ENIDH - EMM - Artigo Científico
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- Influence of Applied Loads on Free Vibrations of Functionally Graded Material Plate–Shell PanelsPublication . Moita, José Simões; Correia, Victor M. Franco; Soares, Cristovão MotaAn analysis of the influence of applied loads on free vibrations of plate and shell panels made of functionally graded materials is analyzed in the present work. Formulations for the static analysis considering geometrically nonlinear behavior, as well as linear buckling and free vibrations analyses are considered. A calculation of the through-thickness stress distribution is also performed. The finite element model is based on a higher order shear deformation theory using a non-conforming flat triangular plate/shell element with three nodes, and eight degrees of freedom per node is used in the numerical implementation. The results obtained with this numerical model are presented, discussed, and compared with alternative solutions published by other authors in some benchmark applications.
- Vibration modes of the Euler–Bernoulli beam equation with singularitiesPublication . Dias, C. Nuno; Jorge, Cristina; Prata, João NunoWe consider the time dependent Euler–Bernoulli beam equation with discontinuous and singular coeffi-cients. Using an extension of the Hörmander product of distributions with non-intersecting singular supports (L. Hörmander, 1983 [25]), we obtain an explicit formulation of the differential problem which is strictly defined within the space of Schwartz distributions. We determine the general structure of its separable solu-tions and prove existence, uniqueness and regularity results under quite general conditions. This formalism is used to study the dynamics of an Euler–Bernoulli beam model with discontinuous flexural stiffness and structural cracks. We consider the cases of simply supported and clamped-clamped boundary conditions and study the relation between the characteristic frequencies of the beam and the position, magnitude and struc-ture of the singularities in the flexural stiffness. Our results are compared with some recent formulations of the same problem.
- On Orthogonal Projections of Symplectic BallsPublication . Dias, C. Nuno; Gosson, A. Maurice; Prata, N. JoãoWe study the orthogonal projections of symplectic balls in R2n on complex subspaces. In particular we show that these projections are themselves symplectic balls under a certain complexity assumption. Our main result is a refinement of a recent very interesting result of Abbondandolo and Matveyev extending the linear version of Gromov’s non-squeezing theorem. We use a conceptually simpler approachwhere the Schur complement of a matrix plays a central role. An application to the partial traces of density matrices is given.
- Noncomoving description of adiabatic radial perturbations of relativistic starsPublication . Luz, Paulo; Carloni, SanteWe study adiabatic, radial perturbations of static, self-gravitating perfect fluids within the theory of general relativity employing a new perturbative formalism. We show that by considering a radially static observer, the description of the perturbations can be greatly simplified with respect to the standard comoving treatment. The new perturbation equations can be solved to derive analytic solutions to the problem for a general class of equilibrium solutions.We discuss the thermodynamic description of the fluid under isotropic frame transformations, showing how, in the radially static, noninertial frame, the stressenergy tensor of the fluid must contain momentum transfer terms. As illustrative examples of the new approach, we study perturbations of equilibrium spacetimes characterized by the Buchdahl I, Heintzmann IIa, Patwardhan-Vaidya IIa, and Tolman VII solutions, computing the first oscillation eigenfrequencies and the associated eigenfunctions. We also analyze the properties of the perturbations of cold neutron stars composed of a perfect fluid verifying the Bethe-Johnson model I equation of state, computing the oscillation eigenfrequencies and the e-folding time.
- The Impact of Curing Temperature and UV Light Intensity on the Performance of Polymer-Dispersed Liquid Crystal Devices Exhibiting a Permanent Memory EffectPublication . Mouquinho, Ana; Sotomayor, JoãoPDLC films, synthesized via polymerization-induced phase separation (PIPS) utilizing both temperature and UV monochromatic radiation, were derived from a blend of E7 nematic liquid crystal (LC) and PolyEGDMA875 (polyethyleneglycoldimethacrylate) oligomers, serving as the precursor for the polymeric matrix. The influence of the curing temperature on thermal polymerization, UV light intensity on photochemical polymerization, and exposure time during these processes on the electro-optical characteristics of PDLC films was thoroughly examined. Observations revealed that employing thermal polymerization during device preparation notably enhanced the permanent memory effect of the PDLC films. Sustained high transparency (TOFF = 45%) over an extended duration at room temperature, even subsequent to voltage cessation, was achieved. This transition initiated from an opaque state (T0 = 0%) through to a transparent state (TMAX = 65%), resulting in a substantial 70% permanent memory effect.
- Pre-Polymer Chain Length: Influence on Permanent Memory Effect of PDLC DevicesPublication . Mouquinho, A.; Barros, M. T.; Sotomayor, J.This study delved into the correlation between the chain length of PEG polymerizable oligomers and the electro-optical properties exhibited by the resultant PDLC films. A range of di(meth)acrylate oligomers derived from polyethylene glycol with varying molecular weights (Mn = 1000, 2000, 4000, and 6000 g mol−1) was synthesized for incorporation as the polymer matrix in PDLC devices. Comprehensive analyses employing 1H-NMR, 13C-NMR, and MALDI-TOF mass spectroscopy were conducted to validate the structure and purity of the synthesized products. The investigation revealed a significant influence of pre-polymer molecular chain length on the thermal properties of the polymer, including amorphousness and crystallinity, which in turn impact the permanent memory effect. Specifically, it was observed that amorphous PEG polymers serve as an ideal matrix for fostering the permanent memory effect in PDLCs. Among the polymerizable PEG oligomers examined, those with a molecular weight of 1000 g/mol yielded polymer chains existing in an amorphous state, exhibiting a glass transition temperature lower than room temperature (−50 °C). This characteristic imparts flexibility and mobility to the polymer matrix chains, facilitating a 37% permanent memory effect. Conversely, longer polymer chains lead to the formation of crystal aggregates, resulting in semi-crystalline polymer matrices. This reduces the malleability of the polymer chains, thereby nullifying the permanent memory effect in the corresponding PDLC devices.
- Fault Diagnosis of Maritime Equipment Using an Intelligent Fuzzy FrameworkPublication . Mendonça, L. F.; Sousa, J. M. C.; Vieira, S. M.The task of automatically and intelligently diagnosing faults in marine equipment is of great significance due to the numerous duties that shipboard professionals must handle. Incorporating automated and intelligent systems on ships allows for more efficient equipment monitoring and better decision-making. This approach has attracted considerable interest in both academia and industry because of its potential for economic savings and improved safety. Several fault diagnosis methods are documented in the literature, often involving mathematical and control theory models. However, due to the inherent complexity of some processes, not all characteristics are precisely known, making mathematical modeling highly challenging. As a result, fault diagnosis often depends on data or heuristic information. Fuzzy logic theory is particularly well suited for processing this type of information. Therefore, this paper employs fuzzy models to diagnose faults in a marine pneumatic servo-actuated valve. The fuzzy models used in fault diagnosis are obtained from the data. These fuzzy models are identified for the normal operation of the marine pneumatic servo-actuated valve, and for each fault, predicting the system’s outputs from the inputs and outputs of the process. The proposed fault diagnosis framework analyzes the discrepancy signals between the outputs of the fuzzy models and the actual process outputs. These discrepancies, known as residuals, help in detecting and isolating equipment faults. The fault isolation process uses an intelligent decision-making approach to determine the specific fault in the system. This method is applied to diagnose abrupt faults in a marine pneumatic servo-actuated valve. The approach presented was used to detect and diagnose three very important faults in the operation of a marine pneumatic servo-actuated valve. The three faults were correctly detected and isolated, and no errors were detected in this detection and isolation process.
- A 1.7-mW −92-dBm Sensitivity Low-IF Receiver in 0.13-um CMOS for Bluetooth LE ApplicationsPublication . Silva-Perreira, Marco.; Sousa, J. T. de; Freire, J. Costa; Vaz, J. CaldinhasThis paper presents a 1.7-mW low-intermediate-frequency receiver design for Bluetooth low-energy (BLE) applications. The design exploits particular aspects of BLE, such as the relaxed in-band interference characteristics, more precisely the C/I1 MHz, and the relatively high-frequency-shift keying modulation index, to deliver a high level of energy efficiency and simplicity to the receiver baseband architecture. Reliable quadrature signals are generated in the RF signal path without consuming energy, which is supported by an inverter-based low-noise amplifier (LNA) that achieves high gain and low noise figure under low-power budgets. A small-signal analysis of low-power inverter-based LNAs is presented offering simple design equations. Seeking an affordable solution, the fabricated prototype is fully integrated into an earlier generation CMOS technology node (0.13 μm), occupying a silicon area smaller than 0.7 mm2. The receiver achieves a sensitivity level of -92 dBm while consuming 1.41 mA from a 1.2-V supply.
- Oil reaching the coast: Is Brazil on the route of international oceanic dumping?Publication . Zacharias , Daniel Constantino; Crespo, Natália Machado; Silva, Natália Pillar da; Rocha, Rosmeri Porfirio da; Gama, Carine Malagolini; Silva, Sergio B.N. Ribeiro e; Harari, JosephAfter the oil spill disaster occurred in 2019, various events of tar balls reaching the Brazilian coast and archipelagos have been reported. The hypothesis here is that the oil/waste dumped in international waters by ships on-route to Cape of Good Hope is reaching the Brazilian coast. On that account, 30-year probabilistic simulations were used to estimate the probability of dumped oil residue reaching the Brazilian coast. The simulations considered three Zones following the South Atlantic route. The results have shown that up to 28.5 % of large ships could dump oil on-route. Inside the Brazilian Exclusive Economic Zone, the probability of dumped oil/waste reaching the coastline is about 62 % and quickly decreases for Dumping Zones 2 and 3. Equatorial and Northeast shores of Brazil are the most vulnerable to oceanic dumping when compared to other regions
- Optimization on Elastoplasticity of Functionally Graded Shells of Revolution, under Axisymmetric LoadingPublication . Moita, J. S.; Araújo, A. L.; Correia, V.F.; Soares, C.M.M.; Herskovits, J.Depending on the load level, structures can experience a material nonlinearity known as elastoplasticity, which has an important role in the behaviour of structures. In order to avoid the elastoplastic behaviour, it is necessary to find the optimal thickness distribution, which corresponds to the minimum mass that provides an elastic behaviour for a certain load level. The elastoplasticity analysis of functionally graded axisymmetric shells under axisymmetric mechanical loading, and the subsequent optimization, was performed by using a simple conical frustum finite element model with two nodal circles; three degrees of freedom per node, which was based on Kirchhoff’s theory allowing for shear deformation; and using a reduced numerical integration procedure that is essential for its success when applied to thin shells. The formulation accounts for the calculation of the displacements and through-thickness stress distribution, including the effective stress. In this work, the thickness was the design variable in the optimization procedure and the mass was the objective function that needed to be minimized subject to a constraint imposed on the effective stress. The optimization solutions were obtained by using a feasible arc interior point gradient-based algorithm. Some illustrative examples were performed, and the corresponding results are presented and discussed.