Abstract: Both hydrogen induced cracking and overload crack initiated at same characteristic distance, r~*,within the plastic zone along the slip line when the plastic zone developed to a critical ex- tent.For the overload crack. K_(IC)=αr~*~(1/2)[σ_F~((n+1)/2n)/σ_(ys)~((1-n)/2n)],σ_F=σ_0+g[2μbσ_(th)/π~2L(1-v)]~(1/2) For the hydrogen induced cracking: K_(IH)=αr~*~(1/2)[σ_F(H)~((n+1)/2n)/σ_(ys)(H)~((1-n)/2n)], σ_F(H)={σ_0(H)+g[2μbσ_(th)(H)/π~2L(1-v)]~(1/2)}/k Hydrogen pomoting the dislocation multiplication and motion would result in σ_0(H)<σ_0, k>1,Therefore,hydrogen promoting the cleavage fracture in titanum aluminide can be due to that hydrogen facilitates the local plastic deformation,which results in σ_F(H)<σ_F and then K_(IH)

Key words: Ti_3Al, intermetallic compound, hydrogen induced crack