NeoHookeanElasticityAutodiff.hpp

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#pragma once
 
#include <polyfem/assembler/GenericElastic.hpp>
#include <polyfem/assembler/MatParams.hpp>
 
namespace polyfem::assembler
{
    class NeoHookeanAutodiff : public GenericElastic<NeoHookeanAutodiff>
    {
    public:
        std::string name() const override { return "NeoHookeanAutodiff"; }
        // Used only for testing, no need to output
        std::map<std::string, ParamFunc> parameters() const override { return std::map<std::string, ParamFunc>(); }
 
        NeoHookeanAutodiff();
 
        // sets material params
        void add_multimaterial(const int index, const json &params, const Units &units) 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 log_det_j = log(polyfem::utils::determinant(def_grad));
            const T val = mu / 2 * ((def_grad * def_grad.transpose()).trace() - size() - 2 * log_det_j) + lambda / 2 * log_det_j * log_det_j;
 
            return val;
        }
 
    private:
        LameParameters params_;
    };
} // namespace polyfem::assembler