Design Optimization of Superelastic Niti Yarns for Energy-Absorbing Textiles

Superelastic nickel–titanium (NiTi) microfilament yarns are promising for energy-absorbing textile systems because they combine high strength, flexibility, and reversible phase transformation with recovery under repeated loading. These characteristics make them attractive for passive adaptive applications such as pressure-redistributing medical textiles, prosthetic liners, and protective wearable systems. However, achieving desired force, stiffness, and energy absorption in superelastic NiTi yarns requires a predictive framework that links yarn geometry and manufacturing parameters to mechanical response. This work addresses a key limitation in an existing analytical model for superelastic SMA yarns: the geometric yarn parameters are not fully independent because the influence of packing density is not explicitly captured. To close this gap, packing density is introduced as a governing design parameter that relates yarn geometry to internal filament arrangement and overall performance. Twisted yarns were manufactured in-house, and a microscopy-based image analysis procedure was developed to experimentally characterize radial packing density across yarn cross sections. Using data from multiple yarn geometries and replicated measurements, an empirical relationship was established between packing density, twist, and normalized radial position. The resulting framework provides a stable and interpretable description of radial packing behavior and creates a path toward more accurate prediction of yarn force response. By linking manufacturable design variables to mechanical performance, this work lays the foundation for optimization-based inverse design of superelastic NiTi yarns tailored for energy absorption in biomedical, protective, and wearable textile applications.