Transverse impact of ballistic fibers and yarns: fiber length-scale finite element modeling and experiments
Date
2016
Authors
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Publisher
University of Delaware
Abstract
Ballistic impact onto flexible textile fabrics is a complicated multi-scale problem owing to the structural hierarchy of the materials, anisotropic material behavior, projectile geometry, impact velocity and boundary conditions. While this subject has been an active area of research for decades, the fundamental mechanisms such as material failure, dynamic response and multi-axial loading occurring at lower length scales during impact are not well understood. This work provides new insights into the fundamental deformation and failure mechanisms during ballistic impact onto textile fabrics at the micron length scale. In this research, a hybrid computational-experimental systematic approach is adopted to understand the mechanisms and deformation modes of high performance polymer fibers, specifically Kevlar KM2, that is widely used in ballistic impact applications. Fiber length-scale 3D finite element (FE) models are developed to better understand and complement the complicated transverse impact experiments. The fiber length-scale study suggests that fibers are subjected to multiaxial stress states including transverse compression, axial tension, axial compression and transverse shear significant enough to cause fibrillation in the fiber during impact. A dispersive flexural wave mode is predicted by the model due to the finite longitudinal shear modulus of the fiber. The flexural wave induces curvature in the fiber significant enough to cause compressive kinking and, in turn, local fibrillation in the fiber. A fiber length-scale yarn model is developed by explicitly modeling all the 400 fibers in a KM2 600 denier yarn. The yarn transverse compression results show that fiber-fiber contact plays a significant role in the spreading and deformation of individual fibers that is consistent with experimental results. When subjected to transverse impact, the model indicates significant transverse compressive strains in the fiber that increase with impact velocity and a flexural wave that induces curvatures in the fibers significant enough to induce compressive kinking and fibrillation. In addition to the transverse wave, a spreading wave develops due to fiber-fiber contact interaction that spreads the fibers to a large extent resulting in non-uniform loading and progressive failure of fibers within the yarn. Guided by the computational models, single-fiber micromechanical experiments for axial compressive kinking and transverse compression deformation modes are developed. The average tensile strength of the kinked fibers is found to be reduced by 7% compared to the virgin fibers. An experimental methodology is developed to determine the single fiber constitutive behavior in quasi-static transverse compression by removing the geometric nonlinearity due to the growing contact area. The fibers exhibit nonlinear inelastic behavior under large compressive strains. The fibers subjected to 60% nominal strains (80% true strains) showed a 20% reduction in average tensile strength compared to the virgin fibers. A nonlinear inelastic constitutive model is implemented as a user defined material (UMAT) suitable for the commercial FE code LS-DYNA explicit analysis. During impact, the inelastic behavior results in a significant reduction in the fiber bounce velocity and a reduction in the projectile-fiber contact forces by 40% compared to an elastic constitutive behavior. The inelastic dissipation and reduced bounce leads to an inelastic collision rather than an elastic collision. The longitudinal shear modulus and the inelastic behavior are found to govern the failure response of the fibers during impact. Modeling the single fiber quasi-static multiaxial loading experiments indicate fiber failure may be initiated based on a gage length dependent maximum axial tensile strain in the fiber. Regardless of the material behavior (elastic or inelastic), fiber length-scale impact models show a gradient in the axial tensile strain (stress) in the fiber cross section at the location of failure consistent with multiaxial loading experimental observations. Fiber-level yarn breaking speed predictions based on a maximum axial tensile strain (stress) criterion are much lower than the breaking speed based on classical theory and they are consistent with experimental measurements. Therefore, the reduction in experimental yarn breaking speed compared to theoretical Smith solution is attributed to the stress concentration and property degradation mechanisms due to multiaxial stress states at the location of failure.