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

Report No. ARL-TR-6403
Authors: Brett D. Sanborn; Tusit T. Weerasooriya
Date/Pages: April 2013; 28 pages
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 Kevlar KM2 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.
Distribution: Approved for public release
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Last Update / Reviewed: April 1, 2013