Mechanical properties and failure mechanisms of 3D six-directional braided composites at elevated and liquid nitrogen temperatures

Abstract 3D six-directional braided carbon/epoxy composites are fabricated successfully, and their mechanical properties and failure mechanisms at room, elevated and liquid nitrogen temperatures are reported. The results show the temperature has an important influence on load-deflection curves and bending properties. All the curves are divided into three stages: initial linear elastic, nonlinear and final fracture stage. At room temperature, the linear characteristic is obvious and properties are excellent. The failure is a gradual process from external to internal due to good reinforcement of transverse yarns, and fibers are sheared and pulled out. At elevated temperatures, the curves decline successively and nonlinear features increase, with decrease of bending properties. The damage is matrix softening and plasticization, interface debonding. At liquid nitrogen temperature, zigzagged fluctuation feature increases and linear elastic characteristic strengthens. Bending properties have been improved significantly and failure is fibers shear fracture and formation of shear bands among fiber/matrix interface.