Issue 29

A Fortini et alii, Frattura ed Integrità Strutturale, 29 (2014) 74-84; DOI: 10.3221/IGF-ESIS.29.08 74 Focussed on: Computational Mechanics and Mechanics of Materials in Italy TWSME of a NiTi strip in free bending conditions: experimental and theoretical approach A. Fortini, M. Merlin, R. Rizzoni Department of Engineering (EnDiF), University of Ferrara, (Italy) annalisa.fortini@unife.it, mattia.merlin@unife.it , raffaella.rizzoni@unife.it S. Marfia Department of Civil and Mechanical Engineering (DiCeM), University of Cassino and Lazio Meridionale, (Italy) marfia@unicas.it A BSTRACT . This paper deals with the two-way shape memory effect (TWSME) induced on a strip of a near- equiatomic NiTi alloy by means of the shape memory cycling training method. This procedure is based on the deformation in martensite state to reach the desired cold shape followed by cycling the temperature from above A f to below M f . To this end, the sample was thermally treated to memorise a bent shape, thermomechanical trained as described and thermally cycled in unloaded conditions in order to study the stability of the induced TWSME. Heating to A f was reached by a hot air stream flow whereas cooling to M f was achieved through natural convection. The evolution of the curvature with the increasing number of cycles was evaluated. The thermomechanical behaviour of the strip undergoing uniform bending was simulated using a one-dimensional phenomenological model based on stress and the temperature as external control variables. Both martensite and austenite volume fractions were chosen as internal parameters and kinetic laws were used in order to describe their evolution during phase transformations. The experimental findings are compared with the model simulation and a numerical prediction based on the approach proposed in [25]. K EYWORDS . NiTi-based alloys; two-way shape memory effect; thermomechanical training; bending. I NTRODUCTION hape memory alloys (SMAs) are an interesting class of metallic materials with the ability of recovering seemingly permanent deformation when they are deformed in the martensitic low temperature phase and subsequently heated to the austenitic high temperature phase. Shape memory effect (SME) and superelasticity (SE) are associated with diffusionless solid-state transformations between two crystallographic phases: the cubic crystal structure, stable at high temperature, and the monoclinic crystal structure, stable at low temperature. The forward transformation from austenite to martensite occurs during cooling, begins at the martensitic start temperature M s and ends at the martensitic finish temperature M f . The reverse transformation from martensite to austenite occurs upon heating, begins at the austenitic start temperature A s and finishes at the austenitic finish temperature A f . Over the last decades a wide variety of SMAs have been investigated and several compositions have been studied, by adding different alloying elements (such as zinc, copper, gold and iron) to existing alloys. Among the different shape memory alloys compositions, the NiTi alloy system is the most extensively studied and used in the greatest number of commercial applications mainly because of its excellent mechanical properties, high corrosion resistance and S