Interaction of an edge dislocation with a thin-film-covered crack

The interaction of an edge dislocation with a thin-®lm-covered crack under mode I and/or mode II loadings is studied in order to investigate the e€ects of a passive ®lm on stress-corrosion cracking. The passive ®lm is modeled to be an ellipse in which there is a crack. The ellipse is embedded in an in®nite medium under remote loadings. Superposition, Cauchy integration, specially constructed functions, and series expansion technique are used to do as much analytical calculation as possible of the stress ®elds in the ®lm and the substrate. The image force and the shielding e€ect are calculated and the critical stress intensity factor for dislocation emission from the ®lm-covered crack tip is investigated on the basis of the Rice±Thomson model. The results show that the slip component of the image force of an edge dislocation in the ®lm increases with increasing the ratio of the ®lm shear modulus over the substrate shear modulus, as does the shielding e€ect of the dislocation on the crack tip. Since the presence of the thin ®lm changes both the image force of the dislocation and the local stress ®eld due to the applied loads, the nominally critical stress intensity factor for dislocation emission is related to the thin ®lm thickness, the properties of the ®lm and the loading conditions. For a given loading mode and crack length, there is a critical value of the ®lm thickness at which the ®lm does not in¯uence the dislocation emission. When the ®lm thickness is smaller than the critical one, a harder thin ®lm makes the dislocation emission easier and a softer ®lm makes the dislocation emission more dicult. The opposite is also true if the ®lm thickness is larger than the critical value for a given crack length