An Experimental Setup for Measuring Unstable Thermo–mechanical Behavior of a Shape Memory Alloy Wire

An experimental arrangement is demonstrated that overcomes some difficulties in thermo–mechanical testing of thin Shape Memory Alloy (SMA) wires under uniaxial tension. It is now well known that stress–induced transformations in some SMAs under uniaxial loading can lead to mechanical instabilities and propagating phase transformation fronts. Critical parameters, such as nucleation barriers are difficult to measure by conventional testing techniques and are often masked by unavoidable stress concentrations at grips. In addition, simultaneous full field measurements of localized deformation and temperature fields are difficult to obtain for different ambient conditions. The current scheme uses a temperature–controlled conduction block and a non–uniform temperature field induced by thermoelectric modules to uncover the underlying thermo–mechanical response of the wire. The approach also allows access for optical and infrared imaging of the specimen deformation and temperature fields. INTRODUCTION Shape Memory Alloys (SMAs), such as NiTi, exhibit two remarkable properties, the shape memory effect and pseudoelasticity (see Fig. 1). The shape memory effect is the material’s ability to erase large mechanically-induced strains (up to 8% ) by moderate increases in temperature (≈10-20oC). Pseudoelasticity refers to the ability of the material in a somewhat higher temperature regime to accommodate strains of this magnitude ∗Address all correspondence to this author. during loading and then recover upon unloading (via a hysteresis loop). The underlying mechanism is a reversible martensitic transformation between solid-state phases, often occurring near room temperature. The transformation can be induced by changes in temperature or by changes in stress due to the strong thermo-mechanical coupling in the material behavior. NiTi’s remarkable behavior arises from the interplay of two phases, a high temperature phase (austenite), having a cubic lattice structure, and a low temperature phase (martensite), having a monoclinic structure (Otsuka et al., 1971). Due to its low degree of symmetry, the martensite phase exists either as a randomly twinned structure (low temperature, low stress state) or a stress-induced detwinned structure that can accommodate relatively large, reversible strains. These properties can be exploited to generate large stresses and deformations compared to other so–called “smart” materials, making it a promising candidate for novel structural applications (see Funakubo (Ed.) (1987), Duerig et al. (1990) and Otsuka and Wayman (Ed.) (1998)). It is now well known that unstable mechanical behavior can occur during stress–induced transformation in uniaxial loaded SMAs (see Shaw and Kyriakides (1995), Liu et al. (1998)), thereby causing an extreme sensitivity of the material response to the ambient environment and loading rate. In particular, the transformation from austenite (A) to martensite (M) and back again during the pseudoelastic response of virgin polycrystalline NiTi occurs through the nucleation and propagation of phase transformation fronts. These events lead to distinctly non–uniform deformation and temperature fields. Local to each transformation front is the generation or absorption of latent heat 1 Copyright  2000 by ASME