Variability of Carbon and Glass Fibers, and the Strength of Aligned Composites

The strength and variability of carbon and glass fibers have been estimated in several ways from tests on single fibers and loose bundles of fibers. These tests involved analyzing the variation of strength with the length of the fibers, the distribution of strengths, multiple fractures of a single fiber, and the shape of the stress/strain curve for a loose bundle. The procedures based on loose bundle tests are considerably easier to perform and provide very similar results. Tows of the same batches of fibers were impregnated with epoxy resin and the tensile strengths of the resulting composites were found to be approximately 2.5 times higher than those of the equivalent loose bundles. A rule-of-mixtures approach is unable to account for this ratio, but two- and three- dimensional probabilistic models provide good predictions of the relative strengths.