Abstract: The relationship between the stress intensity factor K_1~* required for brittle crack initiation and propa- gation and the fractal dimension D_F of the fracture surface was derived,i.e, InK_1~*=(1/2)In2γE'+(1/2)In(d_f/L_0)(1-D_F) The real surface energy can be calculated based on the measured linear relation of the InK_1~* vs D_F, The equation is not only suitable for the overload fracture but also for the delayed fracture,e.g. hydrogen induced cracking (HIC) and stress corrosion cracking (SCC).The experiment results showed that the hydrogen induced delayed cracking occurred in the Ti-24AI-11 Nb alloy during dy- namic charging,and the threshold stress intensity factor was very low,i.e..K_(IH)/K_(IC)= 0.43.The ex- perimental relationship between the stress intensity factor K_1~* and D_F was consistent with the theo- retical equation.

Key words: titanium aluminide, hydrogen induced cracking, tractal dimension