  Isothermal visco-elastodynamics in the Kelvin-Voigt rheology is formulated in
the spatial Eulerian coordinates in terms of velocity and deformation gradient.
A generally nonconvex (possibly also frame-indifferent) stored energy is
admitted. The model involves a nonlinear 2nd-grade nonsimple (multipolar)
viscosity so that the velocity field is well regular. To simplify analytical
arguments, volume variations of the solid material are assumed to be only
rather small so that the mass density is constant, exploiting the concept of
semi-compressible materials. Existence of weak solutions is proved by using the
Galerkin method combined with a suitable regularization, using nontrivial
results about transport by smooth velocity fields.
