The Hencky elastic solid constitutive model.
**Parameters**
- The bulk modulus $`\kappa`$.
- The shear modulus $`\mu`$.
**External variables**
- The deformation gradient $`\mathbf{F}`$.
**Internal variables**
- None.
**Notes**
- The Hencky strain measure is given by $`\mathbf{H}=\tfrac{1}{2}\ln(\mathbf{C})`$.