  We revisit the delamination of a solid adhesive sheet under uniaxial
compression from a flat, rigid substrate. Using energetic considerations and
scaling arguments we show that the phenomenology is governed by three
dimensionless groups, which characterize the level of confinement imposed on
the sheet, as well as its extensibility and bendability. Recognizing that
delamination emerges through a subcritical bifurcation from a planar, uniformly
compressed state, we predict that the dependence of the threshold confinement
level on the extensibility and bendability of the sheet, as well as the
delaminated shape at threshold, varies markedly between two asymptotic regimes
of these parameters. For sheets whose bendability is sufficiently high the
delaminated shape is a large-slope "fold", where the amplitude is proportional
to the imposed confinement. In contrast, for lower values of the bendability
parameter the delaminated shape is a small-slope "ruck", whose amplitude
increases more moderately upon increasing confinement. Realizing that the
instability of the fully laminated state requires a finite extensibility of the
sheet, we introduce a simple model that allows us to construct a bifurcation
diagram that governs the delamination process.
