抄録
In our previous study, we have theoretically derived an equation of σm=ψKICSmf1/2 based on the fracture theory for general hard materials, where σm is the flexural strength of test piece, ψ is a factor, KIC is the fracture toughness and Smf 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 Si3N4 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 KIC estimated by using the above equation was investigated on WC-10mass%Co cemented carbide (the size of test piece is 4×3×36mm3 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 (Sc) increased from 4.7 to 9.3mm2 with increasing σm. However, the ratio of Sc/Smf was almost constant; 0.05-0.06. (2)A linear correlation was found between σm and (Sm ?? +Sc)1/2 in similar manner to σm-Smf1/2 relation. The slope of the correlation line (ψKcIC; KcIC is the fracture toughness estimated by taking into consideration Sc) was nearly the same as that of σm-Smf1/2 correlation line (ψKIC) in accordance with the results that KcIC/KIC expressed as {Smf/(Smf+Sc)}1/2 was 0.97-0.98 and the error estimated by equation (KIC-KcIC)/KcIC is about 2.5%. Therefore, it was concluded that the error due to stopped cracks or Sc is negligible.
