Effect of Random Void Defects on the Mechanical Behavior of C/C Braided Composites

Carbon/carbon (C/C) composites are widely used in engineering due to their superior mechanical performance. However, manufacturing defects, such as voids and micro-cracks, seriously affect the C/C material and may initiate microstructural damage. In this paper, finite element models with random void defects are constructed to analyze the progressive damage behavior of C/C braided composites subjected to uniaxial tension. Because Hashin's criteria may not accurately predict the onset of failure for the braided composites, an improved damage initiation criterion based on Hashin's theory is proposed. A new damage evolvement model that is controlled by the material fracture energy of the textile composites and equivalence displacements is proposed. A Monte Carlo algorithm program is written to randomly assign individual elements as void defects in the carbon fiber yarns, carbon matrix, and fiber yarn-matrix interface. Based on numerical analysis, the influence of the volume fraction of voids in three zones on the mechanical properties of the C/C composites is investigated. The numerical simulation results show that although the braided fiber yarns reduce the effect of random void defects on the elastic modulus and tensile strength, the influence of matrix defects cannot be avoided.