  We study the propagation of guided elastic waves in a highly-stretched
Ecoflex\c{opyright} plate, a nearly incompressible elastomer. The plate is
subjected to a nearly-uniaxial stress with an elongation reaching 120% and we
measure in-plane displacements of the shear horizontal mode SH0 and of the
plate mode S0 coexisting in the low frequency limit. An induced anisotropy is
first observed and characterized by following the phase velocities in two
principal directions. Although these measurements provide an initial stress
estimate, we evidence the limits of the acoustoelastic theory to predict those
phase velocities in a prestressed elastomer. Taking into account the frequency
dependent shear modulus of the elastomer, an experiment-driven fractional
rheological model is added to the theory. This provides a proper prediction of
phase velocities up to 80% elongation.
