Research Papers

Why Impacted Yarns Break at Lower Speed Than Classical Theory Predicts

[+] Author and Article Information
James D. Walker1

 Southwest Research Institute San Antonio, TX 78238james.walker@swri.org

Sidney Chocron

 Southwest Research Institute San Antonio, TX 78238


Corresponding author.

J. Appl. Mech 78(5), 051021 (Aug 08, 2011) (7 pages) doi:10.1115/1.4004328 History: Received November 26, 2010; Revised May 23, 2011; Published August 05, 2011; Online August 08, 2011

Fabrics are an extremely important element of body armors and other armors. Understanding fabrics requires understanding how yarns deform. Classical theory has shown very good agreement with the deformation of a single yarn when impacted transversely. However, the impact speed at which a yarn breaks based on this classical theory is not correct; it has been experimentally noted that yarns break when impacted at a lower speed. This paper explores the mechanism of yarn breakage. The problem of the transverse strike of a yarn by a flat-faced projectile is analytically solved for early times. It is rigorously demonstrated that when a flat-faced projectile strikes a yarn, the minimum impact speed that breaks the yarn will always be at least 11% less than the classical-theory result. It is further shown that when the yarn in front of the projectile “bounces” off the projectile face due to the impact, the impact speed that breaks the yarn is further reduced. If the yarn bounces elastically off the projectile face at twice the impact velocity (the theoretical maximum), there is a 40% reduction in the projectile impact speed that breaks the yarn.

Copyright © 2011 by American Society of Mechanical Engineers
Topics: Yarns , Projectiles , Waves
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Figure 1

Three views of a right circular cylinder flat-faced impactor (moving up the page) striking a single yarn, from an LS-DYNA computation. The bends in the yarn show the transverse waves emanating from the edges of the impactor. The upward motion of the yarn away from the impactor face, referred to as the bounce in the text, is due to the impact.

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Figure 2

The geometry of the yarn motion in the original Lagrangian frame. The prescribed boundary velocity along the z axis is V, and a longitudinal wave and a transverse wave move to the right.

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Figure 3

The geometry of the yarn motion when impacted by the flat face projectile in the original Lagrangian frame (upper) and in the laboratory frame (lower). The projectile velocity in the z direction is V. The speed of the yarn in the z direction after bouncing off the projectile face is V¯. Only the right edge of the projectile is considered; from it, both longitudinal and transverse waves move to the right and to the left. Similar waves emanate from the left edge of the projectile; when they meet at the center the strain in the yarn doubles. Arrows indicate direction of material velocity components.

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Figure 4

The impact speed at which a yarn breaks normalized by cE for three failure strains as a function of the amount of bounce V¯/V due to the impact

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Figure 5

The impact speed at which a yarn breaks normalized by the breaking speed from the classical theory as a function of the amount of bounce V¯/V due to the impact




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