Estudo do comportamento de fios de liga NiTi com memória de forma em regime superelástico: avaliação de fadigas funcional e estrutural.

Abstract

Shape memory alloys (SMA) working in a superelastic regime have been applied in the fields of medicine, dentistry and engineering, and due to the nature of their applications, the response to functional and structural fatigue of these materials when subjected to cyclic demands is of utmost importance for safety in their application. In this sense, the objective of this work is to evaluate the fatigue behavior of NiTi superelastic SMA wires of different diameters (0.3 mm, 0.5 mm, 0.7 mm and 0.9 mm) submitted to mechanical tensile tests stress control. Experiments were performed to determine the functional and structural fatigue properties of these SMA wires, varying the tensile stress and the loading frequency. A Factorial Experimental Planning was applied to the study, and mathematical models that describe the fatigue behavior of these SMA wires were developed. The dissipated energy, the accumulated deformation, the superelastic deformation, and the damping factor were evaluated as a function of the intensity of the mechanical stress and the frequency of loading applied in the cyclic tests, emphasizing the self-heating phenomenon that occurs in these alloys, being higher in the higher frequencies. The number of cycles until rupture was recorded by means of the S-Nf curves. It has been found that the functional characteristics of the wires depend directly on the tensile stress and the test frequency, that the dissipated energy and superelastic deformation decrease with increasing stress peaks and loading frequency, and that the accumulated deformation increases with increasing loading stressess and lower test frequency. In the structural fatigue, for the highest values of tensile stress and loading frequency, the lowest wire life was recorded (number of cycles up to rupture).