Modeling superelastic behavior of shape memory alloys (SMA) has received considerable attention due to SMAs ability to recover large strains with associated loading{unloading hysteresis enabling them to find many applications. In this work, a simple mechanics of materials modeling approach for simulating superelastic responses of SMA components under tension and bending loading conditions is developed. Following Doraiswamy, Rao and Srinivasa's1 approach, the key idea here would be in separating the thermoelastic and the dissipative part of the hysteretic response with a Gibbs potential based formulation which includes both thermal and mechanical loading in the same framework. The dissipative part is then handled by a discrete Preisach model. The model is formulated directly using tensile stress{strain or bending moment{curvature rather than solving for non-homogeneous stress and strains across the specimen cross-sections and then integrating the same especially for bending loading conditions. The model is capable of simulating complex superelastic responses with multiple internal loops and provides an improved treatment for temperature dependence associated with superelastic responses. The model results are verified with experimental results on SMA components like wires and beams at different temperatures.
The issue of material performance over its designed life is of prime concern with designers lately due to increasing use of shape memory alloy (SMA) components in different engineering applications. In this work, a concept of "Driving force amplitude v/s no of cycles" is proposed to analyze functional degradation of SMA components under torsion. The model is formulated using experimentally measurable quantities such as torque and angle of twist with the inclusion of both mechanical and thermal loading in the same framework. Such an approach can potentially substitute the traditional fatigue theories like S-N, epsilon-N theories which primarily use mechanical loading effects with temperature being an external control parameter. Such traditional S-N, epsilon-N fatigue theories work well for capturing superelastic effects at a given temperature but not for shape memory effects or temperature dependent superelastic effects which involves mechanical and thermal coupling. Experiments on SMA extension springs are performed using a custom designed thermomechanical test rig capable of mimicking shape memory effect on thermally activated SMA springs held under constant deformation. For every thermomechanical cycle, load and temperature sensor readings are continually recorded as a function of time using LabVIEW software. The sensor data over the specimen lifetime is used to construct a "Driving force amplitude v/s no of cycles" relationship that can be used as a guideline for analyzing functional degradation of SMA components.
A Multifunctional smart material system consists of two or more different smart material phases in the form
of a hybrid system, in which every phase performs a different but necessary function. In this work, we show
how thermally responsive Shape memory alloys (SMA) and Shape Memory Polymers (SMP) can be combined to
form a Multifunctional Smart Material system (MSMS). The transformation temperatures Mf, Ms, As and Af
of SMA and the glass transition Tg for the SMP play a critical role in designing such a MSMS. We illustrate how
varying the Tg of SMP between the transformation temperatures Mf and Af of SMA results in a multi-state
smart bias system with varying stiffnesses. In addition, we establish guidelines for the volume fractions of the
individual constituents of such MSMSs to form "smart-bias" tools/devices. We further propose various ideas for
smart devices that can operate through three temperature ranges, with one material constituent being passive
and the other active at a given temperature.
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