Multifunctional NiTi spun yarns combine the intriguing shape memory and superelastic effects with classic yarn properties such as softness and pliability. These highly twisted bundles of NiTi microfilament provide increased manufacturability, performance tailorability, and comfort as opposed to monofilament textiles and enable a new class of energy absorbing textiles for medical devices, aerospace, defense, and consumer products. The mechanics of highly twisted NiTi microfilament yarns, however, are not yet fully understood, specifically there currently lacks a fundamental understanding of the impact of twist on the individual filaments and their local response to global yarn straining. This research presents an analytical model for predicting the uniaxial superelastic force response of twist inserted NiTi microfilament yarns by mapping the localized microfilament stress response to yarn displacement. The helical geometry of the filaments within the yarn structure are initially defined using the manufacturing twist, ideal yarn theory, as well as a closed circle packing assumption. Upon loading along the yarn axis, the global strains of the yarn can be related to microfilament strains from the change in geometry of each filament. Experimental data of untwisted microfilament bundles is used to calibrate a multiaxial NiTi material model to calculate the corresponding stresses during yarn loading. The component of axial stress in the direction of the yarn axis is used to calculate the force contribution of each filament. The summation of these forces results in a global applied force response which is compared with experimental data for validation. To validate the predictive analytical model, five yarn variants composing of 400 filaments of 10 μm diameters with twist factors ranging from 0 to 150 were manufactured on an industrial ring spinner. Uniaxial isothermal displacement-controlled testing was performed on each yarn at a temperature above austenite finish. The model was successful in capturing the different stress-strain states of the microfilaments at different radial locations within the yarn. Globally, this resulted in decreased martensitic transformation stresses as well as expanded ranges exhibited in both the model and experimental responses. At higher twists, more variation in microfilament stress-strain states resulted in increased non-uniformity among microfilament transformation kinematics. Globally, this was demonstrated by the decreased distinction between austenitic loading, stress induced martensitic plateau, and completion of the martensitic transformation. This model relates the global yarn response to the microfilament mechanics and provides the foundation for the informed design of NiTi yarns to fully-utilize the smart material properties. By understanding the interrelation of the global force-displacement response and the induced twist, the spun yarn behavior can be designed for desired force-displacement profiles as needed in multifunctional textiles for energy absorption applications.


Analytical Model and Experimental Validation of Superelatic Twist Inserted Niti Microfilament Yarns

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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