Contribuição ao estudo da histerese em ligas com memória de forma.

Abstract

In this work the characterization of the strain-temperature (e x T) and electrical resistancetemperature (R x T ) in the shape memory alloy (SMA) NiTi are studied. Based on this study, the modelling of the hysteresis in the e x T characteristics of a SMA wire was achieved. Shape memory alloys exhibit permanent deformation under mechanical work, and after heating, recover its original shape. This remarkable property motivates the application these materials in many engineering fields, in medical applications and orthodontic applications. These materials are very attractive as thermomechanical actuators for applications in robotics due to the shape memory effect phenomenon that can be reproduced during a large number of thermal cycles. An experimental platform was developed to submit the SMA wire to heating/cooling cycles, under constant load. Using this platform, the (e x T) and (R x T ) characteristics was obtained. The experimental platform used in this work, has three parts: a mechanical structure where a linear variable differential transformer (LVDT) was used to measurement strain of the SMA wire; a voltage/current converter, used for heating SMA wire (Joule effect) and the acquisition system. Cooling of the SMA wire is achieved by free convection. External room temperature disturbances were greatly reduced by embedding the wire in a heat insulating medium. In the absence of an accurate and reliable technique for measurement of the SMA wire temperature, such temperature was estimated by using the thermal balance equation. In this work is proposed an adaptation of the Limiting Loop Proximity (L2P) hysteresis model, originally developed for magnetic hysteresis and adapted to thermal hysteresis in VO2 thin films, to describe the hysteresis in the e x T characteristics of the SMA wire. The models obtained experimentally were used to generate hysteresis loops that were compared with the experimental data. The obtained results are considered satisfactory. However, the developed models reproduce approximately the hysteresis in the e x T characteristics of the SMA wire. The discrepancies between the calculated and the experimental data, can be attributed to limitations in the numeric implementation of the model and to the large asymmetry of curve s x T.