Hencky

struct Hencky <: ConstitutiveModels.AbstractHyperelasticModel{2, 0}

$\psi = \frac{1}{2}\kappa tr\left(\mathbf{E}\right)^2 + \mu\mathbf{E_0}:\mathbf{E_0}$

helmholtz_free_energy(
    _::Hencky,
    props,
    Δt,
    ∇u,
    θ,
    Z,
    args...
) -> Tuple{Any, Any}

initialize_props(_::Hencky, inputs::Dict{String}) -> Any