Thermomechanical training and characterization of shape memory alloy axial actuators
Becker, Marcus Patrick
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Although considerable work has been performed to understand the key mechanisms of Shape Memory Alloy (SMA) behavior, little of this work follows a standard testing protocol, quantifies a conditioning methodology, or develops data appropriate for design of SMA actuators. One major issue that limits the ability of the material from being used directly as an actuator is the large, non-recoverable strains likely to accrue in the material during each training cycle, mechanical or thermal. When mechanical or thermal cycling is performed, a hysteresis curve develops and reaches a steady state strain recovery response. At the point where permanent plastic strain stops growing, or saturates, the SMA has been successfully trained. The focus of this work is oriented toward SMAs in general, but all testing and experimentation was carried out on Nickel-Titanium (NiTi) alloys. The experimentation and testing was performed on a combination of 4 different sizes and 3 different NiTi alloy compositions. Thermomechanical testing was performed to determine critical values to describe the stress-temperature phase space of the materials and parameters to model the applied stress and transformation strain relationship. All material size and alloy combinations were tested in the as-received, or as-machined, and fully annealed state. The results of the training and actuation strain characterization process developed in this work shows that the samples that experienced Transformation Induced Plasticity (TRIP), greater than 2% during the training process and exhibit Two-Way Shape Memory (TWSM) after being fully trained, share a very similar applied stress versus transformation strain curve. This curve is modeled by the Back Stress formulation derived from the Gibbs Free Energy constitutive model by Bo & Lagoudas. The design space created by the Back Stress formulation, recrystallization temperature, and training stress allows SMA materials to be characterized and implemented as stable 1-D actuators. This research formalized a thermomechanical training and characterization method for uniaxial SMA actuators by addressing the interaction between processing, recoverable and non-recoverable deformation. Using various sizes and NiTi alloy combinations, this research develops and evaluates a method to train and characterize a diverse range of SMAs through a set of thermomechanical and physical property measurements.