In the fracture toughness tests of perfectly brittle and crack-rate sensitive ceramics (R∝a^m*, R; the crack resistance force, a; the crack velocity), it has been analytically as well as experimentally clarified that an initial crack begins to extend before a maximum load appears in load-deflection curve. This crack-extension phenomenon was named PSP (psuedo-stable phenomenon) in the previous paper. The amount of the crack-extension due to the PSP depends on the crack-rate sensitivity parameter m^*, the non-dimensional compliance of testing machine EBΛ^* and the initial crack length x₀.
Nevertheless, the fracture toughness value, K_IC has been currently determined by the maximum load and the precracked crack-length measured before testing. In the testing regulation for high performance ceramics in JIS, the K_IC is also evaluated in the same way.
For a single edge cracked 3 point bending specimen of JIS, the amount of crack-extension is numerically calculated as a function of m^*, EBΛ^* and x₀, and it is studied whether or not the K_IC is evaluated exactly. The results obtained show that the K_IC is reasonably evaluated when both m^* and EBΛ^* are small enough, but the K_IC is considerably underestimated when these values are large.