Strength of flexible composites: Damage tolerance in compliant unidirectional fiber reinforced composites

Abstract

The strength of advanced materials and thin-film membranes for gossamer spacecraft may be enhanced by small-diameter fiber reinforcements, where the resulting composite material offers improved damage resistance and greater space environment survivability while maintaining flexibility and low mass. One of the potential advantages of decreasing the fiber diameter in fiber-reinforced composites is the resulting increase of surface area at the fiber-matrix interface, which, for example, may help to compensate for broken fibers and imperfect fiber-matrix bonding. In this paper, the load redistribution from broken fibers to unbroken neighboring fibers in a flexible matrix is investigated using 3D finite element (FE) micromechanical models under longitudinal tensile loading. The fiber load transfer characteristic length (Lc), also known as the ineffective fiber length, of a composite with reduced fiber diameter (RFC) is compared to that of a conventional fiber-diameter composite (CFC) with the same fiber volume fraction. The work presented here also compares fiber diameter scaling effects upon the initiation and evolution of fiber-matrix interfacial damage using a cohesive element fracture mechanics method. Results suggest that for a constant fiber volume fraction, as the fiber diameter is reduced both Lc and the maximum interfacial shear stress decrease in the damage models, while damage tolerance is increased.