Fracture behavior and fracture toughness of pitch-based carbon fiber with an artificial notch introduced by a focused ion beam

Fracture behavior and fracture toughness of pitch-based carbon fiber were studied using the fiber specimens into which an artificial mode I type straight-fronted edge notch was introduced by a focused-ion (Ga ⁺)-beam. Two fiber samples were used for experiment. One was the as-supplied pitch-based fiber (GRANOC XN-35^®, Nippon Graphite Fiber Corporation) and another was the same fiber heat-treated at 3273 K for 3.6 × 10³ s in argon atmosphere. The heat-treatment raised the Youngs modulus from 370 to 840 GPa. The fracture toughness value of the as-supplied fiber was ~1.4 MPa m^1/2 and the apparent fracture toughness value of the heat-treated fiber was ~4.2 MPa m^1/2, which were estimated from the measured values of the strength (σ_f), notch depth (a), and fiber diameter (D) and correction factor. The increase in the apparent fracture toughness value due to the heat-treatment was attributed to the enhanced c-plane array and its cleavage fracture in longitudinal direction ahead of the notch tip. The analysis of the fracture strength - notch depth relation showed that the fracture of the as-supplied fiber obeys the fracture mechanical criterion and the fracture of the heat-treated fiber obeys the net stress criterion.