Modeling the fiber length-scale response of Kevlar KM2 yarn during transverse impact

In this study, transverse impact of a cylindrical projectile onto a 600 denier Kevlar KM2 yarn (400 individual fibers) is studied using a fiber length-scale three-dimensional finite element model to better understand projectile–fiber and fiber–fiber contact interactions on wave propagation and fiber failure within the yarn. A short time scale response indicates significant transverse compressive deformation in the fiber that increases with impact velocity. Fiber-level modeling predicts a flexural wave that induces curvatures in the fibers significant enough to induce compressive fiber kinking and fibrillation. A spreading wave normal to the direction of projectile impact develops and spreads the fibers at high velocity. The models predict bounce velocities of the individual fibers within the yarn that varies based on spatial location. These mechanisms result in non-uniform loading and progressive failure of fibers within the yarn. In addition, the models show a gradient in the axial tensile stress in the fiber cross-section at the location of failure. Current state-of-the-art experimental capabilities in yarn/fabric impact testing do not have the spatial resolution to track individual single-fiber micron length-scale deformations in real time. These fiber-level mechanisms may explain the experimentally observed lower breaking speed for yarns better the classic Smith solution, which assumes yarns are homogenous (i.e. individual fibers and their interactions are not considered) and loaded uniformly in tension (multi-axial loading and stress gradients are neglected).