Resumo em Inglês:

The new mechanical device developed is capable of performing compression tests but also bending, torsion and tensile tests. Its measurement system has the following components: a load cell (0 to 9800 N), a strain gauge signal conditioning board, a plug-in general purpose data acquisition board, and a displacement sensor (0 to 29.7 mm). These components are connected to a personal computer that has specially developed software. The load cell consists of four extensometers, which are sensible to compressive load. The signal conditioning board filters and amplifies the signal, which is acquired by the AD converter, making this information available to be processed by the software. The device is powered by a DC motor (12 to 24 Volts). By setting the motor voltage, it is possible to control its rotation. The displacement sensor was made with a sliding potentiometer, which is pushed by the mobile component of the device while the tests are performed. Finally, the software provides the stress-strain diagram for each test. In order to guarantee reliable results, tests are made with cylindrical samples with same dimensions (radius and length). For the first set of tests, potatoes were studied. The test results for compressive showed an average value of 897.64 kPa. This value seems to be compatible to the ones from the literature. The device proves to be reliable. Moreover, its flexibility allowed testing a variety of fruits and vegetables under compression, tension, torsion and combined loads.

Resumo em Inglês:

The main purpose of this work is the development of computational tools in order to assist the on-line automatic detection of burn in the surface grinding process. Most of the parameters currently employed in the burning recognition (DPO, FKS, DPKS, DIFP, among others) do not incorporate routines for automatic selection of the grinding passes, therefore, requiring the user’s interference for the choice of the active region. Several methods were employed in the passes extraction; however, those with the best results are presented in this article. Tests carried out in a surface-grinding machine have shown the success of the algorithms developed for pass extraction.

Resumo em Inglês:

This work is about the steady state vibration of a rotor bearing system. The rolling bearings are modeled as two degrees of freedom elements where the kinematics of the rolling elements is considered, so are the internal clearance and the Hertz contact nonlinearity. The steady state solution of the system is achieved by the harmonic balance method, validated by a numerical integration using a three-point-centered finite difference scheme. The number of harmonics employed in the harmonic balance method is examined to find the satisfactory quantity of harmonics that can describe the system dynamics. The contact regimes in the rolling elements are also explored.

Resumo em Inglês:

This paperwork presents a Pulse Width Modulation (PWM) speed controller for an electric mini-baja-type car. A battery-fed 1-kW three-phase induction motor provides the electric vehicle traction. The open-loop speed control is implemented with an equal voltage/frequency ratio, in order to maintain a constant amount of torque on all velocities. The PWM is implemented by a low-cost 8-bit microcontroller provided with optimized ROM charts for distinct speed value implementations, synchronized transition between different charts and reduced odd harmonics generation. This technique was implemented using a single passenger mini-baja vehicle, and the essays have shown that its application resulted on reduced current consumption, besides eliminating mechanical parts.

Resumo em Inglês:

This study examines the behavior of well known simple thermal micro-actuators. This work describes some unexplored aspects of the employed methodology to produce these electro-thermal micro-actuators (ETMs). These information could be time saving for new groups to start research and development on electro-thermal actuators. The focus of the applied methodology is the use of the finite element method (FEM) applied to an iterative procedure to predict the behavior of the micro-actuators before production. The computational model is adjusted based on measurements of force and displacement. To be correctly applied, this methodology requires a sensitivity study of the material properties. The results emphasize the need to use FEM with more accurate thermal and mechanical properties, and to consider also the variation of these properties with temperature. We must also consider a careful definition of thermal boundary conditions and the effects of heat convection and radiation, due to deviations between experimental results and FE results. In this investigation, the structural material applied was Nickel, plated at about 1 µm/min. The experimental characterization was carried out in a micro probe station with a CCD (charge-coupled device) camera attached to it, and used a controlled voltage source.

Resumo em Inglês:

This work has as a fundamental objective the numerical study of the effects of synthetic jet actuators on the flow of the boundary layer developed on a flat plate and on a hypothetical airfoil. The aim is to obtain computational data to indicate how these effects may be used as a means of flow control, describing the dynamics of the synthetic jet in the presence of external flow. The present paper uses a spatial Direct Numerical Simulation (DNS) to solve the incompressible Navier-Stokes equations, written in vorticity-velocity formulation. The spatial derivatives are discretized with a sixth order compact finite difference scheme. The Poisson equation for the normal velocity component is solved by an iterative Line Successive Over Relaxation Method and uses a multigrid Full Approximation Scheme to accelerate the convergence. The results of simulations with different values of frequency, amplitude and slot length were analyzed through a temporal Fourier analysis. Through this analysis the decision as to which are the better parameters to delay the separation of the boundary layer is examined.

Resumo em Inglês:

This paper aims to demonstrate the importance of adequately estimating the discretization error intrinsic in the result of any numerical simulation. The problem under consideration is forced convection in a staggered circular tube heat exchanger. The problem is solved to analyze the effect of the distance between the tubes, aiming to optimize the heat exchanger’s geometrical configuration by two Reynolds numbers (50 and 100). The present work did not confirm the existence of an optimal geometrical point for the operation of staggered circular tube heat exchangers, as claimed in a numerical study published in the literature.

Resumo em Inglês:

Mechanical material removal during ultraprecision machining of semiconductors crystals normally induces surface damage. In this article, Raman micro-spectroscopy has been used to probe structural alteration as well as residual stresses in the machined surface generated by single point diamond turning. The damage found is characterized by an amorphous phase in the outmost surface layer. In addition, it is reported, for the first time, the results of in-situ re-crystallization annealing of micromachined silicon monitored by micro-Raman spectroscopy. It is also shown that the annealing heat treatment influenced surface roughness: results were Rmax equal to 24.2 nm and 47.3 nm for the non treated and for the annealed surfaces, respectively.

Resumo em Inglês:

This paper deals with an experimental study of both wear and friction responses of lubricated sliding tests, considering both responses in Stribeck diagram. Analyses concerning the wear coefficient by Archard equation were also done. Tests were oil-bath lubricated, performed through a machine with devices for rotating pin-on-disk and reciprocating pin-on-plate tests. Tested specimens were pins of AISI52100 steel and counter-bodies of AISI8640 steel. Presence of additives and contamination in lubricant oil was investigated under two mechanical loading levels, determined by the velocity/load ratio. Wear was studied by means of optical microscope and dimensional analysis of the worn surfaces. The use of a wear coefficient similar to that of Archard equation for characterizing sliding wear behavior is discussed. Differences were observed in the wear trend of the rotating and the reciprocating tests in terms of Stribeck parameters considerations.

Resumo em Inglês:

Functionally graded materials have an additional length scale associated to the spatial variation of the material property field which competes with the usual geometrical length scale of the boundary value problem. By considering the length scale of nonhomogeneity, this paper presents the weak patch test (rather than the standard one) of the graded element for nonhomogeneous materials to assess convergence of the finite element method (FEM). Both consistency (as the size of elements approach zero, the FEM approximation represents the exact solution) and stability (spurious mechanisms are avoided) conditions are addressed. The specific graded elements considered here are isoparametric quadrilaterals (e.g. 4, 8 and 9-node) considering two dimensional plane and axisymmetric problems. The finite element approximate solutions are compared with exact solutions for nonhomogeneous materials.

Resumo em Inglês:

The aim of this work is to propose a methodology to estimate the fatigue limit of cylindrical contacts under a partial slip regime. Taylor’s point stress method, usually applied to estimate fatigue limit for notched structures, was associated with the Modified Wöhler Curves to define the fretting crack initiation threshold methodology. Twenty-nine tests on cylindrical contacts were selected from the literature and considered to evaluate the quality of the estimates. The results agree well for twenty-three experimental data. As the fatigue limit under fully reversed bending is the fatigue parameter usually available for most metallic alloys, it was also showed how the second fatigue limit needed to calibrate the proposed procedure could be estimated by taking full advantage from other standard predictive methodologies previously devised to estimate the mean stress effect under uniaxial fatigue loading.

Resumo em Inglês:

The aim of this study is to determine conditions for the quasi-steady regime (QSR) of vaporization of Methanol droplets in high pressure Nitrogen. Under the quasi-steady regime, the square of the droplet radius decreases linearly with time (QSR), a simple result that can be easily implemented in numerical codes. Nevertheless, the vaporization rate cannot be described by a simple expression. The vaporization rate depends initially on the quantity of gas dissolved inside the droplet, on the liquid phase expansion and on the mass loss to the gas phase which is controlled by the initial conditions of the droplet. After this initial period and the droplet heating period, when the heat flux from the gas phase is almost completely employed to vaporize the droplet, the vaporization rate can be expressed approximately by a quasi-steady model. As shown by the numerical results, the main unsteadiness source for the droplet vaporization at high pressure is the droplet heating.

Resumo em Inglês:

A magnetic actuator as excitation source in rotative systems is proposed, in order to accomplish modal analysis without contact between the actuator and the rotor. Although the use of electromagnets for applying forces to rotating machinery have been carried out with high performance level (for example, magnetic bearings), the development of a conveniently easy and cheap device for laboratory application presents interesting contribution to experimental methods used in test rigs based on similarity design to rotating machinery. The initial concept of the magnetic actuator proposed here is simple, but enables either the external excitation without contact or the vibration control when associated with a controller system. However, the calibrations of performance characteristics to attend the dynamic demand of the system in not so trivial. Following this focus, the paper brings practical experience and discussion about the development, calibration and performance analysis of a magnetic actuator used for rotating machinery tests. The influence of the electrical current in the actuator coils, the air-gap between actuator and rotative system, the type of surface of the actuator poles (flat or curved) as well as excitation frequency was experimentally verified and compared with theoretical concepts. Force estimation was carried out and compared with the measured force. The estimation was based on the magnetic flux density, measured by hall sensors, or input current and initial air-gap.

Resumo em Inglês:

This paper presents an analytical study to minimize the heat lost during the power stroke of hydrogen-fueled, 4-stroke spark ignition engine. The effects of various design and operating parameters like engine speed, equivalence ratio, ignition timing, compression ratio and spark plug location on heat losses was studied. Then, the effect of heat loss on the engine’s performance and emission characteristics was conducted with aim to minimize it. The study showed that the percentage heat loss decreased with increase in engine speed. Further, changing the compression ratio from 7 to 11 produced change of around 2-3% in percentage heat loss, while changing the spark plug location from periphery to center produced change of around 1-1.5% in percentage heat loss with maximum change occurring at lean mixtures. Changing the ignition timing from 50 to 10 degrees bTDC changed the percentage heat loss by more than 4%. As for the effect of percentage heat loss on the engine performance and emission characteristics, it was found that some of the heat must be allowed to be transferred from the cylinder to enhance the engine performance. This amount of heat loss varies with engine speed and equivalence ratio. It was found to be higher at higher engine speeds. Beyond this specific value, the engine performance and emission characteristics deteriorate.

Resumo em Inglês:

Structures’ load resistance is reduced when subject to high temperatures in fire. Nevertheless, it is not enough to take into account strength and modulus of elasticity reduction. One must know with great accuracy the behaviour of the structures including the realistic boundary conditions at high temperatures, in order to determine for safe and of low cost solutions. This work is intended to analyse the behaviour of steel beams under uniform temperature rising. It covers beams under several uniform load levels and three boundary conditions: simply supported (pin-roller), simply supported with axial restraint (pinned-pinned) and both ends fixed (fixed-fixed). The variation of deflection, critical temperature, bending moment, normal force and stresses, with the temperature is presented. The analyses were made with the aid of ANSYS computer software taking into account material and geometric non-linearities and the variation of the stress-strain diagram with the temperature.