André Cervieri

Thermally induced strains and total shrinkage of the polymethyl-methacrylate cement in simplified models of total hip arthroplasty

An evaluation of transient and stabilized strains in the cement mantle during polymerization was carried out in simplified cemented total hip arthroplasty (THA) model. A mathematical approach combined with a simple finite element simulation was used to compare measured and calculated stabilized strain values and to provide the Von Mises stresses at the stem/cement interface due to shrinkage related to temperature decrease after exothermal reaction. A second similar model was carried out to measure stem/cement/mold interfacial shear strength and dimensional changes of the cement mantle to obtain total shrinkage due to temperature decrease plus cement polymerization. The results indicated that positive strain peaks found during the exothermic stage of polymerization have the potential to produce pre-loading cracking. After the initial expansion, it was observed a progressive strain decrease pattern down to stabilized values that takes place near 2h after the cementation. Even though there is a great deal of dispersion in the measured stabilized strain values, in average those values match quite well with the numerical simulations, indicating 4,7 MPa von Mises interfacial stress due to thermal shrinkage. The total cement shrinkage leads to a negative radial stress of 11 MPa and 14 MPa von Mises interfacial stress. Finally, total shrinkage has the potential to enhance gaps in the cement/mold interface.

Estudo de um motor CC brushless aplicado no acionamento de um carro elétrico de pequeno porte

This work presents the study of an electric motor, which was used to drive a small electric car. The aim was to select a motor with characteristics of suited operation to drive the electric car at a speed range from 12 to 20 km/h. Tests were carried out on the electric car with the purpose of determining its dynamic behavior. The test results allowed to select a DC (Direct Current) brushless electric motor of 12 V and 5.9 A with 77% of maximum efficiency. A theoretical study was developed to determine the best relation of the movement transmission from the motor to the car. By doing that, it was possible to determine the characteristics of the operation of the motor in which it developed its maximum efficiency, when the electric car was started up. The speed ratio value resulted in 64:1. The study showed that to operate a 20 km/h speed, the car requires a mechanical power of 58 W from its electric motor.