Date of Award

Summer 2023

Document Type


Degree Name

Doctor of Philosophy (PhD)


Mechanical Engineering

First Advisor

Moore, John A.

Second Advisor

Erdeniz, Dinc

Third Advisor

Sen, Andrew


Nitinol is an alloy of nickel and titanium, which exhibits outstanding functional properties, such as shape memory and superelastic behavior. Superelastic nitinol wires exhibit recoverable strains that are significantly greater than traditional alloys. In most of its applications, nitinol is exposed to cyclic loads, which results in functional and/or structural fatigue that ultimately leads to failure. The classical fatigue theories do not appropriately address the fatigue performance of superelastic nitinol due to the complex nature of the martensitic transformation. It has been shown that the main fatigue crack initiation sites, other than surfaces, are microstructural inhomogeneities such as voids and non-metallic inclusions. The effect of wire size on fatigue performance of superelastic nitinol is not clearly understood. In this research, nitinol wires with three different sizes are subjected to microstructural analysis and low-cycle fatigue tests to understand the effect of wire size on their phase transformations and fatigue life. For this purpose, advanced non-destructive characterization techniques such as X-ray microtomography (μCT) and far-field High Energy Diffraction Microscopy (ff-HEDM) were utilized. This allows for the acquisition of a comprehensive 3-D map of the distribution of microstructural defects within a material, determination of the crystallographic orientation of the material surrounding these defects, as well as the lattice strain of the grains before and during fatigue testing.This study indicated that the superelastic properties and fatigue response of the nitinol wires is strongly influenced by their size and microstructure. Comparing the fatigue data of the wires and analyzing the macroscopic scale results of the 3-D Digital Image Correlation (DIC) technique revealed that the smaller wires exhibited a better functional performance, although they were more vulnerable to surface defects. Conversely, larger wires experienced a more significant microstructural damage during cycling, which was linked to their cooling rate effects. Overall, the results of this research suggest that wire size and microstructure are crucial factors that must be carefully controlled in the design and development of nitinol-based devices.

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