Effect of Strain Rates and Pre-Twist on Tensile Strength of Kevlar KM2 Single Fiber
Final rept. Feb 2012-Feb 2013
ARMY RESEARCH LAB ABERDEEN PROVING GROUND MD WEAPONS AND MATERIALS RESEARCH DIRECTORATE
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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 fibersyarns 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.