Effect of Strain Rate and Pre-twist on Strength of Single Kevlar® KM2 Fiber

Abstract

High performance fibers such as Kevlar KM2 used in protective vests are loaded primarily in tension at high loading rates. During the weaving and crimping process of fibers/yarns to make fabrics, individual fiber tensile strength may be reduced due to pre-straining of fibers by twisting. To systematically investigate the loading rate effects under different pre-twisted stress states, a miniaturized Kolsky tension bar is used to study single Kevlar KM2 fibers subject to varying levels of shear strain at high strain rates. Low and intermediate strain rate responses of the fibers are investigated using a Bose Electroforce loading system. In this paper, tensile strength as a function of pre-loaded shear strain on a single fiber is presented for different strain rates of loading. Fracture morphology of the fibers is also investigated using a high resolution scanning electron microscope to understand the mechanisms of failure.