Effects of micro-defects on the mechanical properties of CBCFS carbon-bonded carbon fiber composites

Carbon-bonded carbon fiber composites (CBCFs) consisted of chopped carbon fibers and the phenolic resin. The random carbon fiber bonded together with the carbonized phenolic resin and formed the fiber network material with an excellent thermal protective property. As the material forming process inevitably leads to defects within the material, evaluation on the effects of the defects on the mechanical and thermal properties is very necessary. In this paper, the effects of micro-defect on the mechanical properties of CBCFs are investigated by numerical simulation based on a 3D random fiber network model. Two typical defects, broken fiber and broken bonding joints are considered in the simulation. Broken fibers are realized by randomly removing fiber segment between two bonding joints in a perfect model, and failure at the bonding joints is realized by the randomly removing the bonding materials. The numerical simulation results show that the mechanical properties of the CBCFs are more sensitive to the connection failure than the broken fiber. In CBCFs material, fiber is divided into several fiber segments by the bonding joints. The load is transmitted through the bonding joints between the fibers. When one of the fiber segments is broken, the remaining fiber segments still have a load carrying capacity. If bonding joints between fibers fail, the stress in the fiber network is redistributed, and the force path is remarkably changed. In addition, the mechanical properties of fiber are better than those of bonding material which caused more bonding joints come into breakage.