  Crack propagation in viscoelastic materials has been understood with the use
of Barenblatt cohesive models by many authors since the 1970's. In polymers and
metal creep, it is customary to assume that the relaxed modulus is zero, so
that we have typically a crack speed which depends on some power of the stress
intensity factor. Generally, when there is a finite relaxed modulus, it has
been shown that the toughness increases between a value at very low speeds at a
threshold toughness G0, to a very fast fracture value at Ginf, and that the
enhancement factor in infinite systems (where the classical singular fracture
mechanics field dominates) simply corresponds to the ratio of instantaneous to
relaxed elastic moduli.
  Here, we apply a cohesive model for the case of a bimaterial interface
between an elastic and a viscoelastic material, assuming the crack remains at
the interface, and neglect the details of bimaterial singularity. For the case
of a Maxwell material at low speeds the crack propagates with a speed which
depends only on viscosity, and the fourth power of the stress intensity factor,
and not on the elastic moduli of either material. For the Schapery type of
power law material with no relaxation modulus, there are more general results.
For arbitrary viscoelastic materials with nonzero relaxed modulus, we show that
the maximum toughness enhancement will be reduced with respect to that of a
classical viscoelastic crack in homogeneous material.
