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Abstract

The failure mechanisms of Kevlar® 49 fibers, epoxy resin and uniaxial Kevlar® 49 fiber reinforced epoxy composites under tensile loads have been studied using small angle x-ray scattering (SAXS) and scanning electron microscopy (SEM). The behavior of specimens of uniaxial Kevlar® 49 fiber reinforced epoxy with various angles between the fiber direction and the tensile load axis was examined using an Instron Mechanical Tester. SAXS showed the Kevlar® 49 fibers fail due to increases in the volume fraction of microvoids and enlargement of larger microvoids along the fiber axis direction. These changes in microvoids arise from the effect of Poisson's ratio being less than 0.5. Hydrolytic degradation of Kevlar® 49 fibers leads to roughening of the surfaces and decreases in the mass densities. The epoxies and the composites failed in a catastrophic dynamic process, the crack originating from surface flaws or air bubbles entrapped in the epoxies during the curing process. The SEM investigations on the failed composites revealed fiber-splits and fiber-pullouts. The results of mechanical tests showed the composite moduli, the composite ultimate strengths and the elongations at break decrease as the angle between the fibers and loading axis increase. The maximum work theory provides a good fit with the experimental ultimate strength results of the com posites.

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A Dynamic Small Angle X-Ray Scattering Study of Stressed Kevlar® 49/Epoxy Composites

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