Spline model for strain energy

[violetus]

@haileyfromde

# Define the strain energy density function
        def strain_energy_density(u, strain):
            energy = inner(dot(F, strain), grad(u))
            return energy

You need to apply different energy model to get the type of response you want, so the result you are getting is most probably from Neo-Hookean energy model. I am getting a vague idea that what you are attempting to simulate is viscoelasticity therefore I would suggest you to go through this post Issues with a viscoelastic problem.