body armor

body armor - X. Miscellaneous J. Body Armor Information 4....

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X. Miscellaneous J. Body Armor Information 4. Kevlar Aramid Fiber by Julius Chang ( jchang@whidbey.net ) 1. KEVLAR ARAMID FIBER -- Processing, Structure, and Properties Kevlar is an aromatic polyamide, or aramid, fiber introduced in the early 1970s. The chemical composition of Kevlar is poly para-phenyleneterephthalamide (PPD-T). It is made from a condensation reaction of para-phenylene diamine and terephthaloyl chloride. The resultant armomatic polyamide contains aromatic and amide groups. Polymers with high breaking strength often have one or both of these groups. The aromatic ring structure contributes high thermal stability. The para configuration leads to stiff, rigid molecules that contribute high strength and high modulus. Para-aramid fibers belong to a class of materials known as liquid crystalline polymers. When PPD-T solutions are extruded through a spinneret and drawn through an air gap during fiber manufacture, the liquid crystalline domains can orient and align in the flow direction. Kevlar can acquire a high degree of alignment of long, straight ploymer chains parallel to the fiber axis. The structure exhibits anisotropic properties, with higher strength and modulus in the fiber longitudinal direction than in the axial direction. The extruded material also possesses a fibrillar structure. This structure results in poor shear and compression properties for aramid composites. Hydrogen bonds form between the polar amide groups on adjacent chains and they hold the individual Kevlar polymer chains together. Tensile modulus is a function of molecular orientation. As a spun fiber, Kevlar 29 (a high toughness variant) has a modulus of 62 GPa (9 Mpsi). Heat treatment under tension increases crystalline orientation. The resulting fiber, Kevlar 49, has a modulus of 131 GPa. The tensile strength of Kevlar ranges from about 2.6 to 4.1 GPa. This is more than twice that for conventional fibers like Nylon 66. Tensile failure initiates at the fibril ends and propagates via shear failure between the fibrils. Kevlar behaves elastically in tension. In compression, it shows nonlinear, ductile behavior. It exhibits yield at compression strains of 0.3 to 0.5%. This corresponds to formation of structural defects known as kink bands. These bands are related to compressive buckling of the aramid molecules. Aramid fibers are noted for toughness and general damage tolerance. Tensile elongation
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This note was uploaded on 01/22/2012 for the course ME 2733 taught by Professor Meng during the Fall '10 term at LSU.

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body armor - X. Miscellaneous J. Body Armor Information 4....

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