CURVAS DE VIDA EM FADIGA DE BAIXO CICLO DE LIGAS SUPERELÁSTICAS DE NiTi
LOW CYCLE FATIGUE LIFE CURVES OF NiTi SUPERELELASTIC ALLOYS
Figueiredo, Ana Maria G.; Modenesi, Paulo José; Buono, Vicente Tadeu L.
http://dx.doi.org/10.4322/tmm.00302011
Tecnol. Metal. Mater., vol.3, n2, p.55-59, 2006
Resumo
Este trabalho apresenta um estudo da vida em fadiga de baixo ciclo controlada por deformação (curvas ea-Nf) de fios de NiTi rompidos em ensaios de flexão rotativa. Foram ensaiados fios de NiTi superelástico, austenítico estável, bifásico e martensítico estável sob amplitudes de deformação variando de 0,6% a 12%, além de um fio de aço inoxidável austenítico sob amplitudes de deformação de 0,4% a 1,1%. As curvas ea-Nf foram comparadas entre si e com as encontradas na literatura. Os valores de vida em fadiga dos fios de NiTi são superiores aos do aço inoxidável austenítico em todas as condições estudadas. As curvas ea-Nf dos fios de NiTi bifásico e superelástico, sob amplitude de deformação inferiores a 4%, são coerentes com as da literatura e próximas à curva do fio austenítico estável. Sob amplitudes de deformação maiores, a vida em fadiga passa a crescer com a deformação, até que seja atingida a região próxima à curva do fio martensítico estável. Esse comportamento incomum resulta numa curva em forma de "Z". Tais resultados estão possivelmente vinculados à inibição da transformação martensítica na ponta da trinca, devida à redução de volume que a acompanha, até que ocorra transformação generalizada do material.
Palavras-chave
Ligas de NiTi, Superelasticidade, Vida em fadiga
Abstract
This paper presents an analysis of low cycle fatigue life under strain control (ea-Nf curve) of NiTi wires submitted to bending-rotation fatigue. Fatigue tests were carried out on stable austenitic, superelastic, biphasic and stable martensitic NiTi wires, with strain amplitudes from 0.6% to 12%. An austenitic stainless steel wire was also tested for comparison, with strain amplitudes from 0.4% to 1.1%. The resulting ea-Nf curves together with data from the literature are compared. Fatigue life of the NiTi wires were always longer than that of the austenitic stainless steel in all investigated conditions. For strain amplitudes up to 4%, ea-Nf curves for biphasic and superelastic wires are consistent with those values reported in the literature, closely approaching the stable austenitic wire’s curve. For higher strain amplitudes, it is found that fatigue life of superelastic and biphasic wires increases with strain until they approach the fatigue life curve of stable martensite wire. This unusual behavior results in a "Z-shaped" curve for high strain values. It is possibly linked to the inhibition of martensitic transformation ahead of the crack tip, caused by the volume reduction inherent to that phase transformation, until this transformation occurs all over the material.
Keywords
NiTi alloys, Superelasticity, Fatigue life
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