|Ph.D Student||Vollach Shahaf|
|Subject||The Mechanical Response and Phase Transformation Kinetics|
of NiTi under a Rapid Heating Pulse
|Department||Department of Mechanical Engineering||Supervisor||Professor Doron Shilo|
|Full Thesis text|
Shape Memory Alloys (SMA) is a group of smart materials that “remembers” their original shape and recovers it when sufficiently heated. The shape memory effect (SME) is based on solid-solid martensitic phase transformation typically accompanied by considerable stress and strain. In this study we explore a new mode of SMA activation, in which a thin SMA wire is superheated abruptly by an electric pulse of few microseconds. This activation mode opened the way for a new type of SMA actuators that are especially advantageous in one-shot applications.
We developed an original novel experimental system for exploring the thermo-mechanical response of shape memory wires under a rapid heating pulse. The new setup enables the force and displacement generated by the phase transforming wire to be measured with a μs time resolution. In addition, part of the tests incorporated high-speed infrared and visible light photography. The experimental system enables exploring several unique dynamic effects. In particular, stress levels of 1.6 GPa with negligible plastic deformation and elastic strain rates of 103s-1 are observed. The high strain rate, generated by the phase transformation, induces string-like vibrations of the SMA wire that result in strong stress vibrations.
The true kinetics of the reverse martensitic transformation in shape memory alloy wires is studied under conditions at which it is restricted neither by the rate of heat transfer nor by mechanical inertia. Two characteristic times for the transformation are identified and estimated. A model provides a universal expression that fits all experimental measurements performed at different temperatures. The kinetic law predicted by the model indicates that interface velocities are governed by viscous resistance and are thus much slower than the shear wave speed, even under very large driving force values.
The experimental results revealed the existence of a critical plateau stress that determines the performances of high rate SMA actuators. We investigated the effects of temperature and initial stress on the plateau stress. Calculations based on the integration of the Clausius-Clapeyron equation while considering the inhomogeneity of the transformation temperature were in good agreement with the measured data. Our results indicate that the plateau stress represents equilibrium conditions and that actuation performances can be enhanced significantly by increasing the initial stress.
Finally, we provide guidelines for several key rapid actuation properties. By correlating the input heating energy with the output kinetic energy we discovered the optimal operation conditions. A predictive model for the duration of the actuation used a simplified stress-strain model and presented an excellent fit to measured data. An experiment where the same wire was subjected to many consecutive rapid actuations cycles, found no sign of performance degradation and no significant change in the thermo-mechanical behavior.
The experimental system, techniques, calculations, models and findings suggested and developed through the framework of this research are already routinely used in several other research programs.