EXPERIMENTAL AND COMPUTATIONAL METHODS FOR SHAPE MEMORY ALLOYS

Abstract eng:
The unique properties of Shape Memory Alloys (SMA) have drawn the interest and attention of both engineers and scientists alike. The mechanisms that govern these behaviors are still a subject of research and there is a need for the development of reliable constitutive relationships. This work focuses its attention on one such interesting material viz. TiNi that is enjoying the widest application due to its superior memory and structural properties. The first step in developing a constitutive model is to understand the behavior of these materials under combined thermal and mechanical loading cycles. A series of tests were designed and conducted. The testing methodology and specimen preparation for the High Resolution Electron Microscopy is discussed. Finally we present comparisons between our experimental results and those conducted independently by other research groups. A general inelastic framework for the derivation of general three-dimensional thermomechanical constitutive laws for materials undergoing phase transformation is used. The framework is the result of combination of elements from generalized plasticity theory and the theory of continuum damage mechanics. This formulation is capable of accommodating the multiple and interacting loading mechanisms, which is an integral part of the modeling of materials with phase transformations like SMA. This is then used to develop an internal variable thermomechanical constitutive model for SMA materials. The model's predictive capabilities in describing important material characteristics, associated with internal phase transformations are assessed. These include shape memory effect, stress-strain-temperature relations for pseudoelasticity under monotonic and cyclic loading, and energy dissipation of the material that governs its damping behavior.

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