Influence of multiaxial loading on the failure of Kevlar KM2 single fiber

The role of multiaxial loading on the failure of Kevlar® KM2 ballistic fibers during transverse loading is not well understood. Quasi-static experiments reported by Hudspeth M, Li D, Spatola J, et al. (2015) on single KM2 fiber subjected to transverse loading by three indenter geometries over a wide range of loading angles exhibit significant reductions in the average axial tensile failure strain. In this study, a three-dimensional finite element model is developed to predict the degree of multiaxial loading present at the location of fiber failure in these experiments. The model predicts an axial tensile strain concentration within the indenter–fiber contact zone. The fiber is also subjected to multiaxial stress/strain states within the contact zone consisting of axial tension, axial compression, transverse compression and interlaminar shear that can degrade axial tensile failure strain. For a round indenter with a radius much higher than the fiber diameter, the axial tensile strain concentration and multiaxial strain in the fiber are negligible. In the case of a fragment-simulating projectile and a razor indenter, significant axial tensile strain concentrations (2.2–5.9) are predicted and the localized transverse loading results in extensive inelastic deformation within the fiber cross-section. Based on the results, a maximum axial tensile strain failure criterion incorporating the multiaxial loading degradation effects is developed. The failure criterion correlates well with the experimental measurements reported by Hudspeth et al. for all three indenters. Modeling the experiments provides new insights into the tensile failure strain of high-performance ballistic fibers at extremely small gage lengths subject to transverse impact loading.