Chapter 8 Single Fiber Tensile Properties Measured by the Kolsky Bar Using a Direct Fiber Clamping Method J.H. Kim, N.A. Heckert, W.G. McDonough, K.D. Rice, and G.A. Holmes Abstract The Kolsky bar test has been widely used in measuring material behavior under high strain rate conditions. In particular, polymers used in ballistic applications have been characterized by this method to investigate high strain rate behavior during ballistic impact. Research conducted by Cheng et al. (J Eng Mater Technol-Trans ASME 127(2):197–203, 2005) and Lim et al. (J Mater Sci 45(3):652–661, 2010) measured high strain rate properties of single PPTA [Poly (p-phenylene terephthalamide)] fibers and aramid co-polymer fibers by gluing the fiber directly to the Kolsky bar, which is time consuming work and can be affected by wicking of the glue into the fiber gauge length area. Kim et al. (J Mater Sci. doi:10.1007/s10853-013-7142-y, 2013) investigated clamping effects of the glue-tab and direct gripping methods on the single PPTA fiber tensile properties under the quasi-static loading condition and applied the direct grip method for the Kolsky bar test to measure the tensile strengths at a high strain rate (Kim et al., Compos Sci Technol. doi:10.1016/j. compscitech.2012.03.021, 2012). This study extends the measurement capability for the tensile strength, failure strain and modulus that are important parameters that influence performance of soft body armor. Keywords Single fiber tensile test • High strain rate • PPTA • Soft body armor • Direct fiber grip 8.1 Introduction Important parameters influencing the ballistic performance of soft body armors are the mechanical properties of high strength fibers used in the armors. Although fibers deform at high rates during ballistic impact, the mechanical properties of these fibers are often characterized under quasi-static test conditions. To measure fiber properties under high strain rate (HSR) conditions, Kolsky bar tests [1, 2] have been utilized. However, testing at these rates requires relatively short fiber lengths compared to quasi-static testing. The gripping method in these studies consisted of gluing a fiber on the Kolsky bar. However, as emphasized by a study on clamp effects [3] and experimental results with sub-millimeter long fibers [4], tensile tests with short fibers were shown to be highly influenced by the gripping condition. Therefore, a direct clamp method for measuring tensile strengths [5] is recommended when using the Kolsky bar test to minimize the complexity inherent in the gluing method and to increase the test throughput. A recent study comparing fiber strengths, moduli, and failure strains from tests that used the glue-tab method and the direct grip method found that the direct grip method to be more reliable for short fiber lengths ( 10mm) [6]. One important aspect to understanding soft body armor performance is determining the mechanism of impact energy transfer and dissipation during impact. Strain energy dissipation in fibers of soft body armor subjected to ballistic impact J.H. Kim • W.G. McDonough • G.A. Holmes (*) Materials Science and Engineering Division (M/S 8541), National Institute of Standards and Technology, Gaithersburg, MD 20899, USA e-mail: gale.holmes@nist.gov N.A. Heckert Statistical Engineering Division (M/S 8980), National Institute of Standards and Technology, Gaithersburg, MD 20899, USA K.D. Rice Materials Measurement Science Division (M/S 8102), National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States. B. Song et al. (eds.), Dynamic Behavior of Materials, Volume 1: Proceedings of the 2013 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-00771-7_8, #The Society for Experimental Mechanics, Inc. 2014 69
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