IsochoricNeoHookean.hpp

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#pragma once
 
#include <polyfem/assembler/GenericElastic.hpp>
#include <polyfem/assembler/MatParams.hpp>
 
namespace polyfem::assembler
{
    class IsochoricNeoHookean : public GenericElastic<IsochoricNeoHookean>
    {
    public:
        IsochoricNeoHookean();
 
        // sets material params
        void add_multimaterial(const int index, const json &params, const Units &units) override;
 
        std::string name() const override { return "IsochoricNeoHookean"; }
        std::map<std::string, ParamFunc> parameters() const override;
 
        template <typename T>
        T elastic_energy(
            const RowVectorNd &p,
            const double t,
            const int el_id,
            const DefGradMatrix<T> &def_grad) const
        {
            double lambda, mu;
            params_.lambda_mu(p, p, t, el_id, lambda, mu);
 
            const T J = polyfem::utils::determinant(def_grad);
            const auto Cbar = (def_grad.transpose() * def_grad / pow(J, 2.0 / 3.0)).eval();
            const T I1_bar = Cbar.trace();
 
            // Isochoric NK strain energy: Psi_iso = mu/2 * (I1_bar - 3)
            return (mu / 2.0) * (I1_bar - T(3.0));
        }
 
    private:
        LameParameters params_;
    };
} // namespace polyfem::assembler