Viscoelastic solid constitutive models cannot be defined by a Helmholtz free energy density and a viscous dissipation function.
These constitutive models are therefore defined by a relation for the stress as a function of the deformation gradient and rate.
Consequently, the rate tangent stiffness associated with the first Piola-Kirchhoff stress is not symmetric for these models.
```math
\mathcal{U}_{iJkL} \neq \mathcal{U}_{kLiJ}
```