{"id":{"repo_id":"texas","oai_identifier":"oai:repositories.lib.utexas.edu:2152/131108"},"canonical_url":"https://search.dev.ndltd.org/etd/texas/oai:repositories.lib.utexas.edu:2152/131108","repository":{"repo_id":"texas","name":"University of Texas","base_url":"https://repositories.lib.utexas.edu/server/oai/request"},"display":{"title":"On the thermomechanics of localizing pseudoelastic transformations in NiTi materials and structures","abstract":"Nearly equiatomic NiTi alloys undergo reversible solid-state transformations around room temperature, which are activated by the application of stress or by modest temperature changes. The forward transformation causes about 6% strain and the release of about 12 kJ/kg of latent heat; both are recovered during the reverse transformation (pseudoelasticity). In a structure, the transformations occur inhomogeneously with two deformation regimes coexisting, separated by sharp fronts. Moreover, latent heat is released/absorbed at the fronts causing temperature gradients which can affect the evolution of transformation. This study aims, first, to quantify the transformation behavior over the pseudoelastic temperature regime and use the results to calibrate a constitutive model. Then, the interaction between the latent heat and the surrounding environment, and its effect on the transformation stress and the spatiotemporal evolution of the induced inhomogeneous deformation is studied using experiments and transient finite element analyses. Isothermal experiments establish the transformation behavior in the absence of latent heat-related effects. They are conducted on thin-walled tubes in a temperature-controlled liquid bath. The stress-elongation responses typically trace closed hystereses, which allow determination of the transformation stresses and strains. The thermomechanical interactions and their effects on structural behavior are studied through constant stress thermal cycles on NiTi tubes (isobaric cycles). The experiments are conducted in a custom testing facility, that allows precise application of thermomechanical loads on small structures. Axial loads can be applied by a tensile frame and thermal loading by a feedback-controlled circulating airstream. The transparent windows of the device enable the monitoring of the specimen using digital image correlation. The results provide the transformation temperature and strain at different stress levels and the evolution of the associated inhomogeneous deformation, which usually takes the form of helical bands. It is demonstrated that the transformation rate is reduced or increased to match the rate that the latent heat is added or removed by the airstream. This complex thermomechanical interaction forms the main test platform for evaluating the constitutive model and structural analyses that follow. A recently developed fully coupled thermomechanical constitutive model for SMAs is extended to the needs of the problem. The model postulates a Helmholtz free energy with transformation strain and entropy as internal variables. Its key features are the modeling of the forward and reverse transformations using a single surface in the stress-temperature space that obeys kinematic hardening, and the adoption of softening over the transformations to model the observed inhomogeneous deformation. The model is extended to accommodate different elastic moduli for the two phases and the dependence of transformation strain and entropy on temperature. It is calibrated to the isothermal experiments and integrated as a material subroutine into a static displacement transient temperature finite element analysis to simulate the isobaric experiments. The analysis includes a finely meshed tube with radially constrained ends. A small thickness depression at one end is used to trigger the transformations. The heat exchange with the airstream is modeled by a convection boundary condition on the outer surface. The convection coefficient is chosen so that the calculated rate of transformation matches the measured one. The analyses reproduce the transformation temperatures, strains, and inhomogeneous deformation patterns. Moreover, the transformation rate is reproduced for several cases due to the correct calibration of the convection coefficient. The results point to the challenges of modeling thermomechanically coupled systems in the presence of instabilities and provide a platform for evaluating constitutive models for SMAs.","abstract_html":"Nearly equiatomic NiTi alloys undergo reversible solid-state transformations around room temperature, which are activated by the application of stress or by modest temperature changes. The forward transformation causes about 6% strain and the release of about 12 kJ/kg of latent heat; both are recovered during the reverse transformation (pseudoelasticity). In a structure, the transformations occur inhomogeneously with two deformation regimes coexisting, separated by sharp fronts. Moreover, latent heat is released/absorbed at the fronts causing temperature gradients which can affect the evolution of transformation. This study aims, first, to quantify the transformation behavior over the pseudoelastic temperature regime and use the results to calibrate a constitutive model. Then, the interaction between the latent heat and the surrounding environment, and its effect on the transformation stress and the spatiotemporal evolution of the induced inhomogeneous deformation is studied using experiments and transient finite element analyses. Isothermal experiments establish the transformation behavior in the absence of latent heat-related effects. They are conducted on thin-walled tubes in a temperature-controlled liquid bath. The stress-elongation responses typically trace closed hystereses, which allow determination of the transformation stresses and strains. The thermomechanical interactions and their effects on structural behavior are studied through constant stress thermal cycles on NiTi tubes (isobaric cycles). The experiments are conducted in a custom testing facility, that allows precise application of thermomechanical loads on small structures. Axial loads can be applied by a tensile frame and thermal loading by a feedback-controlled circulating airstream. The transparent windows of the device enable the monitoring of the specimen using digital image correlation. The results provide the transformation temperature and strain at different stress levels and the evolution of the associated inhomogeneous deformation, which usually takes the form of helical bands. It is demonstrated that the transformation rate is reduced or increased to match the rate that the latent heat is added or removed by the airstream. This complex thermomechanical interaction forms the main test platform for evaluating the constitutive model and structural analyses that follow. A recently developed fully coupled thermomechanical constitutive model for SMAs is extended to the needs of the problem. The model postulates a Helmholtz free energy with transformation strain and entropy as internal variables. Its key features are the modeling of the forward and reverse transformations using a single surface in the stress-temperature space that obeys kinematic hardening, and the adoption of softening over the transformations to model the observed inhomogeneous deformation. The model is extended to accommodate different elastic moduli for the two phases and the dependence of transformation strain and entropy on temperature. It is calibrated to the isothermal experiments and integrated as a material subroutine into a static displacement transient temperature finite element analysis to simulate the isobaric experiments. The analysis includes a finely meshed tube with radially constrained ends. A small thickness depression at one end is used to trigger the transformations. The heat exchange with the airstream is modeled by a convection boundary condition on the outer surface. The convection coefficient is chosen so that the calculated rate of transformation matches the measured one. The analyses reproduce the transformation temperatures, strains, and inhomogeneous deformation patterns. Moreover, the transformation rate is reproduced for several cases due to the correct calibration of the convection coefficient. The results point to the challenges of modeling thermomechanically coupled systems in the presence of instabilities and provide a platform for evaluating constitutive models for SMAs.","abstract_has_math":false,"creators":["Tsimpoukis, Solon"],"institution":"The University of Texas at Austin","degree_name":"Doctor of Philosophy","degree_level":"Doctoral","degree_discipline":"Engineering Mechanics","degree_department":null,"school":null,"contributors":[],"advisors":["Kyriakides, S."],"committee_chairs":[],"committee_members":["Landis, Chad M","Ravi-Chandar, Krishnas","Sanoja, Gabriel"],"year":2024,"date_issued":"2024-08","date_published":"2024-08","updated_at":"2026-07-24T05:01:24Z","subjects":["Multiphysics analysis","Full-field methods","Latent heat","Phase transformation","Thermomechanical experiments","Inhomonogeneous deformation"],"languages":[],"rights":[],"rights_urls":[],"identifier_entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://doi.org/10.26153/tsw/58454"],"render_values":[{"text":"https://doi.org/10.26153/tsw/58454","href":"https://doi.org/10.26153/tsw/58454","code":true}]}]},"links":{"outbound_url":"https://hdl.handle.net/2152/131108","outbound_label":"Handle","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Kyriakides, S."]},{"key":"dc:contributor.committeemember","label":"Committee Member","values":["Landis, Chad M","Ravi-Chandar, Krishnas","Sanoja, Gabriel"]},{"key":"dc:creator","label":"Author","values":["Tsimpoukis, Solon"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.accessioned","label":"Dc Date Accessioned","values":["2025-02-11T00:48:15Z"]},{"key":"dc:date.available","label":"Dc Date Available","values":["2025-02-11T00:48:15Z"]},{"key":"dc:date.issued","label":"Date","values":["2024-08"]},{"key":"dc:type","label":"Dc Type","values":["Thesis"]},{"key":"thesis:degree_discipline","label":"Discipline","values":["Engineering Mechanics"]},{"key":"thesis:degree_level","label":"Degree Level","values":["Doctoral"]},{"key":"thesis:degree_name","label":"Degree Name","values":["Doctor of Philosophy"]},{"key":"thesis:institution_name","label":"Thesis Institution Name","values":["The University of Texas at Austin"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Multiphysics analysis","Full-field methods","Latent heat","Phase transformation","Thermomechanical experiments","Inhomonogeneous deformation"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://hdl.handle.net/2152/131108","https://doi.org/10.26153/tsw/58454"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["Nearly equiatomic NiTi alloys undergo reversible solid-state transformations around room temperature, which are activated by the application of stress or by modest temperature changes. The forward transformation causes about 6% strain and the release of about 12 kJ/kg of latent heat; both are recovered during the reverse transformation (pseudoelasticity). In a structure, the transformations occur inhomogeneously with two deformation regimes coexisting, separated by sharp fronts. Moreover, latent heat is released/absorbed at the fronts causing temperature gradients which can affect the evolution of transformation. This study aims, first, to quantify the transformation behavior over the pseudoelastic temperature regime and use the results to calibrate a constitutive model. Then, the interaction between the latent heat and the surrounding environment, and its effect on the transformation stress and the spatiotemporal evolution of the induced inhomogeneous deformation is studied using experiments and transient finite element analyses. Isothermal experiments establish the transformation behavior in the absence of latent heat-related effects. They are conducted on thin-walled tubes in a temperature-controlled liquid bath. The stress-elongation responses typically trace closed hystereses, which allow determination of the transformation stresses and strains. The thermomechanical interactions and their effects on structural behavior are studied through constant stress thermal cycles on NiTi tubes (isobaric cycles). The experiments are conducted in a custom testing facility, that allows precise application of thermomechanical loads on small structures. Axial loads can be applied by a tensile frame and thermal loading by a feedback-controlled circulating airstream. The transparent windows of the device enable the monitoring of the specimen using digital image correlation. The results provide the transformation temperature and strain at different stress levels and the evolution of the associated inhomogeneous deformation, which usually takes the form of helical bands. It is demonstrated that the transformation rate is reduced or increased to match the rate that the latent heat is added or removed by the airstream. This complex thermomechanical interaction forms the main test platform for evaluating the constitutive model and structural analyses that follow. A recently developed fully coupled thermomechanical constitutive model for SMAs is extended to the needs of the problem. The model postulates a Helmholtz free energy with transformation strain and entropy as internal variables. Its key features are the modeling of the forward and reverse transformations using a single surface in the stress-temperature space that obeys kinematic hardening, and the adoption of softening over the transformations to model the observed inhomogeneous deformation. The model is extended to accommodate different elastic moduli for the two phases and the dependence of transformation strain and entropy on temperature. It is calibrated to the isothermal experiments and integrated as a material subroutine into a static displacement transient temperature finite element analysis to simulate the isobaric experiments. The analysis includes a finely meshed tube with radially constrained ends. A small thickness depression at one end is used to trigger the transformations. The heat exchange with the airstream is modeled by a convection boundary condition on the outer surface. The convection coefficient is chosen so that the calculated rate of transformation matches the measured one. The analyses reproduce the transformation temperatures, strains, and inhomogeneous deformation patterns. Moreover, the transformation rate is reproduced for several cases due to the correct calibration of the convection coefficient. The results point to the challenges of modeling thermomechanically coupled systems in the presence of instabilities and provide a platform for evaluating constitutive models for SMAs."]},{"key":"dc:format.mimetype","label":"Dc Format Mimetype","values":["application/pdf"]},{"key":"dc:title","label":"Title","values":["On the thermomechanics of localizing pseudoelastic transformations in NiTi materials and structures"]}]}],"canonical_facts":{"dc:contributor.advisor":["Kyriakides, S."],"dc:contributor.committeemember":["Landis, Chad M","Ravi-Chandar, Krishnas","Sanoja, Gabriel"],"dc:creator":["Tsimpoukis, Solon"],"dc:date.accessioned":["2025-02-11T00:48:15Z"],"dc:date.available":["2025-02-11T00:48:15Z"],"dc:date.issued":["2024-08"],"dc:description.abstract":["Nearly equiatomic NiTi alloys undergo reversible solid-state transformations around room temperature, which are activated by the application of stress or by modest temperature changes. The forward transformation causes about 6% strain and the release of about 12 kJ/kg of latent heat; both are recovered during the reverse transformation (pseudoelasticity). In a structure, the transformations occur inhomogeneously with two deformation regimes coexisting, separated by sharp fronts. Moreover, latent heat is released/absorbed at the fronts causing temperature gradients which can affect the evolution of transformation. This study aims, first, to quantify the transformation behavior over the pseudoelastic temperature regime and use the results to calibrate a constitutive model. Then, the interaction between the latent heat and the surrounding environment, and its effect on the transformation stress and the spatiotemporal evolution of the induced inhomogeneous deformation is studied using experiments and transient finite element analyses. Isothermal experiments establish the transformation behavior in the absence of latent heat-related effects. They are conducted on thin-walled tubes in a temperature-controlled liquid bath. The stress-elongation responses typically trace closed hystereses, which allow determination of the transformation stresses and strains. The thermomechanical interactions and their effects on structural behavior are studied through constant stress thermal cycles on NiTi tubes (isobaric cycles). The experiments are conducted in a custom testing facility, that allows precise application of thermomechanical loads on small structures. Axial loads can be applied by a tensile frame and thermal loading by a feedback-controlled circulating airstream. The transparent windows of the device enable the monitoring of the specimen using digital image correlation. The results provide the transformation temperature and strain at different stress levels and the evolution of the associated inhomogeneous deformation, which usually takes the form of helical bands. It is demonstrated that the transformation rate is reduced or increased to match the rate that the latent heat is added or removed by the airstream. This complex thermomechanical interaction forms the main test platform for evaluating the constitutive model and structural analyses that follow. A recently developed fully coupled thermomechanical constitutive model for SMAs is extended to the needs of the problem. The model postulates a Helmholtz free energy with transformation strain and entropy as internal variables. Its key features are the modeling of the forward and reverse transformations using a single surface in the stress-temperature space that obeys kinematic hardening, and the adoption of softening over the transformations to model the observed inhomogeneous deformation. The model is extended to accommodate different elastic moduli for the two phases and the dependence of transformation strain and entropy on temperature. It is calibrated to the isothermal experiments and integrated as a material subroutine into a static displacement transient temperature finite element analysis to simulate the isobaric experiments. The analysis includes a finely meshed tube with radially constrained ends. A small thickness depression at one end is used to trigger the transformations. The heat exchange with the airstream is modeled by a convection boundary condition on the outer surface. The convection coefficient is chosen so that the calculated rate of transformation matches the measured one. The analyses reproduce the transformation temperatures, strains, and inhomogeneous deformation patterns. Moreover, the transformation rate is reproduced for several cases due to the correct calibration of the convection coefficient. The results point to the challenges of modeling thermomechanically coupled systems in the presence of instabilities and provide a platform for evaluating constitutive models for SMAs."],"dc:format.mimetype":["application/pdf"],"dc:identifier.uri":["https://hdl.handle.net/2152/131108","https://doi.org/10.26153/tsw/58454"],"dc:subject":["Multiphysics analysis","Full-field methods","Latent heat","Phase transformation","Thermomechanical experiments","Inhomonogeneous deformation"],"dc:title":["On the thermomechanics of localizing pseudoelastic transformations in NiTi materials and structures"],"dc:type":["Thesis"],"thesis:degree_discipline":["Engineering Mechanics"],"thesis:degree_level":["Doctoral"],"thesis:degree_name":["Doctor of Philosophy"],"thesis:institution_name":["The University of Texas at Austin"]},"updated_at":"2026-07-24T05:01:24Z"}