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Theory Manual Version 3.6
 Subsection 5.2.6: Holmes-Mow Up Section 5.2: Compressible Materials Subsection 5.2.8: Donnan Equilibrium Swelling 

5.2.7 Conewise Linear Elasticity

Curnier et al. [36] formulated a model for describing bimodular elastic solids exhibiting orthotropic material symmetry. This can be derived from the following hyperelastic strain-energy function: where is the structural tensor corresponding to one of the three mutually orthogonal planes of symmetry whose unit outward normal is ( . The bimodular response is described by The material constants are the three shear moduli , three tensile moduli , three compressive moduli , and three moduli ( , where . The second Piola-Kirchhoff stress can be derived from this strain energy density function: The material elasticity tensor is then given by, It is important to note that although this model is objective, it should only be used for small strains. For large strains, the response may be unrealistic. The Cauchy stress is where and . The spatial elasticity tensor is In the special case of cubic symmetry the number of material constants reduces to four,
 Subsection 5.2.6: Holmes-Mow Up Section 5.2: Compressible Materials Subsection 5.2.8: Donnan Equilibrium Swelling