Stress state effect on mechanical behaviour of shape memory alloys : Experimental characterisation and modelling

Abstract

Abstract. As engineering components utilising the beneficial properties of shape memory alloys (SMA) become geometrically complex and as applications involve combinations of loading states, a full evaluation of the effects of stress state on stress-strain response of these materials becomes critical for the success of these applications. Such evaluation is required to establish reliable constitutive relationship to model the complex thermomechanical behaviour of SMAs. This paper is intended to present a non-exhaustive overview on studies and results dealing with experimental characterisation and modelling of the effects of stress state on mechanical behaviour of shape memory alloys. In that respect, our presentation is mainly focused on one hand on experimental results on superelastic and ferroelastic deformation of polycrystalline alloys and on the other hand on the modelling at the macroscopic level, yielding critical reorientation or transformation constitutive equations. INTRODUCTIONShape memory alloys (SMAs) are known to exhibit a range of novel thermomechanical properties due to thermoelastic martensitic transformations. The superelasticity is a quasi-elastic deformation far beyond the conventional elastic limit of the material when deformed at certain temperature above A p the finishing temperature of the reverse martensite-to-austenite transformation. This phenomenon is associated with stress-induced martensitic (SIM) transformation. If the temperature is lowered to below M p the finishing temperature of the forward austenite-to-martensite transformation, the deformation proceeds by martensite reorientation. Martensite may deformed by reorientation forth and back with alternating stresses in opposite directions. This deformation mode is known as the ferroelasticity, in recognition of its phenomenological similarity to ferromagnetism. The one-way shape memory effect is observed when a specimen deformed by martensite reorientation reverts to its original shape upon heating to above A f Numerous experimental works have been performed in the past three decades aiming at characterising the thermo-mechanical behaviour in order to provide experimental evidences for the establishment of constitutive models. These experimental studies have been devoted to both single-crystal and polycrystal specimens, mainly using Cu-based and NiTi SMAs. Most mechanical testing has been carried out in tension using wire specimens. Less work has been done to characterise properties under other stress states. As engineering components become geometrically complex and as applications involve combinations of loading states, it becomes important to characterise material properties in different stress states. This will allow the establishment of reliable constitutive equations to model the complex thermomechanical behaviour of SMAs.The present paper is intended to give a non-exhaustive overview on the studies and results dealing with experimental characterisation and modelling of the influence of stress state on mechanical behaviour of...

Heat Source Processing for Localized Deformation with Non-Constant Thermal Conductivity. Application to Superelastic Tensile Tests of NiTi Shape Memory Alloys

“…NiTi SMAs have been extensively used in many engineering applications, especially in biomedical products (including stents and catheters) where the superelasticity of these alloys is required [7]. In spite of the ever-increasing interest in SMA modeling, there are still many unsolved problems with the predictability of the thermomechanical responses of this material [8]. One of the main limitations is the deformation localization that appears systematically during the stress plateau in tensile tests [9][10][11][12][13][14][15][16].…”