Effect of fiber waviness and cluster on effective elastic properties of fiber-reinforced composites

Carbon fiber-reinforced polymers (CFRPs) are widely applied in lightweight automotive design due to their high specific stiffness and strength. However, it is difficult to achieve an ideal performance due to defects from manufacturing process. For example, fiber waviness and cluster are two typical manufacture defects in the process of Liquid Composite Molding (LCM). The effective elastic modulus of CFRPs with fiber waviness is studied by finite element method, and the stress-transfer mechanisms between the fibers and matrix is explained theoretically, the modified bridging model, which correlates the stress state in fibers with that in matrix. Additionally, the influence of fiber cluster on the effective elastic modulus of CFRPs is also investigated by finite element method. When the volume fraction of fibers in matrix is higher than 20%, the fiber waviness will decrease the longitudinal modulus E₁₁ greatly, but also increase the transverse modulus E22 slightly. For CFRPs with cured fibers, the proposed modified bridging model has higher prediction accuracy than the rule of mixture. When the cluster degree is less than 1%, the effective elastic modulus of CFRPs with fibers distributed randomly will not be influenced obviously. For straight fibers, the reinforcement effect on elastic modulus ₁₁ and shear modulus G₁₂ decreases slightly with increasing cluster degree.