ElementADHyperelastic.h

// From ElementADPK2:
virtual void computeSecondPiolaKirchhoff(Vec3DiffMatrix3x3 &matS, const Vec3DiffMatrix3x3 &matF, const MaterialParams &matParams) const {
        
        //*****************ASSIGNMENT 4 START HERE ***********************
        //Compute Strain
        // Vec3DiffMatrix3x3 matE =
        //Compute Second Piola Kirchoff Stress (S)
        // matS = 

        // matF is the deformation matrix
        Vec3DiffMatrix3x3 matE = 0.5 * matF.transposed() * matF - Vec3DiffMatrix3x3::id();
        //Vec3DiffMatrix3x3 matE = 0.5 * (matF.transposed() + matF) - Vec3DiffMatrix3x3::id();
        matS = matParams.getLambda() * matE.trace() * Vec3DiffMatrix3x3::id() + 2 * matParams.getMu() * matE;
        //*****************ASSIGNMENT 4 END HERE **************************
}