Functional and Structural Fatigue of a Pseudoelastic NiTi Alloy within the Stress-Induced Transformation Regime

Tuesday, May 21, 2013
OREA Pryamida Hotel
Dr. Carmine Maletta , University of Calabria, Arcavacata Rende (CS), Italy
Mr. Emanuele Sgambitterra , University of Calabria, Arcavacata Rende (CS), Italy
Prof. Franco Furgiuele , University of Calabria, Arcavacata Rende (CS), Italy
Dr. Riccardo Casati , National Research Council - Institute for Energetics and Interphases (CNR-IENI), Lecco, Italy
Dr. Ausonio Tuissi , CNR IENI Istituto per l'Energetica e le Interfasi, Lecco, Italy
The fatigue properties of a pseudoelastic Ni-rich Nickel-Titanium Alloy within the stress-induced transformation regime have been analysed in this investigation. In particular, strain controlled fatigue tests have been carried, with maximum deformation within the stress-strain transformation plateau of the alloy, by using flat dog-bone shaped specimens obtained from as-received NiTi sheets by electro discharge machining. Due to the evolution of the stress-strain response of the alloy in the first mechanical cycles, the tests have been executed in two subsequent steps: i) material stabilization and ii) fatigue life estimation.

The first step was carried up to a stable mechanical response of the alloy and a variable strain ratio was adopted, in order to avoid compression stresses during unloading, due to the accumulation of residual deformations. The evolution of several functional damage parameters have been analyzed, such as the residual and recovery deformations, the dissipated and recovery energies, the direct transformation stresses and the Young’s modulus. The results revealed marked evolutions of these parameters but stabilization always occurs between 100 and 200 mechanical cycles. Subsequently, the stabilized specimens have been subjected to strain controlled fatigue tests, under a fixed strain ratio, up to complete failure.

The strain-life data have been analyzed within the framework of a recent phenomenological model, based on a modified Coffin-Manson approach [1]. Finally, the fracture surfaces have been analysed by scanning electron microscopy (SEM) in order to analyze the stable and unstable crack growth mechanisms.

[1] C. Maletta, E. Sgambitterra, F. Furgiuele, R. Casati2 and A. Tuissi, Fatigue of pseudoelastic NiTi within the stress-induced transformation regime: a modified Coffin–Manson approach, Smart Mater. Struct. 21 112001

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