In our previous study, we have theoretically derived an equation of σ
m=ψK
ICS
mf1/2 based on the fracture theory for general hard materials, where σ
m is the flexural strength of test piece, ψ is a factor, K
IC is the fracture toughness and S
mf is the total macroscopic area of fracture surfaces of all fragments of one test piece, and also experimentally confirmed it's validity on two kinds of hard materials, i.e., WC-Co cemented carbides and Si
3N
4 base ceramics. In this study, the error due to stopped cracks (cracks which do not cause the formation of fragments, but generate internal fracture surfaces) in the relative value of K
IC estimated by using the above equation was investigated on WC-10mass%Co cemented carbide (the size of test piece is 4×3×36mm
3 and the span of ii e is 30mm).
The results obtained were as follows; (1)The total macroscopic area of internal fracture surfaces formed by stopped cracks (S
c) increased from 4.7 to 9.3mm
2 with increasing σ
m. However, the ratio of S
c/S
mf was almost constant; 0.05-0.06. (2)A linear correlation was found between σ
m and (S
m ?? +S
c)
1/2 in similar manner to σ
m-S
mf1/2 relation. The slope of the correlation line (ψK
cIC; K
cIC is the fracture toughness estimated by taking into consideration S
c) was nearly the same as that of σm-Smf1/2 correlation line (ψK
IC) in accordance with the results that K
cIC/K
IC expressed as {S
mf/(S
mf+S
c)}
1/2 was 0.97-0.98 and the error estimated by equation (K
IC-K
cIC)/K
cIC is about 2.5%. Therefore, it was concluded that the error due to stopped cracks or S
c is negligible.
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