Evaluation of Fracture Toughness Using SENB Specimens with Precrack Produced by Growing Microcrack Around Vickers' Indent

Fracture toughness of hot-pressed silicon nitride was measured using the three-point bending test of a SENB specimen with precrack developed from Vickers' indent. Indentation loads used were 10, 30 and 50kgf. The effect of indentation load on the evaluation of fracture toughness was investigated. Fracture toughness values were found to decrease with increasing indentation load. The heat treatment of specimens (at 1000°C for 1h at 10-5Torr) after indentation reduced the dependence of the fracture toughness evaluation on indentation load. It was shown that fracture toughness values were influenced by residual stresses caused by Vickers indentation even after precrack was developed.