Jucélio Tomás Pereira

Mobile and Personal Communications: Acts and Beyond

In Europe, Mobile and Personal Communications have always been considered a key driver for growth and innovation, as well as being a necessary building block of the Wireless Information Society. Since 1988, European Union (EU)-funded RD projects have been working towards the development of the next generations of mobile communication concepts, systems and networks.

Viscoelastic Relaxation Modulus Characterization Using Prony Series

The mechanical behavior of viscoelastic materials is influenced, among other factors, by parameters like time and temperature. The present paper proposes a methodology for a thermorheologically and piezorheologically simple characterization of viscoelastic materials in the time domain based on experimental data using Prony Series and a mixed optimization technique based on Genetic Algorithms and Nonlinear Programming. The text discusses the influence of pressure and temperature on the mechanical behavior of those materials. The results are compared to experimental data in order to validate the methodology. The final results are very promising and the methodology proves to be effective in the identification of viscoelastic materials.

A three-dimensional finite element study on the stress distribution pattern of two prosthetic abutments for external hexagon implants.

Objective: The purpose of this study was to evaluate the mechanical behavior of two different straight prosthetic abutments (one- and two-piece) for external hex butt-joint connection implants using three-dimensional finite element analysis (3D-FEA). Materials and Methods: Two 3D-FEA models were designed, one for the two-piece prosthetic abutment (2 mm in height, two-piece mini-conical abutment, Neodent) and another one for the one-piece abutment (2 mm in height, Slim Fit one-piece mini-conical abutment, Neodent), with their corresponding screws and implants (Titamax Ti, 3.75 diameter by 13 mm in length, Neodent). The model simulated the single restoration of a lower premolar using data from a computerized tomography of a mandible. The preload (20 N) after torque application for installation of the abutment and an occlusal loading were simulated. The occlusal load was simulated using average physiological bite force and direction (114.6 N in the axial direction, 17.1 N in the lingual direction and 23.4 N toward the mesial at an angle of 75° to the occlusal plan). Results: The regions with the highest von Mises stress results were at the bottom of the initial two threads of both prosthetic abutments that were tested. The one-piece prosthetic abutment presented a more homogeneous behavior of stress distribution when compared with the two-piece abutment. Conclusions: Under the simulated chewing loads, the von Mises stresses for both tested prosthetic-abutments were within the tensile strength values of the materials analyzed which thus supports the clinical use of both prosthetic abutments.

Fiber Bragg grating tuning with notch-type spring device

This paper presents a notch-type spring mechanism based on the traction principle for tuning fiber Bragg gratings, which magnifies the displacement of piezoelectric actuators. Practical tuning ranges up to 8 nm and tuning times below 20 ms are demonstrated. Oscillations that affect the device due to the rapid voltage change applied to the actuator are mitigated using either an electronic filter or a viscoelastic neutralizer technique.

Numerical evaluation of bone remodelling and adaptation considering different hip prosthesis designs

Background: The change in mechanical properties of femoral cortical bone tissue surrounding the stem of the hip endoprosthesis is one of the causes of implant instability. We present an analysis used to determine the best conditions for long‐term functioning of the bone–implant system, which will lead to improvement of treatment results. Methods: In the present paper, a finite element method coupled with a bone remodelling model is used to evaluate how different three‐dimensional prosthesis models influence distribution of the density of bone tissue. The remodelling process begins after the density field is obtained from a computed tomography scan. Then, an isotropic Stanford model is employed to solve the bone remodelling process and verify bone tissue adaptation in relation to different prosthesis models. Findings: The study results show that the long‐stem models tend not to transmit loads to proximal regions of bone, which causes the stress‐shielding effect. Short stems or application in the calcar region provide a favourable environment for transfer of loads to the proximal region, which allows for maintenance of bone density and, in some cases, for a positive variation, which causes absence of the aseptic loosening of an implant. In the case of hip resurfacing, bone mineral density changes slightly and is closest to an intact femur. Interpretation: Installation of an implant modifies density distribution and stress field in the bone. Thus, bone tissue is stimulated in a different way than before total hip replacement, which evidences Wolffs law, according to which bone tissue adapts itself to the loads imposed on it. The results suggest that potential stress shielding in the proximal femur and cortical hypertrophy in the distal femur may, in part, be reduced through the use of shorter stems, instead of long ones, provided stem fixation is adequate. HighlightsFortran subroutine of femur bone behaviour in case of bone adaptation is developed.Effect of different types of prosthesis on the bone remodelling is studied and quantified.Was obtained change in femur bone density after two years of hip replacementRisk of stress–shielding effect yielded by long‐stem models is discussed.Was detected possibility reduce stress shielding in case of using short stem implants

Indirect Identification of the Complex Poisson's Ratio in Fractional Viscoelasticity

The use of viscoelastic materials (VEMs) has becoming more and more frequent both as vibration control in general or as parts of structural components. In all applications, the mechanical behavior of such materials can be predicted by the complex moduli (Young’s, shear or volumetric) and the complex Poisson’s ratio. Over recent decades, various methodologies have been presented aiming at characterizing complex moduli. On the other hand, the indirect identification of the Poisson’s ratio, in the frequency domain, proves to be underexplored. The present paper discusses two computational methodologies in order to obtain, indirectly, the complex Poisson’s ratio in linear and thermorheologically simple solid VEMs. The first of them uses a traditional methodology, which individually identifies the complex Young’s and the shear moduli and, from them, one obtains the complex Poisson’s ratio. The second methodology – proposed in the present paper and called ‘integrated’ – obtains the complex Poisson’s ratio through a simultaneous identification of those two complex moduli. Both methodologies start from a set of experimental points of the complex moduli in the frequency domain, carried out at different temperatures. From those points, a hybrid optimization technique is applied (Genetic Algorithms and Non-Linear Programming) in order to obtain the parameters of the constitutive models for the VEM under analysis. For the experiments described here, the integrated methodology proves to be very promising and with a great application potential.