We determine the general form of a rotationally invariant non-symmetric stiffness matrix of a viscoelastic medium. It is described by three additional parameters in comparison with a rotationally invariant symmetric stiffness matrix of a transversely isotropic (uniaxial) viscoelastic medium. As a consequence, we find that the stiffness matrix of an isotropic viscoelastic medium is always symmetric.
Viscoelastic media, stiffness tensor, elastic moduli, transverse isotropy, symmetry axis.
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