Shape memory alloy (SMA) can exhibit interesting features such as the diverse material behaviors according to the induced temperature and stress. SMA changes its material properties progressively under cyclic loading conditions and finally reaches stable path(state) after a certain number of stress/temperature loading-unloading cycles, so called 'training' completion. The presence of permanent deformation, due to plastic strains or irreversible martensite variants during the material training, shifts the material characteristic curves of SMA wire. In this study, SMA wires that have been in a stable state through the training are used. Stress-strain curve of SMA wire at different temperature levels are measured. In addition, we observe other important effects such as the effect of mechanical/thermal training, rate effect according to thermal cycle times or strain rates, etc. Until now, the rate effect is not considered significantly in the SMA research and only extremely slow time rate is considered in most SMA experiments. It is common to use rate independent constitutive relations in the modeling and simulation of SMA behaviors. Therefore to make the actuators using an SMA wire which has the fast response or short-time thermal cycle environment, rate dependency should be properly considered. The result of two-way experiment at each (short or long) cycle time in phenomenological aspect shows that stress-strain-temperature relations and hysteresis characteristics depend upon the cycle time. In short-time cycle, strain-temperature curve moves in counterclockwise and the size of hysteresis envelop is large. As the time rate of the thermal cycle increases, the size of the hysteresis envelop is getting smaller and strain-temperature curve moves along the clockwise direction above a certain thermal cycle time. Above that thermal cycle time, hysteresis trajectory is fixed in the stable state. These new effects of SMA are investigated and the effect would be explained qualitatively. The present work presents the experimental test using 1-D SMA wire after training completion by mechanical/thermal cycling. Through these tests, we measure the characteristics of SMA. With the estimated SMA properties and effects, we compare the experimental results with the simulation results based on the SMA constitutive equation including the training and thermal rate effect.
A structure using the two-way shape memory effect (TWSME) returns to its initial shape by increasing or decreasing temperature under initial residual stress. Through the thermo-mechanical constitutive equation of shape memory alloy (SMA) proposed by Lagoudas et al., we simulate the behavior of a double actuator in which two SMA wires are attached to the tip of bar under the initially given residual stress. Through the numerical results conducted in the present study, the proposed actuator device is suitable for repeated actuation. The simulation algorithm proposed in the present study can be applied extensively to the analysis of the assembled system of SMA-actuator and host structure in practical applications.
One way Shape Memory Effect (SME) is not suitable mechanism for application to the repeated actuation of an Shape Memory Alloy(SMA) wire because the host structure does not return to its initial shape after it cools down. In the present study, the two-way SME under residual stress is considered. A structure using the two-way effect returns to its initial shape by increasing or decreasing temperature under an initially given residual stress. A thermo-mechanical constitutive equation of SMA proposed by Lagoudas et al. was employed in the present study. Laminated composite beams and plates are considered as simple morphing structural components. The modeling of beams and plates are based on first-order shear deformable laminated composite beam and plate theories with large deflections. Numerical results of fully coupled SMA-composite structures are presented. The proposed actuation mechanism based on the two-way SMA effect and a simulation algorithm can be used as a powerful morphing mechanism and simulation tool for structures.