Microscale finite element analysis for predicting effects of air voids on mechanical properties of single fiber bundle in composites

Air voids produced in the manufacturing process have significant influence on the mechanical performances of single fiber bundle in carbon fiber-reinforced composites. The microscale finite element model of fiber bundle with voids is developed to predict mechanical properties and failure mechanisms. The failure responses of fiber and matrix in bundle are initiated by the maximum stress and Stassi failure criteria. The sudden stiffness degradation law is adopted to capture brittle behaviors. Both available theoretical models for voids and non-voids are used to validate the numerical model without voids and with different percentages of voids including 0.15, 0.5, 1, 2, 3, 4 and 5%. Effects of void contents on stress-strain responses of bundle are studied. The matrix damage development is studied regarding transverse compression and out-of-plane shear loading. The micro-stress analysis of matrix and voids in three-RUC model under different loadings is performed. The simulated elasticity and strength properties correlate well with theoretical results for voids and non-voids. The strengths and moduli gradually decrease with increasing void contents except longitudinal properties. The stress concentrations of bundle are mainly influenced by the loading direction.