MooneyRivlinElasticity.hpp

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#pragma once
 
#include <polyfem/assembler/GenericElastic.hpp>
#include <polyfem/assembler/MatParams.hpp>
 
// non linear MooneyRivlin material model
namespace polyfem::assembler
{
    class MooneyRivlinElasticity : public GenericElastic<MooneyRivlinElasticity>
    {
    public:
        MooneyRivlinElasticity();
 
        // sets material params
        void add_multimaterial(const int index, const json &params, const Units &units) override;
 
        const GenericMatParam &c1() const { return c1_; }
        const GenericMatParam &c2() const { return c2_; }
        const GenericMatParam &k() const { return k_; }
 
        std::string name() const override { return "MooneyRivlin"; }
        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
        {
            const double c1 = c1_(p, t, el_id);
            const double c2 = c2_(p, t, el_id);
            const double k = k_(p, t, el_id);
 
            const T J = polyfem::utils::determinant(def_grad);
            const T log_J = log(J);
 
            const auto F_tilde = def_grad / pow(J, 1.0 / size());
            const auto C_tilde = F_tilde * F_tilde.transpose();
            const auto I1_tilde = first_invariant(C_tilde);
            const auto I2_tilde = second_invariant(C_tilde);
 
            const T val = c1 * (I1_tilde - size()) + c2 * (I2_tilde - size()) + k / 2 * (log_J * log_J);
 
            return val;
        }
 
    private:
        GenericMatParam c1_;
        GenericMatParam c2_;
        GenericMatParam k_;
    };
} // namespace polyfem::assembler