Influence of fiber volume fraction on mode III interlaminar fracture toughness of glass/epoxy composites

The influence of fiber volume fraction on the mode III interlaminar fracture toughness, GIIIC, of a glass/epoxy composite was examined using the edge-cracked torsion specimen. It was found that GIIIC corresponding to onset of crack propagation substantially increased with decreasing fiber volume fraction. The fracture surfaces of broken specimens were inspected in optical and scanning electron microscopes. The higher initiation fracture toughness for the low fiber volume fraction composite is attributed to the increased matrix volume allowing for increased plastic deformation energy dissipation in the matrix resin prior to macroscopic crack propagation. In addition to the primary delamination crack, both the low and high fiber volume fraction specimens displayed intralaminar matrix cracks in the 90° plies adjacent to the crack plane, and such cracks were particularly numerous in the low fiber volume fraction specimens which contribute to the resistance to crack propagation.