- A cohesive zone type interface model, taking full account of finite geometry changes, is used to study the decohesion of a viscoplastic block from a rigid substrate. The specific boundary value problem analyzed is a plane strain one with the imposed loading corresponding to overall uniaxial straining. The imperfection takes the form of a non-bonded portion of the interface. Dimensional considerations introduce a characteristic length into the formulation and the decohesion mode shifts from more or less uniform separation along the bond line to crack-like propagation as the ratio of block size to characteristic length increases. Field distributions prior to and accompanying propagation are displayed. 1990 Kluwer Academic Publishers.