INVESTIGATION OF FRACTAL DIMENSIONS OF HYDROGEN-INDUCED BRITTLE-FRACTURE OF TITANIUM ALUMINIDE

A relation between the stress intensity factor K1 required for brittl e crack initiation and propagation and the fractal dimension D(F) of t he fracture surface was derived, i.e. lnK1 = (1/2)ln 2gammaE' + (1/2) ln(d(f)/L0)(1 - D(F) where d(f) is the fracture unit, L0 is the length of the straight projection line of the fracture profile, gamma is the real surface energy, and E' = E (plane stress) or E/(1 - nu2) (plane strain). The real surface energy can be calculated on the basis of the measured linear relation of ln K(I) vs. D(F). The equation is not on ly suitable for overload fracture but also for delayed fracture, e.g. hydrogen-induced cracking and stress corrosion cracking. The experimen t results showed that the hydrogen-induced delayed cracking occurred i n Ti-24Al-11Nb alloy during dynamic charging, and the threshold stress intensity factor was very low, i.e. K(IH)/K(IC) = 0.43. The experimen tal relation between the stress intensity factor K(I) and D(F) was co nsistent with the theoretical equation.