In the previous studies on the fracture of hard or brittle materials such as hardmetals, cermets, ceramics and glass whose un-notched test piece fractures to several fragments, we have semi-theoretically derived and experimentally verified a new equation which correlates the total macroscopic fracture surface area (S_mf) of all fragments of each un-notched test piece and its transverse-rupture strength (σ_m) with the fracture toughness (K_IC) of the material: σ_m=ΨK_ICS_mf^1/2. Here, Ψ is a factor including test piece size and test method, etc., and the value of Ψ was clarified to hardly depend on the kind of materials.
In this study, we investigated whether this equation can be applied to ductile hot tool alloy steels which have extremely higher KIC and fracture not to several fragments, but to only two fragments even in the case of un-notched test piece, differing from the above hard or brittle materials. As the result, it was concluded that the equation substantially can be applied also to such ductile steels if a sharply notched test piece showing a brittle fracture mode is used.