Abstract
A precise fracture toughness testing method in ceramics is required to be established. In the present study, requirements to obtain the valid fracture toughness values of sintered silicon nitride ceramic was examined in the static three-point bending test and instrumented impact test. Moreover, it is generally considered that the toughness of ceramics is influenced by microcracking or phase transformation at the crack tip process zone. It is important, therefore, to understand and clarify the properties of the process zone. Measurement of residual stress by the X-ray method and transmission electron microscope (TEM) observation were carried out for this purpose.
It was shown that fracture toughness was not affected by crack length (a) to specimen width (W) ratio a⁄W and span length (S) to specimen width ratio S⁄W. However, the fracture toughness was affected by specimen thickness (B) and notch root radius (ρ). Static and dynamic fracture toughnesses increased with increasing process zone size.
The valid fracture toughness value was obtained by precracked type specimen thicker than 4 mm. This condition was represented by B≥70(KIC⁄σmc)2, where σmc shows a local critical fracture stress at the process zone.
