The role of flaw geometry in thin film delamination from two-dimensional interface flaws along free edges

Delamination from planar interface edge flaws between a thin film and a semi-infinite substrate is examined to determine the roles of flaw width and depth relative to the film thickness. The flaws have curved and straight sections, and the crack front intersects the free edge at a right angle. Three-dimensional finite element models are used to extract local energy release rates and mode-mixity angles along the entire crack front. This paper focuses the crack dimensions required to reach steady state, wherein energy release rates are independent of flaw dimensions along the entire crack front. Results indicate that moderate elastic mismatch, although affecting mode mixity, plays a small role in determining the crack aspect ratios required to reach steady state. For wide cracks, the energy release rate for crack advance into the film interior approaches the plane-strain steady-state value when the half-width of the crack is approximately four times its depth (for cracks whose depths is several times the film thickness). For narrow cracks, the energy release rate near the free edge is significantly greater than the plane-strain steady-state result, and reaches a steady state when the depth approximately 10 times its width (for widths several time the film thickness). The results imply that delamination from wide cracks is reasonably accurately predicted via plane-strain analyses. Conversely, two-dimensional models are incapable of accurately predicting delamination from narrow cracks, which have a tendency to widen into flaws with more balanced aspect ratios (i.e. without growth in the depth direction).