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Abstract
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Compliant textile garments developed for protection have been produced for thousands of years – from layers of linen utilized by the ancient Greeks through layers of silk worn by medieval Japanese samurai. In the late 19th century the U.S. military also used silk body armor. During World War II, the “flak jacket”, constructed of ballistic nylon, was developed. However, none of these early protective textiles could resist a high velocity bullet. In the late 1960s, Du Pont invented Kevlar, an aramid material, the first bullet-resistant fiber. This ushered in a new era of textile armor that continues to progress toward more efficient systems, offering exceptional protection against bullet penetration. While Kevlar was the first ballistic material from which modern body armor could be constructed, there are presently several other fiber types used in bullet-resistant armor, such as Twaron®, Dyneema®, Spectra®, and Zylon®.
Fabric penetration resistance is determined not only by fiber physical properties, such as strength, modulus, density, viscosity and thermal properties, but also by fabric structural geometry. How a ballistic textile is manufactured and its resulting configuration directly influences its behaviours.
Since the 1960s, numerous numerical methods for simulating textile penetration have been developed. However, the design and development of textile armor systems have been driven primarily by experiments and experience. Numerical models typically have not been used to predict design of armor systems. Rather, they have been used to provide insight into the relative importance of specific material parameters that influence performance. Although textile performance is directly related to the manufacturing process and its resulting fabric architecture, most of the important details related to fabric micro-structure and filament-level physics, such as yarn denier, end count, tow structures, filament spatial paths, and fiber-to-fiber interaction were not accurately modeled in previous numerical techniques. As a result, these models could not establish a quantitative relation among material properties, manufacturing process, fabric micro-geometry, and ballistic performance.
A micro-scale computational tool, based upon an explicit digital element method (DEM), has been developed for numerical simulation of ballistic impact and penetration of textile fabrics. In this approach, each yarn is digitized as an assembly of digital fibers. Each digital fiber is further digitized into a short digital rod element chain connected by frictionless pins (nodes). A search is conducted to find contacts between adjacent digital fibers. If a contact is detected, compressive and frictional forces between fibers will be determined, based upon contact stiffness and friction coefficient. Nodal forces are calculated for each time step. Nodal displacements are determined using an explicit procedure. Because the digital element approach operates on a sub-yarn micro-scale, one can determine textile penetration resistance based upon sub-yarn scale properties, such as inter-fiber compression, friction, and fiber strength. Research presented in this paper includes three parts. First, the explicit digital element algorithm used in dynamic simulation is explained. Second, the approach is used to generate 2-D woven fabric micro-geometries and to simulate ballistic penetration processes. Third, numerical results are compared to high resolution experimental impact and ballistic test data. |