SAXS study of failure mechanisms in Kevlar [superscript R] fiber reinforced epoxy composites

The failure mechanisms of the Kevlar^R 49 fibers, the epoxies and the uniaxial Kevlar^R 49 fiber reinforced epoxy composites under tensile loads have been studied using the 10 meter Small Angle X-ray Scattering (SAXS) facility at the National Center for Small Angle Scattering Research and the scanning electron microscope (SEM) at The University of Tennessee, Knoxville. The behavior of specimens of uniaxial Kevlar^R 49 fiber reinforced epoxy with various fiber orientations has also been studied under tensile load using an Instron Mechanical Tester.

The results show that the Kevlar^R 49 fibers fail due to increases in the volume fractions of microvoids and enlargements of larger microvoids along the fiber axis direction. Degradation of Kevlar^R 49 fibers was accompanied by roughening of the surfaces and by decreases in the mass densities. The epoxies and the composites failed in a catastrophic dynamic process, the crack originating from the surface flaws or the 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 that 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 fitted well with the experimental ultimate strength results of the composites.