Effect of Strain Rates and Pre-Twist on Tensile Strength of Kevlar KM2 Single Fiber

Abstract

Abstract : High-performance fibers such as Kevlar (Kevlar is a registered trademark of DuPont Company) KM2 used in protective vests are loaded primarily in tension at high-loading rates during impact. During the weaving and crimping process of fibers/yarns to make fabrics, individual fiber tensile strength may become reduced due to pre-straining of fibers by twisting. To systematically investigate the effect of loading rate on the fiber strength under different pre-twisted stress states, a miniaturized Kolsky tension bar is used to study single KevlarKM2 fibers subject to varying levels of preset shear strain at high- (1200 s( 1)) strain rates. Low- (0.001 s( 1)) and intermediate- (1 s( 1)) strain rate responses of the fibers are investigated using a Bose Electroforce loading system. In this report, tensile strength is presented as a function of preloaded shear strain on a single fiber for different strain rates of loading. The fiber behaved similarly at all investigated strain rates. Below the threshold shear strain of 0.15, the fiber retained 95% of the original untwisted virgin strength. Fracture morphology of the fibers is also investigated using a high-resolution scanning electron microscope to understand the mechanisms of failure.