Experimental characterization of Ni-Ti Shape Memory alloy wires under uniaxial loading conditions

Shape Memory Alloys (SMAs) have shown promise as high-force, high-displacement actuators. Critical issues such as path-dependence, predictability and sensitivity to testing conditions, however, need to be addressed in order to design controllable actuators using SMAs. This paper presents research aimed at addressing design issues involved in the application of SMAs, particularly as actuators. Quasistatic experiments at constant stress, strain and temperature are consolidated on a critical stress-temperature diagram to delineate the regions of stability of the various phases of the material. The critical points from these quasistatic tests are found to be in excellent agreement with each other, and correlate relatively well with the constitutive models for SMA thermomechanical behavior. It is also observed that the stress-strain behavior of the material is a function of the imposed strain rate on the material. However, once the strain rate drops to a quasistatic value, the material relaxes to the quasistatic stress state, irrespective of the transient state it was in. This shows that quasistatic predictions are sufficient to predict the behavior of an SMA actuator undergoing only temporary loading at high strain rates. When the material undergoes continuous loading at higher strain rates, however, the strain rate effects become quite significant. The strain rate dependency is likely to be decisive in determining the dynamic behavior of the material in applications involving cyclic loading and unloading at higher strain rates with no quasistatic period in the interim.It is argued that modeling approaches incorporating strain rate effects are required for design of repeatable actuators only in some applications.