Fadiga funcional e estrutural de fios de ligas NiTi com memória de forma em regime superelástico.

Abstract

The rapid growth in the use of shape memory alloys (SMA) working in superelastic regime, especially NiTi, for applications covering the fields of medicine, dentistry and engineering, makes a more specific knowledge of the behavior of these materials when subjected to cyclic mechanical stresses of fundamental importance. Further, the answer to both structural and functional fatigue of these materials is of extreme importance for safety during applications. The tests performed in this work have focused on evaluating the fatigue behavior of NiTi SMA subjected to uniaxial stress-controlled cyclic loadings. More specifically, the main objective was to determine the functional and structural properties of fatigue in superelastic NiTi SMA wires of circular and square cross section (diameter of 0.5 mm and 0.40 mm side). The evolution of dissipated energy, the transformation stresses, accumulated strain and superelastic strain were determined according to the applied stress and frequency of the mechanical tests, emphasizing functional fatigue of the material. In addition, the number of cycles to failure was recorded through Wohler curves, and it was verified that the circular NiTi SMA wires has a lifetime longer than the ones with square section. It was also noted that the performance characteristics of the two NiTi SMA wires depend directly on the applied tensile stress and test frequency. It was observed that the dissipated energy (ED) decreases with the increasing peak stress and loading frequency, while the cumulative strain (ep), superelastic strain (SSE), stresses and transformation temperatures increases with increasing tensile load and frequency.