The elastic-plastic fracture toughness J_Ic tests by the stretched zone method were performed to obtain J_Ic values for machine structural steels. The validity of the J_Ic values obtained was evaluated, and the effects of the heat treatment conditions and microstructure of steels on the J_Ic values were investigated by use of streched zone width, SZW.
The results obtained are summarized as follows.
(1) The inclination of the blunting line in SZW versus _J curve is hardly affected by the condition of heat treatment for SCM440 steels, while it is affected obviously for S45C steels.
(2) The relation between SZW_C and σ_B varies with the difference of microstructure and with the ratio of ferrite area to pearlite area in ferrite-pearlite steels.
(3) SZW_C versus J_Ic/σ_fs diagram (in log. scale) can be arranged by two linear lines according to the difference of microstructure.