Ceramics are typical brittle materials, and fracture occurs owing to crack initiation caused by stress. Therefore, it is very important to measure the fracture toughness for evaluating the mechanical properties of ceramics. In this work, the fracture toughness of various kinds of ceramics were measured by the bending fracture of notched beams, and also the effect of testing conditions were examined from the stress-distribution of two dimensional photoelasticity experiment with epoxy-resin models. As a result, the notch depth was an effective factor on the
KIc value, and the notch width was the most effective one. The effect of testing span was also recognized in the test of silicon carbide, And energy release ratio
gIc is calculated from the obtained
KIc value and experimental elastic constant of ceramics which was measured by means of ultrasonic pulse echo method. HPSN shows the biggest value of
gIc from 160 to 220J/m
2 whereas the
gIc of HPSiC was the smallest value 33J/m
2, and this result indicates that the latter material fracture easily. In general, the
KIc values of ceramics increase as the width of notch become larger, the testing span smaller, and the depth of notch larger. It is because these factors mentioned above increase the fracture load relatively. In the three-point bending test, the smallest value of
KIc for ceramics was obtained from the testing condition in the notch width and depth of 0.3 and 1.25mm, the testing span of 40mm, and specimen size of 5×5×60mm long, in this study. The ceramic materials, the
KIc values of which are liable to increase, have larger values of
E/σ
b, and this result showed that the elastic properties gave the effect on the measurement of
KIc values for ceramics.
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