  The problem of the detachment of a sufficiently large flat indenter from a
plane adhesive viscoelastic strip of thickness "b" is studied. For any given
retraction speed, three different detachment regimes are found: (i) for very
small "b" the detachment stress is constant and equal to the theoretical
strength of the interface, (ii) for intermediate values of "b" the detachment
stress decays approximately as b^(-1/2), (iii) for thick layers a constant
detachment stress is obtained corresponding to case the punch is detaching from
a halfplane. By using the boundary element method a comprehensive numerical
study is performed which assumes a linear viscoelastic material with a single
relaxation time and a Lennard-Jones force-separation law. Pull-off stress is
found to consistently and monotonically increase with unloading rate, but to be
almost insensitive to the history of the contact. Due to viscoelasticity,
unloading at high enough retraction velocity may allow punches of macroscopic
size to reach the theoretical strength of the interface. Finally, a corrective
term in Greenwood or Persson theories considering finite size effects is
proposed. Theoretical and numerical results are found in very good agreement.
