Elastic constants of three-dimensional orthotropic composites with platelet/ribbon reinforcement

Unidirectional plane-parallel (UPP) platelet-or ribbon-reinforced composites have three orthogonal planes of symmetry. Their elastic behavior is characteristic of three-dimensional (3D) orthotropic materials, i.e. they possess nine independent elastic constants. Unlike the situation in unidirectional fiber composites, in which extreme behavior is observed along two orthogonal directions, the elastic constants (such as Young's modulus) along three orthogonal directions can differ significantly. An approach is proposed for the calculation of the Young's moduli of such 3D composite materials. The specific cases of unidirectional platelet- and ribbon-reinforced composites are presented as examples of particular relevance to advanced composite structures of present-day interest. While fiber-reinforced composites show lower specific strength and stiffness properties in directions perpendicular to the oriented fiber direction, UPP composites reinforced with platelets or ribbons offer superior stiffness properties in the plane of the lamina.