Fracture toughness of PAN-based carbon fibers estimated from strength–mirror size relation

Fracture toughness ( K IC) of representative high-strength type PAN (polyacrylonitrile)-based carbon fibers, Torayca™ T300 and T800H, with or without artificial surface defects, were estimated to be ca. 1 MPam 1/2 from the tensile strength vs. fracture mirror size relation, assuming a constant crack-to-mirror size ratio. The corresponding critical energy release rate ( Γ) was ca. 7.4 J m −2, which was close to the value derived from the reported surface energies for a graphite crystal. Similar K IC values were obtained for the old-type PAN-based carbon fibers from the reported data by the use of the present estimation procedure.