  A non-linear history-dependent cohesive zone model of crack propagation in
linear elastic and visco-elastic materials is presented. The viscoelasticity is
described by a linear Volterra integral operator in time. The normal stress on
the cohesive zone satisfies the history dependent yield condition, given by a
non-linear Abel-type integral operator. The crack starts propagating, breaking
the cohesive zone, when the crack tip opening reaches a prescribed critical
value. A numerical algorithm for computing the evolution of the crack and
cohesive zone in time is discussed along with some numerical results.
