Strength and fracture toughness of carbon fibre polyester composites

Fracturing of carbon fibre/polyester composites has been studied by means of mechanical testing and scanning electron microscopy. Carbon fibres were surface-treated in several ways so as to vary the interlaminar shear strength of the composites, and the effect of this variation on the work of fracture was determined by means of Charpy V-notch impact tests and slow three-point bend tests on notched specimens of triangular cross-section. The effect of moisture on the fracture toughness was also studied by measuring toughness and interlaminar shear strength after exposure to steam. Improvement of the fibre/resin bond results, as expected, in an increase in the brittleness of composites and it appears that a purely mechanical bond, such as might be obtained by acid-etching the fibre surface, is less proof against deterioration in humid atmospheres than a chemical bond, such as can be obtained by the use of coupling agents. Estimates of the magnitude of various contributions to the fracture toughness show that in carbon-fibre-reinforced resins the effect of increasing the stiffness or load-bearing ability of the matrix and the work done against friction in pulling broken fibres out of the matrix contribute approximately one fifth and four fifths, respectively, of the total work of fracture.