Longitudinal and transverse moduli and strengths of low angle 3-D braided composites

This paper presents the results of an experimental study conducted on a class of three-dimensional braided graphite-epoxy composites as well as the results of a modeling study to relate the microstructural variables such as volume fraction of fibers and braid angles to the anisotropic elastic moduli and strengths of these composites. In the experimental program, composite samples with a range of volume fractions (0.2-0.45) and a range of braid angles (0 degrees-30 degrees) were produced and tested in simple compression in both longitudinal and transverse directions. Such data is currently unavailable in literature on a single material system. Extreme care was taken in processing to minimize porosity and non-homogenieties in microstructure, and in proper strain gaging to ensure the reliability and reproducibility of our measurements. In the modeling study, the measured elastic moduli were compared against the currently employed isotrain and isostress models for these materials. At the present time, it is widely reported in literature that the predictions of isostrain model are fairly accurate. This study revealed that the isostrain model predictions can be in significant error and that a weighted average of the isostrain and isostress model predictions yielded the best comparisons against the experiments. For the strength of the composite in the longitudinal and transverse directions, correlations expressing these properties as a function of volume fraction and braid angle were motivated by micromechanics and experimental observations. The predictive capability of these correlations for the selected class of 3-D braided composites was found to be reasonable.