EFFECTS OF SURFACE-DIFFUSION AND RESOLVED SHEAR-STRESS INTENSITY FACTOR ON ENVIRONMENTALLY ASSISTED CRACKING

Hydrogen induced crack-tip plastic deformation has been known as the p rimary mechanism of hydrogen assisted cracking and stress corrosion cr acking. It has been systematically shown that the same mechanism of en vironmentally assisted crack-tip dislocation emission causes hydrogen assisted cracking, stress corrosion cracking, and liquid metal embritt lement cracking. An embrittling chemical species has to reach a crack tip in order to accelerate crack growth. Very close to a sharp crack t ip, surface diffusion is shown to be the dominant transport process of embrittling species for stage-II crack growth. The role of surface di ffusion in stage II crack growth is analyzed. The constant cracking ve locity is proportional to the surface diffusion coefficient of an embr ittling species and inversely proportional to a length parameter, rho, which is related to the transport process upstream. Dislocation emiss ion at a crack tip is driven by crack-tip resolved shear stress. Crack -tip resolved shear stress field is characterized by resolved shear st ress intensity factor, K-RSS. K-RSS is defined, the procedure for its calculation outlined, and its applications to crack-tip dislocation em ission and environmentally assisted cracking discussed.