Abstract: Because of the technical diffculty to create an ideal plannar crack with zcro root radius, macronotchedspecimen are of ten prepared for testing the fracture toughness of ceramics. In this paper, the quantitativeexpression of fracture toughness of ceramics and notch radius was investigated. Based on the theory thatthere exists a fracturc proccss zone in front of crack tip and average stress fracture model, a relationbetween fracture toughness and notch radius was derived, with this relation, the fracture toughness of ceramics can be predicted from tested value of macronotched specimens. Combined the fracture process zone with the experiment results from the available literature, it is showed that the scale of fracture process zone is four times of average grain diameter. Compared with the experimental results, we have a conclusion that when the notch radius reach to the average grain diameter, the tested results can yield the true value K_IC under 6% deviation.

Key words: fracture toughness, notch radius, ceramics