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

The interaction of an edge dislocation with a thin-film-covered crack under mode I and/or mode II loadings is studied in order to investigate the effects of a passive film on stress-corrosion cracking. The passive film is modeled to be an ellipse in which there is a crack. The ellipse is embedded in an infinite 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 fields in the film and the substrate. The image force and the shielding effect are calculated and the critical stress intensity factor for dislocation emission from the film-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 film increases with increasing the ratio of the film shear modulus over the substrate shear modulus, as does the shielding effect of the dislocation on the crack tip. Since the presence of the thin film changes both the image force of the dislocation and the local stress field due to the applied loads, the nominally critical stress intensity factor for dislocation emission is related to the thin film thickness, the properties of the film and the loading conditions. For a given loading mode and crack length, there is a critical value of the film thickness at which the film does not influence the dislocation emission. When the film thickness is smaller than the critical one, a harder thin film makes the dislocation emission easier and a softer film makes the dislocation emission more difficult. The opposite is also true if the film thickness is larger than the critical value for a given crack length. (C) 2000 Elsevier Science Ltd. All rights reserved.