Antonio Almeida Silva

Study of the Complex Stiffness of a Vibratory Mechanical System with Shape Memory Alloy Coil Spring Actuator

The vibration control is an important area in the dynamic of structures that seeks to reduce the amplitude of structural responses in certain critical frequency ranges. Currently, the scientific development leads to the application of some actuators and sensors technologically superior comparing to the same features available on the market. For developing these advanced sensors and actuators, smart materials that can change their mechanical properties when subjected to certain thermomechanical loads can be employed. In this context, Shape memory alloys (SMAs) may be used for developing dynamic vibration dampers which are capable of acting on the system providing proper tuning of the excitation frequency and the natural frequency. This paper aims to analyze the behavior of the stiffness and damping of a SMA helical coil spring actuator coupled to a mechanical system of one degree of freedom (1 DOF) subjected to an unbalanced excitement force and a temperature control system. By analyzing the effect of these parameters on the structural response and considering the concept of complex stiffness, it can be possible to predict the systems behavior within certain acceptable ranges of vibration, already in the design phase.

Vibration Attenuation in an Epoxy Smart Composite Beam with Embedded NiTi Shape Memory Wires

Shape memory alloys (SMA) are thermo-responsive materials where deformation can be induced and recovered through temperature changes. Therefore, SMA are considered smart materials. In this work, an epoxy beam reinforced by NiTi SMA wires was developed. This active composite contains five pre-trained NiTi SMA wire actuators, evenly distributed along the neutral plane of the epoxy beam, which can be activated by resistive heating. The results of different ways for electrical activation of the smart composite in a simply clamped mode are discussed. It was possible to demonstrate the viability of this concept for attenuation of mechanical vibrations by controlled electrical heating of the NiTi wire actuators.

Experimental Characterization of Hydrogen Embrittlement in API 5L X60 and API 5L X80 Steels

The present work aims to study the hydrogen embrittlement process in API 5L X60 and API 5L X80 steels. The tests were performed using two kinds of hydrogen sources to work with two conditions of hydrogen damage: environmental hydrogen embrittlement and internal hydrogen embrittlement. The mechanical behavior of API 5L X60 and API 5L X80 steels in tensile tests, with and without hydrogen, were studied. Under environmental hydrogen embrittlement conditions, the API 5L X60 steel presented a softening process observed by the decrease in yield strength and increase in its deformation. The API 5L X80 steel was more susceptible to the phenomenon due the deformation decrease of hydrogenated samples. In notched samples, both steels were susceptible to embrittlement as shown by the decrease in elongation. Under internal hydrogen embrittlement conditions, in both steels the changes in deformation were significant and can be attributed to changes in the hydrogen trapping due to the hydrogenation process used, the chemical composition and microstructure. It was observed that the fracture surface morphology of hydrogenated samples of both steels was ductile by microvoids coalescence, and that the distribution of dimples per unit area was higher in the API 5L X60 steel. It can be concluded, as reported in the literature, that the reversible hydrogen trapping observable in environmental hydrogen embrittlement is more damaging than irreversible hydrogen trapping, observable in internal hydrogen embrittlement.

Dynamical Analysis Applied to Passive Control of Vibrations in a Structural Model Incorporating SMA-SE Coil Springs

Mechanical vibrations are severe phenomena of the physical world. These oscillations may become undesirable and may cause temporary and even irreversible damage to the system. There are several techniques to minimizing these vibration effects ranging from passive methods to the use of controllers with smart materials. In this sense, this study aims to analyze a passive vibration control system installed in a structure that simulates two-floor buildings. This system based on the incorporation of one SMA-SE (Superelastic Shape Memory Alloys) coil springs configuration for energy dissipation and the addition of damping. Modal analysis was performed using analytical, numerical, and experimental methods. In an experimental basis, response amplitudes were analyzed for free and forced vibrations in different configurations. As compared with the structure configuration with steel spring, the forced vibrations FRF (Frequency Response Function) analysis showed a reduction in displacement transmissibility of up to 51% for the first modal shape and 73% for the second mode in the SMA-SE coil spring configuration. As for damping, there was a considerable increase in the order of 59% in the first mode and 119% in the second, for the SMA-SE springs configuration.

Estudo das tensões residuais de juntas soldadas de aço API 5L X80

This study aimed to determine the levels of residual stresses due to welding processes in welded joints of API 5L X80 steel. Initially were executed the welded joint using the Shielded Metal Arc Welding (SMAW) and Gas Tungsten Arc Welding (GTAW) processes, with different consumables and employing an interpass temperature of 175oC. Three different welded joints were obtained. The residual stress measurements were carried out by x-ray diffraction using a portable diffractometer and software for analyzing the results. In the upper region, it was observed that to higher heat inputs the residual stresses levels were more compressive and more displaced were the tensile stresses peaks. In the lower region of the welded joints, it was found that for higher heat input, were higher levels of tensile residual stresses in the region of the Weld Metal (WM) and the lower the level of tensile residual stresses in the Heat Affected Zone (HAZ).

Application of elastic fracture and damage mechanics models for numerical simulation of hydrogen embrittlement in steels

Purpose – The purpose of this paper is to present a numerical simulation of the hydrogen atomic effect on the steels fracture toughness, as well as on crack propagation using fracture mechanics and continuous damage mechanics models.Design/methodology/approach – The simulation was performed in an idealized elastic specimen with an edge crack loaded in the tensile opening mode, in a plane strain state. In order to simulate the effect of hydrogen in the steel, the stress intensity factor ahead of the crack tip in the hydrogenated material was obtained. The damage model was applied to simulate the growth and crack propagation being considered only two damage components: a mechanical damage produced by a static load and a non‐mechanical damage produced by the hydrogen.Findings – The simulation results showed that the changes in the stress field at the crack tip and the reduction in the time of growth and crack propagation due to hydrogen effect occur. These results showed a good correlation and consistency wi...

MPEG-4 Codec Performance using a Fast Integer IDCT

Several standardized video coding algorithms use a well known discrete cosine transform (DCT) at the encoder to remove redundancy from video random processes. Its inverse, the IDCT is present at the decoder as well as at the encoder loop. The accuracy of this inverse transform, required to avoid large drift between the encoder and decoder, is defined in an IEEE standard which will be withdrawn from MPEG and ITU standardized codecs. Due to the huge number of computations required to compute the FDCT/IDCT (forward/inverse DCT) pair, reduction of their complexity is essential to speed up the video processing. In this paper, we propose a fast integer IDCT calculation method. Additionally, we insert it into the MPEG-4 (Part 2) reference software in order to validate our proposed method. The testing results using two different video sequences, at QCIF resolution, show similar PSNR average values between the reference MPEG-4 and the proposed MPEG-4 codec

Effect of Reversible Hydrogen Trapping on Crack Propagation in the API 5CT P110 Steel - A Numerical Simulation

This paper presents a numerical simulation of the reversible hydrogen trapping effect on crack propagation in the API 5CT P110 steel using a model based on a synthesis of fracture mechanics and continuum damage mechanics. The trapping term at the diffusion equation of this model was replaced by the equivalent term of McNab & Fosters model. Was simulated an C(T) specimen loaded in the mode I, in linear elastic regime, in plane strain state, and under the action of a static mechanical loading and hydrogen effect. The simulations showed that the material degradation ahead of crack tip increases with increasing in hydrogen concentration due the trapping with low interaction energies. Furthermore, the process of onset and crack growth in material with reversible traps is faster than the material free of traps. These results show a good correlation and consistency with macroscopic observations of the trapping effect, providing a better understanding of the hydrogen embrittlement in structural steels.

Computational Analysis of a Concept of Rear Suspension System for Off- Road Vehicle

The studies in the field of vehicle dynamics have been performed since the beginnings of the automobile industry, however in relation to advancements in embedded technologies for greater security and performance of vehicles in adverse conditions, research on this subject, gained a greater intensity in recent decades. Several research groups have sought to understand and model the real dynamic behavior of vehicles subject to the conditions imposed by the irregularities of the soil, in order to obtain suspensions of innovative concepts for vehicles that have high reliability and dynamic performance car. This paper presents a study for a cinematic concept of independent rear suspension, applied to off-road vehicles (off road). Thus, the analysis was performed using the software MSC Adams car, a type of vehicle suspension traditional double wishbone front with direction of tendency geometry to the oversteer effect and rear suspension arms overlaid with guiding bar. In these simulations allows perform an analysis of the behavior of the change in camber and convergence of the rear wheels during work vertical of the suspension by the method of kinematic analysis, using the theory of instantaneous centers of rotation. The concept developed is a modification of suspension of overlapping arms (double wishbone), with an addition of a guiding bar. The simulation results show the combined effect of the variation in camber and especially the variation in the effect of convergence in the oversteer of the vehicle, which was verified in experimental tests on a vehicle off-road mini-baja. Furthermore it has been found beneficial effect at the rolling of the chassis on the difference of change at the convergence of the wheels, which involves a tendency of exit from the rear of the vehicle. Based on these results verified the validity of the applicability on the suspension of arms superimposed with guiding bar for off-road vehicles, especially the mini-baja, which one of main characteristics is sought to obtain the ability to perform at the lowest curves possible radius.

Fragilización por Hidrógeno de los Aceros API 5L X60 y API 5L X80

Se ha determinado el comportamiento mecanico de los aceros API 5L grados X60 y X80, fragilizados por hidrogeno ambiental y por hidrogeno interno. El potencial y la densidad de corriente para la generacion del hidrogeno fueron determinados por polarizacion potenciodinamica, y el tiempo de saturacion estimado a partir de datos de difusividad y solubilidad obtenidos en ensayos de permeacion con una celula electroquimica. La carga de hidrogeno para reproducir las condiciones de la fragilizacion por hidrogeno ambiental y fragilizacion por hidrogeno interno fue realizada generando un gradiente de concentracion para el primer caso, y generando gradientes de concentracion y de tensiones para el segundo. Luego de los ensayos de traccion se verifico que, en las condiciones definidas en este trabajo, el modo de fractura en ambos tipos de fragilizacion fue predominantemente ductil. Ademas, los dos aceros mostraron una susceptibilidad baja o moderada a la fragilizacion por hidrogeno ambiental, no siendo susceptibles a la fragilizacion por hidrogeno interno.