Thermomechanical Experiments and Modeling of Shape Memory Alloy Tension Springs; Critical and Liberative Analyses of Engineering Educational Systems

Abstract

This thesis contains two areas of research. The first part contributes to the field of engineering mechanics. This research investigates the thermomechanical shape memory behavior of shape memory alloy (SMA) springs. As SMA springs undergo stress- and/or temperature-induced phase transformation, they experience varying combinations of axial, bending, and torsional stresses. A deeper understanding of the complex, three-dimensional state of stress within SMA springs, in combination with models of their actuation, will enable expanded application of SMA springs as actuators in a variety of industries. As such, the overall objective of this work is to comprehensively understand and model the thermomechanical behavior of SMA springs.

Experiments are performed on commercially-available SMA tension springs in order to explore the force-stretch-temperature design space. Two primary types of experiments are performed, isothermal loading and constant-load thermal actuation, on both virgin and cycled specimens. Experimental results are used to derive thermomechanical models that both describe the homogenized uniaxial behavior and the local, multi-axial distributions of stress and strain in the wire cross-section. Experimental studies of repeated thermal cycling are used to derive models both for virgin SMA springs and for their use in applications that require repetitive actuation.

Over the course of this work, detailed experiments enhance the understanding of the fundamental mechanics behind the actuation of SMA springs as well as the crystallographic phases through which the material passes. Additionally, thermomechanical models of SMA spring actuators enable expanded applications in aerospace, biomedical, and other industries.

The second part of this thesis addresses issues of equity within engineering educational systems. Higher educational programs in engineering today are seeking to correct disproportionately low enrollment and success rates of minoritized students. However, most diversity-related programming fails to address systems of structural oppression that cause particular students to be underrepresented in higher education. In this work, critical and liberative theories are applied to engineering educational systems to re-frame the problems with and goals of diversity, equity, and inclusion within engineering education from the lenses of Freirean critical theory, which is class-based, and other anti-oppressive theories based on race, gender, and sexual orientation.

This educational research has theoretical, quantitative, and qualitative components. To contribute to existing critical and liberative theories, two new models are proposed, one situating these theories relative to one another within the broader classification of identity-based theories, and another that provides example of the application of these theories to engineering systems. In a quantitative study, the disproportionately low outcomes of marginalized undergraduate students studying engineering at a highly selective public university are examined. Qualitative analyses of multiply marginalized students at this university determine some of the mechanisms through which these outcomes are occurring.

Societal structures, such as sexism, racism, and capitalism, have filtering effects that impact the opportunities available to individuals with marginalized identities throughout their lives. In order to correct inequities in engineering education, the role of current educational systems in the perpetuation of systems of oppression must be addressed. As incremental interventions fail to address the roots of these problems, they cannot remove the obstacles facing marginalized students. The true solution lies in the collective liberation of all marginalized people from oppressive social structures. This work thus examines the role engineering education can and must play in the liberation of humanity from structural oppression.