Tensile properties and constitutive modeling of Kevlar29 fibers: From filaments to bundles

•Tensile strength and modulus of Kevlar29 fiber bundles increase with strain rate, while fracture strain and toughness decrease. Filaments exhibit higher strength, fracture strain, and toughness than bundles, but lower tensile modulus.•Under quasi-static loading, fibers show fibrillation or shear flow. Dynamic loading introduces brittle fractures, shifting from fibrillation to brittle fracture with increasing strain rate.•Developed viscoelastic and multiscale constitutive models based on Weibull distribution, accurately predicting tensile performance under varying strain rates and quantifying fiber bundle damage. Models validated through numerical simulations. This paper experimentally investigates the tensile properties of Kevlar29 filaments and fiber bundles, revealing the size and strain rate effects on the mechanical properties of fiber bundles. The fracture modes and mechanisms of the fibers were analyzed, and viscoelastic constitutive models at the fiber bundle scale and constitutive models for filaments-bundles based on the Weibull distribution were established. The results show that Kevlar29 fiber bundles exhibit significant strain rate effects: as the strain rate increases, tensile strength and modulus increase, while fracture strain and toughness decrease. Under quasi-static loading, the fracture modes of the fibers are mainly fibrillation or shear flow, but as the strain rate increases, the failure modes shift towards brittle fracture. Both constitutive models can accurately predict the tensile properties of fiber bundles at different strain rates. The accuracy and applicability of the filament-to-bundle constitutive model were verified through numerical simulations, demonstrating that the model better describes the size effect of fibers and quantifies the damage to the fiber bundles.