Date of Award

7-17-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Systems Engineering

First Advisor

Abhijit Bhattacharyya

Abstract

Interruption of mechanical loading of Shape Memory Alloys (SMAs) exhibits a creep-like phenomenon that is manifested as positive accumulation or negative accumulation in strain (i.e. strain recovery) when stress is held constant during loading or unloading, respectively. Conversely, this effect is also found as stress relaxation/stiffening during the loading/unloading when the strain is arrested. An apparently similar phenomenon, reported in literature by many researchers as caused by thermomechanical coupling, was found to exist in shape memory alloy materials during pseudoelastic deformation as well as shape memory effect. However, experiments conducted in our laboratory on the material indicated that the phenomenon is demonstrated even during reorientation loading, where thermomechanical coupling is absent. Stress-strain tests for pseudoelastic deformation of shape memory wires, at strain rates that can be considered quasistatic and for which the effect of thermo-mechanical coupling is found to be negligible, also revealed the presence of pseudocreep. Our hypothesis regarding this phenomenon is that apart from the thermomechanical coupling, there is an evolution of the microstructure with the movement of phase fronts/interfaces, and that the load needed to sustain a phase transformation may be different than the load needed to maintain a certain state of the microstructure, even under isothermal conditions. Since the load required in maintaining a microstructure can be realized only under rest, the stress seems to settles its value during isothermal deformation to its value at rest, when the loading is interrupted at constant strain. We refer to this phenomenon as pseudocreep. If this indeed is true, its implications can be many. Firstly, this delineates regions in stress-temperature phase space where different phases are stable. Secondly, the rest stress can be used as a basis on which other effects can be built upon to develop a more accurate model as well as to obtain accurate calculations for energy utilization when the SMA is incorporated as the active element in an actuator. Using a non-contact multi-videoextensometric technique for determining strains in segments of a wire, we have detected change in strain, during stress arrest, in regions of the wire suggesting phase front movement during pseudocreep. Experiments on pseudocreep during strain and stress arrest of isothermal pseudoelasticity and reorientation are reported for Nickel-Titanium shape memory alloy wires. Motivated by an overstress model reported in the literature for plasticity, we propose a simple thermomechanical model of pseudocreep in shape memory alloys.

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