With their impressive individual properties, carbon nanotubes should form high-performance fibers. We explored the roles of nanotube length and structure, fiber density, and nanotube orientation in achieving optimum mechanical properties. We found that carbon nanotube fiber, spun directly and continuously from gas phase as an aerogel, combines high strength and high stiffness (axial elastic modulus), with an energy to breakage (toughness) considerably greater than that of any commercial high-strength fiber. Different levels of carbon nanotube orientation, fiber density, and mechanical properties can be achieved by drawing the aerogel at various winding rates. The mechanical data obtained demonstrate the considerable potential of carbon nanotube assemblies in the quest for maximal mechanical performance. The statistical aspects of the mechanical data reveal the deleterious effect of defects and indicate strategies for future work.

Following a brief review of prior work on fabric-based armor systems, the system effects that occur during the ballistic impact of woven fabric body armor materials are discussed from a conceptual framework developed to relate single yarn impact mechanics to fabric impact mechanics. The consequence of assembling yarns into single-ply fabric structures is discussed from this perspective. A steep strain gradient along yarns in the region of the transverse deflection of the fabric is related to the constraint imposed on them by neighboring yarns. Striking and residual velocity data, collected for single-ply fabric systems of Spectra®, Kevlar® 29, and nylon with various different yarn deniers and weave types, are used to establish the response of spaced armor systems. The system effects of assembling fabric plies into body armor systems are determined by comparing the response of spaced armor systems to actual multiple-ply systems. There is a pronounced decrease in energy absorption capacity for the Spectra and nylon systems; this deleterious effect is ascribed to increased transverse stresses and possible interference of the deflection characteristics of fabric plies by subsequent plies.

Dragline silk from the spider, Nephila clavipes, was characterized b y thermal analysis (TGA, DSC, DMA), computational modeling, scanning electron microscopy and by quasi-static as well as high rates of strain. Thermal stability to about 230°C was observed by TGA, two transitions by D M A , -75"C, representative of localized motion in the amorphous domain, and a main chain motion associated with partial melt at 210°C. Tensile tests indicated average initial modulus, ultimate tensile strength and ultimate tensile strain of 22 GPa, 1.1 GPa and 9%, respectively. The corresponding properties of the best fibers tested were 60 GPa, 2.9 GPa and 11 %, respectively. High strain rates (>50,000%/ see) indicated similar mechanical properties to the average values indicated above. Microscopy showed compressive and tensile strains to failure of 34%. Computational modeling yielded a cystal modulus of 200 GPa.

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