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 vari ables such as volume fraction of fibers and braid angles to the anisot ropic elastic moduli and strengths of these composites. In the experim ental program, composite samples with a range of volume fractions (0.2 -0.45) and a range of braid angles (0 degrees-30 degrees) were produce d and tested in simple compression in both longitudinal and transverse directions. Such data is currently unavailable in literature on a sin gle 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 measureme nts. 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 t hat the predictions of isostrain model are fairly accurate. This study revealed that the isostrain model predictions can be in significant e rror 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 direc tions, correlations expressing these properties as a function of volum e fraction and braid angle were motivated by micromechanics and experi mental observations. The predictive capability of these correlations f or the selected class of 3-D braided composites was found to be reason able.