Quantifying damage at multiple loading rates to Kevlar KM2 fibers due to weaving, finishing, and pre-twist

Abstract

Understanding Kevlar's mechanical response at strain rates relevant to impact events is essential for the development of numerical simulations of impact events. The strength and orientation of the individual filaments may become compromised as a result of crimping, weaving, or finishing processes required to weave the Kevlar yarn into the fabric used for protective equipment. To elucidate and quantify any damage to the fibers as a result of the weaving or post treatment finishing process, single fibers were extracted from the warp and weft directions of plain woven, hydrophobically treated Kevlar cloth. The strength of these fibers was measured over a wide range of strain rates and compared with fibers extracted from an unwoven yarn. The tensile response was also measured from single fibers subjected to varying levels of shear strain. The tensile strength of the fibers was evaluated at 0.001 s−1, 1 s−1, and approximately 1000 s−1 using a Bose Electroforce test setup and a Hopkinson tension bar modified for fiber experiments. A wide range of gage lengths was investigated to find the effect of defect distribution on the tensile strength of the woven fibers. The results show that fibers taken from the weft direction of the woven fabric decreased in strength 3%–8% compared with the unwoven fiber. The warp fibers were a minimum of 20% weaker than unwoven and weft fibers at all loading rates. Twisted fibers retained 93% of untwisted tensile strength up to a shear strain of about 0.10–0.15. Measured Young's modulus as a function of strain rate and tensile strength as a function of gage length in relation to defect distribution are also presented in this paper.