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The characterization of thermal-induced minor hysteresis loops of shape memory alloys using DSC techniques
Guanghan Wang, Xin Xiang Jiang and Darius Nikanpour
Shape memory alloys are increasingly used under complex cyclic actuation situations, which may involve either the complete (major) or partial (minor) phase transformations loops. Due to the strong-dependency of the thermal and mechanical properties on the partial fraction of co-existing phases, namely the Martensite and Austenite, and the thermodynamic state of phase transformation, the physics of minor loops are more difficult to be described than those major loops. Furthermore, they cannot be easily predicted by computational modeling techniques. The phase transformations of SMA are processes of energy transfers, from thermodynamic potential to other forms. The thermophysical properties can therefore denote the thermodynamic state of the materials. The variations of those properties can, therefore, denote the changes of energy states, and thus can be used to describe the phase transformation processes. The data of the process-dependent thermophysical properties can hence be one of the resources needed in order to understand the thermo-physical processes and to reliably predict the phenomena using computational modeling techniques.
This article will examine different types of minor hysteresis loops with respect to the thermodynamic states. Measurement results will be presented to explore the characteristics of thermal energy transfer of minor loops for a SMA material during a variety of different major and minor cycling processes. The experiments are conducted using Differential Scanning Calorimetry (DSC) technique.
Keywords: Shape memory alloy, phase transformation, minor loops, energy transfer, DSC, NiTinol.