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.

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.

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 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.

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.

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.

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.

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.

Urbcork is an urban furniture line developed at the Faculty of Engineering of the University of Porto (FEUP) for the collection produced by Leets Urban Design. It can be highlighted for its sustainable properties and inclusive design concerns.

Urban furniture is a product to be used by a large number of people in a public space. It has a high level of wear and ageing due to the intensive use, solar exposure and weather conditions.

During the project development there was a conceptual idea about using cork as the main material for the seat and the back. The opportunity of developing a prototype brought the question about how to keep cork properties after a long exposure on exterior.

The main line of investigation was about how to protect cork keeping its natural appearance and extending its properties and conservation over time. Laboratory tests held at CTCOR and FEUP were conducted in order to understand the behaviour of cork on extreme conditions. The results are not yet successful, but with more tests, the product can be well accepted in the international market.

Adhesive bonding is a viable technique to reduce weight and complexity in structures. Additionally, this joining technique is also a common repair method for metal and composite structures. However, a generalized lack of confidence in the fatigue and long-term behaviour of bonded joints hinder their wider application. Suitable strength prediction techniques must be available for the application of adhesive bonding, and these can be based on mechanics of materials, conventional fracture mechanics or damage mechanics. These two last methodologies require the knowledge of the fracture toughness (G_C) of materials. Being damage mechanics-based, Cohesive Zone Modelling (CZM) analyses coupled with Finite Elements (FE) are under investigation. In this work, CZM laws were estimated in shear for a brittle adhesive (Araldite^® AV138) and high-strength aluminium adherends, considering the End-Notched Flexure (ENF) test geometry. The CZM laws were obtained by an inverse methodology based on curve fitting, which made possible the precise estimation of the adhesive joints’ behaviour. It was concluded that a unique set of shear fracture toughness (G_IIC) and shear cohesive strength (t_s⁰) exists for each specimen that accurately reproduces the adhesive layer behaviour. With this information, the accurate strength prediction of adhesive joints in shear is made possible by CZM.