Effects of Process Zone and Specimen Geometry on Fracture Toughness of Silicon Nitride Ceramic
A precise fracture toughness testing method in ceramics should be established in an early period.
In the present study, requirements to obtain the valid fracture toughness values of sintered silicon nitride ceramic are 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 a crack tip process zone. It is important, therefore, to understand and clarify the feature of process zone. Measurement of residual stress by X-ray method and transmission electron microscope(TEM) observation are 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 the process zone size.
The valid fracture toughness value was obtained by precracked type specimen thicker than 4mm. This condition was represented by B≥70(KIC/σmc)2, where σmc is a local critical fracture stress at the process zone.
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