A theoretical analysis of hydrogen-assisted intergranular brittle fracture of high strength steels has been made. In this analysis the functional relationship between the cohesive energy and hydrogen coverage was derived for the case of the solute equilibrium constraint during the decohering process. In the presence of the triaxial stress field, this relationship was also evaluated and discussed. The variation of the threshold stress intensity $K_{th}$ with hydrogen fugacity was calculated using a criterion for hydrogen-assisted intergranular fracture, and was also considered to be related to the effects of several material parameters, such as trap binding energy of grain boudary, yield strength and work hardening exponent. In particular the fracture mode transition of hydrogen-assisted cracking was discussed as related to the effects of hydrogen on the $K_{th}$ necessary for occurrence of the respective fracture modes.
By using this concept, the interpretative analyses of the susceptibility index to stress-corrosion cracking (SCC), $(K_IC-K_{ISCC})/K_{IC}$, and of stress-corrosion (SC) crack propagation rate v have been proposed in order to characterize the susceptibility to SCC of commercial Al-Zn-Mg-Cu (AA 7075) alloy. For this purpose, the crack propagation rate vs. stress intensity factor relationships were measured for the various heat treatment conditions. The both quantities, $(K_{IC}-K_{ISCC})/K_{IC}$ and v, are well-established as measures of SCC susceptibility. The former is physically based upon the plastic deformation that usually accompanies the SC crack propagation, and the latter is associated with the $K_{th}^0$ or $K_{ISCC}$. The index of $(K_{IC}-K_{ISCC})/K_{IC}$ is not physically associated with the SC crack propagation rate. Considering the value of both indices of SCC susceptibility of Al-Zn-MG-Cu alloy in the aqueous 3.5 wt.% NaCl solution, peak-aged specimen and over-aged or under-aged specimen decrease in the $(K_{IC}-K_{ISCC})/K_{IC}$ value in that order, whereas peak-aged, over-aged and under-aged specimens decrease in the v value in that order.
And SC crack propagation in AISI 4340 steel has been studied with 2 mm thick single edge-notched (SEN) specimens under constant load condition as a function of applied potential and tempering conditions in an aqueous 3.5 wt.% NaCl solution at 30 C. The SC crack length was estimated by using the electrical potential method. As the amount of cathodic polarization increased, the SC crack propagation rate increased. Anodic polarization yielded the opposite results. These polarization effects on the SC crack propagation were discussed in terms of the absorbed hydrogen resulting from a cathodic reaction on the specimen surface. The SC cracks propagated by intergranular mode through most of inner region, but shear lips were formed at the near subsurface, irrespective of applied potential and tempering temperature. This is explaned in terms of the stress state dependency of hydrogen behaviour. Experimental evidences above support the theory that SC crack propagation is controlled by hydrogen embrittlement process. The SC crack propagation rate decreased in the order of 300, 200, and 400 C-tempered specimens. This was discussed in terms of the microstructural and yield strength effects.
Iodine-induced SCC of Zircaloy-4 plate specimens has been studied by constant elongation rate test (CERT) and U-bend test methods. In order to systematically evaluate effects of stress states on the SCC behaviour, four kinds of specimens were prepared from as-annealed Zircaloy-4 plates. The uniaxial tensile and SEN specimens fractured in the ductile manner, not by SCC. However, SCC resulted in the plane strain tensile specimens and deep-notched U-bend specimens As strain rate was lowered, the susceptibility to SCC increased. The SC cracks propagated by the transgranular brittle fracture mode for as-annealed specimens, whereas by the inter-and transgranular mixed fracture mode for re-annealed specimens. The SCC processes of Zircaloy-4 in iodine gas were discussed in terms of the effects of stress states, the strain rates, and the yield strengths. The present experimental results suggest that the iodine-induced SCC processes can be explained by an iodine-diffusion model.
