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((TM)) 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 (Gamma) 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. (C) 2003 Elsevier Science Ltd. All rights reserved.