls of this type of components makes difficult to cast net shape parts; so, near-net shape components with machining allowance are often produced to improve mould filling (better fluidity). To solve this technological limitation, the chemical milling process is used to eliminate this layer and the machining allowance. In bibliography there are only a few systematic studies about the influence of chemical milling in TiAl surface castings. So, this experimental work seeks to contribute to understand the influence of this finishing process on the TiAl castings surface quality (dimensional accuracy, roughness and microhardness) and intends to establish which of the two chemical solutions tested is better for chemical milling of TiAl castings.

Additive manufacturing (AM) has been considered one of the best processes to manufacture components with complex geometries, many times impossible to achieve with traditional processes, such as moulds with conformal cooling. Binder Jetting (BJ) technology uses an ink-jet printing head that deposits an adhesive liquid, layer by layer, to bind a powder material that can be ceramic, metallic, or other, which allows manufacturing parts to be used in research and industry.

The aim of this work is to study the possibility of using BJ to produce plaster moulds for directly cast metallic parts at a lower cost than with metallic AM processes, using different types of infiltrates and post-processing parameters to improve the mechanical and thermal strength of moulds in order to be able to cast an aluminium alloy. The mechanical and thermal resistance of moulds with a thickness range of 2.5-4 mm were analysed, as well as the surface roughness of metal samples, and compared with those obtained by traditional processes. Although all the moulds had good heat resistance during the casting, some did not have enough mechanical strength to withstand the metalostatic pressure, especially those with walls of 2.5 to 3.5 mm.

Three brands of rotary nickel-titanium endodontic instruments with some comparable geometric features but with different cross-section (similar tip size and same taper of .04) were selected for this study: MTwo .04(35) (VDW GmbH, Germany), K3 .04(30) (SybronEndo, Mexico) and K3XF .04(30) (SybronEndo, Mexico). K3XF is made from novel R-phase heat-treated metal alloy while all other files were made from traditional Ni-Ti alloy. The instruments were analysed under the following conditions: i) as-received (AR), ii) heat treated at 350 °C, iii) heat treated at 400 °C. The transformation temperatures were determined by differential scanning calorimeter (DSC). Compared to conventionally K3 and MTwo files, R-phase heat treatment K3XF file showed higher transformation temperatures. These results also showed that heat treatments increase the transformation temperatures.

Reciprocating sliding tests were performed in order to find the effect of lubricant properties and other variables on the coefficient of friction of the steel sheets “Cold Rolled HSLA 380” in dry and lubricated conditions. For this material, tribology tests were performed with different lubricants. Test results showed that values of the coefficient of friction presented different patterns. The coefficient of friction varies over a wide range with different lubricating conditions and different sliding velocities. For some sliding velocities, the coefficient of friction is stable and lower, while for others it is unstable and higher.