Ciência & Tecnologia dos Materiais最新文献

The development of eco-friendly thin hybrid films on metallic surfaces has been becoming an alternative for convectional pretreatment processes. Silane coatings obtained via sol-gel technology are widely known, due to its hybrid nature interface (organicinorganic) facilitates interaction with metallic surface. The bis-1,2-(triethoxysilyl)ethane (BTSE) is a silane whose molecule contains six ethoxy groups and, after hydrolysed and cured, form a crosslinked filme that binds to the metal surface by covalent bonds to form a barrier film with excellent protective properties. However, after long periods of exposure, this film becomes susceptible to passage of electrolytes and sensitive to mechanical stresses. For this reason it is required the addition of plasticisers, such as poly(ethylene glycol) (PEG) to increase performance by barrier effect of the films. The objective of this work is to develop hybrid films and evaluate the influence of the concentration of BTSE silane and the plasticiser PEG 1500 on the electrochemical and mechanical properties when applied over galvanized steel substrate. The films were obtained by dip-coating and subjected to thermal curing. The hydrophobicity of the films were determined by contact angle measurements and the morphology was evaluated by scanning electron microscopy (SEM) and by profilometry. The electrochemical behavior of the films were evaluated by open circuit potential monitoring (OCP), polarisation curves and electrochemical impedance spectroscopy (EIS). The mechanical behavior of the films were evaluated by ball-on-plate test. Results showed that the hybrid films presented small surface irregularities, but no cracking or peeling, independently of the proportions of BTSE and PEG. The surface roughness of the samples showed no appreciable variation after the application of films. It was observed that the films have a higher hydrophobicity than the surface of the metal substrate. These results favors the corrosion protection of the substrate, because hydrophobic surfaces repel the electrolyte action. All the studied films showed good electrochemical performance, protecting the metal substrate of the corrosion process. However, it was observed that the film with a lower concentration of BTSE and higher PEG concentration showed the best corrosion performance, when compared to other films studied. Furthermore, it is also observed that films with lower concentration of BTSE showed greater resistance to abrasion. Based on these results, it is considered these films were a alternative for protection of galvanized steel in potentially aggressive environment.

Peripheral nerve regeneration following severe events is still a challenging topic in the regenerative medicine field, especially when nerve tissue is lost and direct suturing is not feasible. Given the limited success observed in currently available techniques, researchers have been putting efforts towards the development and optimization of these techniques, aiming at the best recovery chances for affected patients. The present work explores the combination of two methods of synthetic biomaterial tubes functionalization: the effect of electroconductive biomaterials and its association to an active cellular system. A tube-guide comprised of polyvinyl alcohol (PVA) loaded with COOH-functionalized multiwall carbon nanotubes (CNTs) was produced and studied alone or in combination with a cellular system of mesenchymal stem cells (MSCs) isolated from the umbilical cord Wharton's jelly (WJ). Tube-guides were assessed for in vitro cytocompatibility to the WJ MSCs and tested for in vivo performance in a neurotmesis rodent model. Animals were assigned to either PVA-CNTs, PVA-CNTs-MSCs, graft or end-to-end reconstruction groups and assessed after 20 weeks of regeneration. Structural analysis revealed overall more evident recovery in the PVA-CNTs-MSCs group, with significant differences to the cell-free group and similar to End-to-End repaired and Grafted groups. Surprisingly, PVA-CNTs-MSCs did no benefit neurogenic muscle atrophy recovery. Overall, the electrofucntionalized tube-guides post an interesting option for nerve reconstruction alone or in combination to MSCs cellular systems.

The traditional method of manufacturing sandwich structures using a structural adhesive to achieve face-sheets to core bonding maintains widespread use. Most research addressing experimental performance of sandwich structures focuses on core and face-sheet related parameters, and only marginal attention is directed to the mechanical properties of the thin, low content, adhesive layer – it usually suffices that it can provide effective face-sheet/core bonding. The present work studies sandwich structures with a cork agglomerate core and resin infusion processed skins consisting of +/-45° glass fibre fabric reinforced epoxy resin. A polyurethane structural adhesive was used to assemble the structure. The focus of the study was the influence on sandwich performance with respect to a modification of the adhesive with multiwall carbon nanotubes, exploring the possibility of an increase in both shear and adhesion strength of the modified adhesive. The sandwich structure was evaluated with respect to four-point bending and low velocity impact tests. In addition, scanning electron microscopy analysis was used to examine the adhesive layer morphology. The results were analysed to determine how the addition of ca. 0.4 wt.% of carbon nanotubes to the adhesive effectively influenced failure behaviour and damage events in both flexural and impact testing.

The development of high damping materials for noise reduction and attenuation of vibration on structural applications, such as automotive and aerospace industries, has been investigated. This experimental study is concerned with the damping capacity (tan delta) and dynamic Young's modulus (|E*|) of Silicon carbide (SiC) reinforced aluminium (Al) matrix composite. AlSi-SiCp composite was produced by hot pressing technique. Damping capacity and dynamic Young's modulus of composite and unreinforced AlSi alloy were studied using a dynamic mechanical analyser (DMA), over a temperature range of room temperature-400 °C (during heating and cooling phases), at 1 and 20 Hz. AlSi-SiC_p composite showed higher damping capacity and dynamic Young's modulus than the AlSi unreinforced alloy. Furthermore, damping capacity was found to increase with temperature, while modulus decreases. The possible damping mechanisms are presented and discussed.

The design of rotating steel shafts is a classical mechanical engineering problem. Since the recognition of fatigue as a major source of failures in shafts, many different criteria for fatigue design of rotating steel shafts have been put forward. Two commonly used approaches are based on the Soderberg criterion and on the DIN 743 approach. However, in the vast and ever growing literature on fatigue design, comparisons of these two procedures, based on concrete examples, are not commonly available. Therefore, a clear need exists for a comparison of the two approaches. This article analyses these two approaches considering a simple and common case. This case is a transition in diameter of a steel shaft, assumed as the critical cross section where bending and torsion moments are applied. Contrary to expectation, substantial differences were found between the two approaches, including in the fatigue correction factors.

The goal of this research is to study the influence of the ratio of an explosive composed of 80% ANFO and 20% matrix on the quality of dissimilar explosive welds of Cu-DHP copper to aluminium alloy 5083-H11, in flat configuration. It is analysed the influence of four explosive ratios (1.4, 1.8, 2.3 and 2.6) on the microstructure and mechanical properties of welds. It was observed that the increase in the explosive ratio gives rise to an increase of the collision point velocity (Vc) and the impact velocity (Vp) and consequently reduces the thickness of the flying plate after welding as well as produces wavy interfaces of greater amplitude. Microstructural analysis showed the formation of hard and brittle intermetallic compounds in the interface region, more obvious in welds made with higher ratio of explosive.

The increasing environmental concern about waste materials and the necessity of improving the performance of asphalt mixtures prompted the study of incorporating different waste materials in conventional bitumen. The reuse of waste materials can present benefits at an environmental and economic level, and some wastes can be used to improve the pavement performance. Thus, the purpose of this study is to evaluate the incorporation of different waste materials in bitumen, namely waste motor oil and different polymers. In order to accomplish this goal, 10% of waste motor oil and 5% of polymers (high density polyethylene, crumb rubber and styrene-butadiene-styrene) were added to a conventional bitumen and the resulting modified bitumens were characterized through basic and rheological tests. From this work, it can be concluded that the incorporation of different waste materials improves some important properties of the conventional bitumen. Such improvements might indicate a good behaviour at medium/high temperatures and an increase of fatigue and rutting resistance. Therefore, these modified bitumens with waste materials can contribute to a sustainable development of road paving industry due to their performance and environmental advantages.

Ultra-high performance concrete is a kind of high-tech composite material which shows superb characteristics such as self- compactness, compressive strength higher than 150 MPa, and exceptional durability performances compared to other kinds of concrete. In this research, compared to known commercially available UHPCs, a type of UHPC paste with greener pozzolans was developed. In this regard, cement and silica fume, as two main constituents of the prevalent UHPC compositions and particularly with high cost and environmental impacts, were replaced by fly ash as a waste material. It was found that the highest fluidity and strength could be achieved with 13% and 16% of fly ash substitution, respectively. Furthermore, ultra-fine fly ash with mean particle size of 4.48 μm showed its applicability to be used in UHPC with 20 wt.% cement substitution resulting in a paste with 153 MPa compressive strength and 37.5 cm flow diameter. Moreover, addition of at least 5% silica fume seems to be a prerequisite regarding strength gain of UHPC paste. Metakaolin as another pozzolanic material was studied. Although it improved the paste strength, it demonstrated lower fluidity and showed its inability to be applied in UHPC with required high workability.

Numerical simulations of sheet metal forming processes need the establishment of highly reliable results, which in turn need the accurate identification of mechanical properties. In this paper a study is presented on the choice of the characterization function of flow stress-strain curve of sheet metal materials, as well as the selection of the best yield locus, based on experimental uniaxial tensile and biaxial hydraulic bulge tests performed on dual-phase steels of industrial interest. To obtain a better characterization of the hardening curve, a combination is made using the uniaxial tensile test data with biaxial hydraulic bulge test results, since bulge test covers a larger range of plastic strain when compared to tensile test. Since the two flow curves have different strain paths, they can’t be directly compared or combined. Therefore, it is necessary a transformation of flow stress-strain curve provided from biaxial bulge test into equivalent stress-strain curve. Different methodologies were applied to transform biaxial stress-strain curve to an equivalent one and the different results are compared and evaluated.

Welding of niobium and tantalum materials is currently conducted in vacuum or inert gas chambers, which limits the size of the equipment to be produced. The objective of this study is to investigate the means to make TIG welding on these materials in atmospheric environment and analyse the effect of heat input on the properties of welds. It was found that it is possible to make TIG welds on these materials in atmospheric environment, provided that adequate protection is ensured on the face and root sides of welds. The increase of weld heat input coarsens the microstructure in melted and heat affected zones of both materials; increases also hardness in melted zones and tensile strength of welds, and there is not an obvious loss of ductility in heat affected zones.