Modeling UD composite stiffness reduction due to multiple fiber breaks and interface debonding

Stiffness of a unidirectional composite (UD) containing fiber breaks with partial debonding is analyzed. Using divergence theorem exact relationships are obtained which link the entire stiffness matrix of the damaged UD composite with two robust parameters from the local solution: average opening displacement (COD) of the fiber break and its sliding displacement (CSD). Both are normalized with respect to the size of the fiber crack and to the far field stress in the fiber. The effect of the partial fiber/matrix debonding at the interface is included in the model through increasing crack opening. Using parametric inspection of obtained expressions it is shown that CSD does not affect the longitudinal stiffness and the COD effect on transverse and shear modulus and Poisson's ratio of the damaged composite is negligible. It motivated the decision to focus on COD and the longitudinal modulus. In this paper cracks are considered as non-interactive which makes the stiffness predictions conservative in a large fiber crack density region.The dependence of COD on fiber and matrix properties, fiber content, debond length etc is described by simple fitting functions obtained from extensive FEM based parametric analysis. FEM analysis is performed using a model which consists of three concentric cylinders: a) broken fiber; b) matrix cylinder; c) large cylinder of effective composite cylinder. The observed trends are described by simple fitting functions which with a high accuracy describe COD's of perfectly bonded and partially debonded cracks.Simulations performed for carbon and glass fiber polymer composites show the significance of debond length on the stiffness reduction.Godkänd; 2007; 20071128 (ysko