Compressive damage in two-step braided thermoplastic composites

This work examines compression-induced damage in a 3D, braided carbon/nylon 6 composite. The two-step braiding was used to form the fabric, and the yarn used was a carbon tow commingled with nylon 6 powder. About 82% of overall fibers were arranged along the axial direction, and the rest were the braiding fibers. A matched mold was designed for melting the resin and pressing the fabric into a predetermined thickness for the resulting composite. The molding temperature was varied to examine its influence on the compressive properties. The compressive tests were carried out by using an end-supported, end-loaded fixture designed for thicker specimens. The results showed that the molding temperature greatly affected the composite modulus and strength. Microscopic observations revealed that the induced damage modes differ from those in 3D thermosetting composites. Miniscopic failure of the yarns appears to be the dominant mode. Two types of damage were observed. One is miniscopic kink-band caused by the through-thickness shear stress; the other is the folding of the axial yarns into a corrugation form. These modes are related to the 3D structure and the interfacial bonding condition. Some key features of the capability of energy absorption of this material are also discussed.