ESTG - Artigos em revistas internacionais
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- Impact of traffic loads on flexible pavements considering Ecuador’s traffic and pavement conditionPublication . Almeida, Arminda; Moreira, John J. M.; Silva, João Pedro; Viteri, Carlos G. V.This paper presents a study regarding the consequences of traffic loads, including the ones due to traffic overload and traffic efficiency, on the damage of flexible road pavements. It was based on a real traffic data collection done in the city of Portoviejo, Ecuador. The study starts by defining the typical Ecuadorian fleet composition and heavy-traffic stream which comprises overloaded vehicles. The mixed heavy-traffic stream is then simulated in a load generating software for two road condition scenarios mimicking a well and a badly-maintained road. Two theoretical heavy-traffic streams were simulated, one without overloaded vehicles and a high-efficient one, therefore this methodology can be easily adapted for other situations. The results show a clear tendency towards a significant increase in the road pavement damage underlying the need to promote actions aimed at reducing traffic overloading thus improving road condition. Enforcement can decrease the pavement damage at least 33% and its condition by 30–40% which in a normal life span of 20 years would represent 7–8 years. This study also draws the attention of the authorities to the implications of high-efficient heavy-traffic streams.
- Selective Laser Melting of Ti6Al4V sub-millimetric cellular structures: Prediction of dimensional deviations and mechanical performancePublication . Bartolomeu, F.; Costa, M. M.; Alves, N.; Miranda, G.; Silva, F. S.Ti6Al4V sub-millimetric cellular structures arise as promising solutions concerning the progress of conventional orthopedic implants due to its ability to address a combination of mechanical, physical and topological properties. Such ability can improve the interaction between implant materials and surrounding bone leading to long-term successful orthopedic implants. Selective Laser Melting (SLM) capability to produce high quality Ti6Al4V porous implants is in great demand towards orthopedic biomaterials. In this study, Ti6Al4V cellular structures were designed, modeled, SLM produced and characterized targeting orthopedic implants. For that purpose, a set of tools is proposed to overcome SLM limited accuracy to produce porous biomaterials with desired dimensions and mechanical properties. Morphological analyses were performed to evaluate the dimensional deviations noticed between the model CAD and the SLM produced structures. Tensile tests were carried out to estimate the elastic modulus of the Ti6Al4V cellular structures. The present work proposes a design methodology showing the linear correlations found for the dimensions, the porosity and the elastic modulus when comparing the model CAD designs with Ti6Al4V structures by SLM.
- Over-the-Air Calibration of Active Antenna Arrays Using MultisinePublication . Jordão, Marina; Belo, Daniel; Caldeirinha, Rafael F. S.; Oliveira, Arnaldo S. R.; Carvalho, Nuno Borges deA novel over-the-air (OTA) beamforming calibration procedure is proposed, which can be applied to different types of multiple-input multiple-output (MIMO) transmitting antennas. The method consists of using two tones on each antenna element, the main tone, and the tickle tone. The latter one operates in a different frequency for each antenna element and is used to obtain the optimum phase that needs to be set for each element to generate a beam in the desired direction, at the main tone frequency. The novel method was applied to linear, planar, 3-D antennas, and noncollocated multiantennas to validate its applicability to different antenna types. Line-of-Sight (LOS) and non-LOS experimental results demonstrate that the method can be used to retrieve the required phase to fully control the antenna radiation pattern. An optimum radiation pattern and lowest error vector magnitude (EVM) can be achieved even under high multipath effects/non-LOS conditions. A comparison between the performance and the application of the array factor (AF) theory is presented for practical indoor scenarios, demonstrating the effectiveness of the method. It is demonstrated that the method may be an alternative to the calibration of nonstandard MIMO antennas, where the AF computation is complex or not possible.
- Assessment of the mechanical properties of nanoclays enhanced low Tg epoxy resinsPublication . Ferreira, J. A. M.; Reis, P. N. B.; Costa, J. D. M.; Capela, C.The mechanical properties of the nanocomposites are dependent, not only of the clays content but, also, of the resin type and manufacturing process. In this context, the present study intends to develop a systematic study involving a low glass transition temperature (Tg) and low permeability epoxy resin (SR 1500 and the hardener SD 2503) with a commercially Nanomer I30 E nanoclays. Two dispersion processes were compared (direct (DM) and indirect method (IDM)) in terms of mechanical properties, as well as the influence of nanoclay content and hydro aging effect. It was possible to observe that the composites obtained by the indirect method present lower mechanical properties than the neat resin because there is residual acetone. For DM composites the tensile strength, fracture toughness and the specific energy absorbed by impact decreases with the reinforcement content, caused by particle agglomerates. Elastic modulus, at 25 °C, increases significantly and Tg increases slightly with the addition of nanoclays. Hydro aging promotes a progressive decreasing of the tensile strength and fracture toughness, with the clay content, reaching about 15 % and 7 %, respectively, for 6 wt% of nanoclays. On the other hand, a small increasing on specific energy absorbed was observed.
- Schreier split epimorphisms between monoidsPublication . Bourn, Dominique; Martins-Ferreira, Nelson; Montoli, Andrea; Sobral, ManuelaWe explore some properties of Schreier split epimorphisms between monoids, which correspond to monoid actions. In particular, we prove that the split short five lemma holds for monoids, when it is restricted to Schreier split epimorphisms, and that any Schreier reflexive relation is transitive, partially recovering in monoids a classical property of Mal'tsev varieties.
- SmartWalk BAN: Using Body Area Networks to Encourage Older Adults to Perform Physical ActivityPublication . Bastos, David; Ribeiro, José; Silva, Fernando; Rodrigues. Mário; Silva, Anabela G.; Queirós, Alexandra; Fernández-Caballero, Antonio; Rocha, Nelson Pacheco; Pereira, AntónioDue to the demographic ageing of the world’s population and the respective consequences, it is necessary to guarantee that older adults can be active and maintain their independence and autonomy for longer. The aim of the SmartWalk system is to promote walks in the city in order to stimulate physically active lifestyles. Body area networks are used to aggregate data collected by different type of sensors, which are transmitted to a server to support informed decisions of caregivers when planning physical activities for their care receivers. This article presents the SmartWalk system and reports an experimental setup that was developed to assess the performance of the current implementation and the respective critical components. According to the results, the SmartWalk system presents good performance in terms of battery usage, data upload, capacity to recover from connectivity failures and wireless coverage of its body area network.
- Application of "thematic analysis" to a set of businesses success stories in the internationalization processPublication . Brás, Gonçalo Nuno RodriguesThis current work is an example of applying the methodology of "thematic analysis" to a set of business success stories in the process of internationalization. It is assumed that these cases, as a hypothesis, constitute discourses obtained appropriately allowing synthesizing and relating some of the basic features inherent in the internationalization of companies. Results showed the prevalence of stimuli intrinsic to the company, to the detriment of extrinsic stimuli, and lower incidence that companies expose their barriers to internationalization. In the area of internationalization of firms, it confirms the association of entrepreneurial characteristics and the fundamental export nature of Portuguese business companies.
- Effect of water and fiber length on the mechanical properties of polypropylene matrix compositesPublication . Ferreira, N.; Capela, C.; Ferreira, J. M.; Costa, J. M.This paper presents the results of a current study on polypropylene matrix composites processed by injection, with two different glass fiber lengths and five different volume fractions. Physical and mechanical properties were obtained, namely flexural strength, stiffness modulus and fracture toughness. The mechanical properties of the composites increased significantly with the increase of the fibers volume fraction in agreement with the Counto model. The effect of water immersion time was also analysed. Immersion in water promotes a marked decrease in mechanical properties in the early seven-ten days, and afterwards tends to stabilize. Water causes a decrease of the relative strength which increases with fiber volume fraction and reaches about 29 % and 32 % for 20 % of 4.5 mm fiber length and for 25 % of 12 mm fiber length respectively, after 28 days immersion in water. Fracture toughness increases with fiber volume fraction and is always higher for 12 mm fiber length composites than for 4.5 mm fiber length composites.
- Development and Evaluation of an Outdoor Multisensory AR System for Cultural HeritagePublication . Marto, Anabela; Melo, Miguel; Gonçalves, Alexandrino; Bessa, MaximinoEnhancing tourist visits to cultural heritage sites by making use of mobile augmented reality has been a tendency in the last few years, presenting mainly audiovisual experiences. However, these explorations using only visuals and sounds, or narratives, do not allow users to be presented with, for example, a particular smell that can be important to feel engaged or to better understand the history of the site. This article pursues the goal of creating an experience that puts the user in a scene planned to evoke several stimuli with SensiMAR prototype - a Multisensory Augmented Reality system that aims to be used in cultural heritage outdoors. When using SensiMAR, the user will be involved with visual reconstructions, surrounded by the soundscape of ancient times, and is exposed to a particular smell very common that time. Given the novelty of this proposal, ascertaining the usability of such a system was raised as a foremost demand. Thus, in addition to its development and implementation specifications, an experimental study was conducted to evaluate the usability of the system in end-users' perspective. The results obtained from random visitors of an archaeological site were analysed according to their sex, age, previous experience with augmented reality technology, and provided condition - audiovisual condition, and multisensory condition, with visual, audio, and smell stimuli. Results were collected from a total of 67 participants and show that this multisensory prototype achieved good usability results across all groups. No statistically differences were found, demonstrating good usability of the SensiMAR system regardless of their sex, age, previous experience with the technology or provided condition.
- Imperfections and Modelling of the Weld Bead Profile of Laser Butt Joints in HSLA Steel Thin PlatePublication . Riofrío, Patricio G.; Ferreira, José A. M.; Capela, Carlos A.In many applications that use high strength steels, structural integrity depends greatly on weld quality. Imperfections and the weld bead geometry are influencing factors on mechanical properties of the welded joints but, especially in the fatigue strength, they cause a great decrease. The proper knowledge of these two factors is important from the nominal stress approach to the fracture mechanics approaches. Studies concerning the profile and imperfections of the weld bead in laser welding for thin plates of high strength steels are scarce. In this work, these two aspects are covered for five series single and double-welded joints, butt joints in a 3 mm thick HSLA steel, welded in a small range of welding parameters. The actual profiles captured with profilometer were modeled with proposed geometric parameters achieving an adequate fit with values of the coefficient of determination ℜ2 greater than 0.9000. Description of imperfections includes the distributions of porosity and undercuts. The evaluation of the weld quality, taking as guide the ISO 13919-1 standard determined B and D levels for the welded series while based on the stress-concentrating effect, showed a greater detriment in those series with undercuts and excessive penetration. The analysis of variance validated the results of the different combinations of laser welding parameters and showed, for the factorial experimental design, a more significant effect of the welding speed.