Effective bending modulus of thin-ply composites with non-uniform fibre spacing

Composite materials are gaining increasing interest for design purposes in many industries such as aeronautical and automotive, not only because of their reduced weight, but also due to their enhanced mechanical properties. The heterogeneity inherent in the nature of composites arises not only because of the mismatch in fibre/matrix properties but also due to the fibre arrangement within the matrix. Previous studies have shown that classical homogeneous elasticity models can either underestimate or overestimate stiffness properties for small ply thicknesses. As such, both positive (stiffer) and negative (more compliant) size effects become increasingly prevalent with decreasing ply thickness, when considering regular distributions of fibres within a matrix material. Current work addresses the effects of irregular fibre spacing on size effects and identifies a characteristic length that can be used to represent the heterogeneity in an equivalent homogeneous micropolar model. Results show that, based on the type of irregular fibre spacing, both kinds of positive and negative size effect arise in a composite ply. As a result, the bending characteristic length embedded in micropolar theory can be defined even for the difficult case whereby fibres are stiffer than matrix depending on the nature of the irregular fibre spacing.