Structural Properties of Braided Graphite/Epoxy Composites

Abstract

The mechanical, impact, and fatigue properties of graphite/epoxy composites manufactured using a general braiding process are being evaluated for possible flight vehicle applications. This new process achieves a fully integrated, multi-dimensional orientation of the fibers and allows the braiding of complex shapes, such as I-beams and cylinders. Motivating this study is the desire to improve the impact resistance, thickness-direction strength, and overcome the delamination tendencies of conventional, laminated composites. Two styles of braided test coupons have been fabricated and tested. Style I is the basic (1 × 1 × 1) braid pattern. Style II is constructed by holding half of the yarns as straight columns and braiding the rest of the yarns about the fixed yarns. Some difficulties were encountered developing the processing methods to vacuum draw the hot melt Hercules 3501 resin into the “Celion 12000” graphite braided preforms and obtain a satisfactory autoclave cure. Results show the braid to have similar strength and elastic properties to corresponding, angle-plied laminates while greatly limiting the extent of impact damage. The braid does not increase the impact damage threshold, however. Tests performed on braided specimens with 6.35 mm (¼ in.) diameter open holes show no tensile strength reduction because of the hole. Bearing strength, transverse strength, and transverse stiffness properties are lower than laminated composites. Tensile fatigue properties are similar to conventional laminates, but compressive fatigue performance is worse.