In order to establish strength evaluation method of the high strength notch material which has an inclusion, tensile tests of material which have various notch radius and fracture toughness tests about high strength casting high speed steel materials were performed. The validity of considering that an inclusion is equivalent to a crack and considering the influence which the interference between an inclusion and a notch has on tensile strength in fracture mechanics using stress intensity factor were examined. Tensile fracture conditions were evaluated by defining one formulas which calculate the stress intensity factor which used √<area> parameter model as the tensile fracture conditions K_<IB>(ρ). As a result of the test, as for the high strength notch material containing many inclusions, the notch root maximum elastic stress at the time of the fracture varied, and it was shown clearly that the cause was because an inclusion size varies. Moreover, a strength evaluation with little scatter was possible by making K_<IB>(ρ) into fracture conditions. That is, it was shown clearly by considering that an inclusion is equivalent to a crack that it is effective to consider the influence which the interference between an inclusion and a notch has on tensile strength in fracture mechanics using a stress intensity factor.
