Experimental and Numerical Investigation of the Fracture Toughness of Shape Memory Alloys



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Shape Memory Alloys (SMAs) are intermetallics with unique properties emanating from a reversible diussionless solid to solid phase transformation from a stable at high temperatures phase (austenite) to a stable at low temperatures phase (martensite). Attributed to their e cacy to generate large recoverable strains by undergoing thermomecanically-induced phase transformation, SMAs emerge as leading preference for various applications in aerospace, outer space, transportation, construction, and biomedical industry. To ensure ascendancy of the SMAs for such applications, understanding their failure behavior and standardization of fracture toughness measurements are imperative in the realm of fracture mechanics methods to provide structural integrity assessment, damage tolerance design, performance evaluation, quality assurance, among others. The fracture response of SMAs is rather complex due to (re)orientation of martensite variants, reversibility of phase transformation, transformation-induced plasticity, latent heat e ects, and possible co-existence of cleavage fracture and ductile tearing. SMAs display stable crack growth, a fracture toughening response attributed to energy dissipation due to phase transformation occurring close to the crack tip, under nominally isothermal conditions, similarly to other dissipative material systems, as well as under \actuation" loading such as isobaric thermal variations. The aim of this dissertation is (i) to propose necessary modifications to the existing ASTM standards, which have been developed for conventional structural metals, for the experimental measurement of fracture toughness of SMAs; (ii) experimentally examine the effect of the reversibility of phase transformation on the transformation-induced fracture toughness enhancement; (iii) propose a path-independent contour integral for describing the driving force for crack growth in SMAs under thermomechanical (actuation) loading paths, which collapses to the J-integral under nominally-isothermal conditions, and experimentally measure the fracture toughness under isobaric conditions; and (iv) investigate void growth and coalescence in precipitation-hardened SMAs by unit cell simulations and, by comparison to the available experimental data, draw conclusions on their importance on the fracture response of these materials.



Shape memory alloys, fracture toughness, void growth and coalescence, isobaric actuation, NiTi,


Portions of this document appear in: Makkar, Jasdeep, and Theocharis Baxevanis. "Notes on the experimental measurement of fracture toughness of shape memory alloys." Journal of Intelligent Material Systems and Structures 31, no. 3 (2020): 475-483; and in: Makkar, J., B. Young, I. Karaman, and T. Baxevanis. "Experimental observations of “reversible” transformation toughening." Scripta Materialia 191 (2021): 81-85; and in: Makkar, J., B. Young, I. Karaman, and T. Baxevanis. "Fracture resistance of shape memory alloys under thermomechanical loading." Engineering Fracture Mechanics 258 (2021): 108059; and in: Makkar, J., and T. Baxevanis. "On the fracture response of shape memory alloys by void growth and coalescence." Mechanics of Materials 153 (2021): 103682.