Failure behavior of three-axis woven carbon/carbon composites under compressive and transverse shear loads

This work examines the responses of three-dimensional carbon/carbon composites under axial compression and transverse shear. By using a 3D weaving technique, two types of preforms with different bundle sizes of the weaving yarns were prepared for assessing its influence on the failure behavior. Carbon yarns were arranged in a three-axis, orthogonal form with interlacing loops on outer surfaces. The carbon matrix was added by using a phenolic resin as the matrix precursor. Resin transfer molding was used for resin impregnation, followed by the curing and carbonization of the resin. The matrix filling was repeated up to five cycles, and the efficiency of the matrix filling was examined. Special fixtures were designed for applying the loads to the 3D composites. Microscopic observations on the induced damage were carried out. The axial yarns were found to undergo bending fracture in the compressive tests. The yarn imperfection, rather than the fiber misalignment, was the major failure-determining factor. The bending of the yarns is analyzed, and the critical value of imperfection that leads to bending fracture is given. The transverse shear resulted in complex but intriguing damage modes, which are closely related to the surface loops and through-thickness yarns. Some keys for preform design to best withstanding the shear are discussed.