Measurement of Fracture Toughness and Effect of Loading Rate on its Value in PSZ and Si<SUB>3</SUB>N<SUB>4</SUB> Ceramics

Although there are many researches on static fracture toughness tests of ceramics in the literature, the testing method has not yet been standardized. On the other hand, only a few reports dealt with dynamic fracture toughness tests of ceramics have been observed. Moreover, it becomes a problem that conventional methods of static fracture toughness testing have been applied mainly to specimens without precracking. Therefore, the values of static fracture toughness obtained by the methods are lacking in reliability. Moreower, it is very difficult to measure the real fracture load in the dynamic fracture toughness test because of a remarkable inertial loading effect.In the present study, static fracture toughness of ceramics was measured by a three-point bending test using precracked specimens. The results were compared with those obtained by the conventional methods. Moreover, the dynamic fracture toughness of ceramics was measured by the impact response curve method using a strain gage attached to the specimen in the instrumented Charpy impact test.As a result, it has been shown that the static fracture toughness test using precracked specimens can evaluate the lower limits of static fracture toughness in contrast with the conventional methods. Dynamic fracture toughness Kd in partially stabilized zirconia has been observed to increase monotonously with loading rate \dotK, while Kd in Si3N4 decreases at first and then increases with increase of \dotK. This phenomenon suggests the existence of a minimum Kd value in ceramics. It has also been made clear that a rather good correlation exists between Kd and the total absorbed energy Et of the unnotched impact test specimen when the latter value is corrected by the fracture unit distance.