In the past study on the transverse-rupture strength (σ
m) of WC-10mass%Co hardmetal, the authors had derived an equation of σ
d-1=σ
0-1+Kα
1/2 (σ
d; the external stress which operated on the fracture origin at the moment of fracture, σ
0; the intrinsic fracture strength, K; 2σ
0-1ρ
-1/2, ρ; the effective tip radius of microstructural defect which acted as the fracture origin, a; the half of the major-axis length of the fracture origin), and also had clarified that this equation holds on various kinds of hard materials. Furthermore, the slope value (K) of the σ
d-1-α
1/2 regression line was suggested to have a correlation with the fracture toughness (K
IC) in a similar way as the slope of σ
m-S
mf1/2 regression line (S
mf; total macroscopic fracture surface area of fragments). In this study, this suggestion was investigated on four kinds of hard or brittle materials such as high speed steel, WC-10mass%Co hardmetal, Si
3N
4 ceramics and Mn-Zn ferrite. It was found that the K has surely a strong correlation with K
IC, and K
IC can be expressed by an unified equation of K
IC??0.7K
-1.
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