Factors controlling the strength of carbon fibres in tension

We have investigated the fracture mechanisms of different types of carbon fibres, in terms of skin-core differences in single fibres, flaw size and fracture toughness. The fibre strength distribution was measured precisely using the fragmentation test for single-fibre composites. The failure probability for intermediate/high modulus types fibres was found to be constant with fibre strength in the range 2–4 GPa, but in contrast the strength scatter for high modulus type fibres was reduced. The fracture toughness of the carbon fibres, determined by introducing notches with lengths in range 60–200 nm, was found to be about 1.1 MPa m1/2. The average flaw size of the carbon fibres increased with increasing fibre modulus, suggesting that the crack growth of surface flaws on the tens-of-nm scale occurred. This appears to be the main reason for the reduction in tensile strength during the carbonisation treatment.