Advanced elasticity/Neo-Hookean material
The response of a neo-Hookean material, or hyperelastic material, to an applied stress differs from that of a linear elastic material. While a linear elastic material has a linear relationship between applied stress and strain, a neo-Hookean material does not. A hyperelastic material will initially be linear, but at a certain point, the stress-strain curve will plateau due to the release of energy as heat while straining the material. Then, at another point, the w:elastic modulus of the material will increase again.
This hyperelasticity, or rubber elasticity, is often observed in polymers. Cross-linked polymers will act in this way because initially the polymer chains can move relative to each other when a stress is applied. However, at a certain point the polymer chains will be stretched to the maximum point that the covalent cross links will allow, and this will cause a dramatic increase in the elastic modulus of the material. One can also use thermodynamics to explain the elasticity of polymers.
Neo-Hookean Solid Model 
The model of neo-Hookean solid assumes that the extra stresses due to deformation are proportional to Finger tensor:
The strain energy for this model is:
Usually the model is used for incompressible media.
The model was proposed by w:Ronald Rivlin in 1948.
Uni-axial extension 
Under uni-axial extension from the definition of Finger tensor:
where is the elongation in the w:stretch ratio in the -direction.
Assuming no traction on the sides, , so:
where is the strain.
The equation above is for the true stress (ratio of the elongation force to deformed cross-section), for w:engineering stress the equation is:
For small deformations we will have:
Thus, the equivalent w:Young's modulus of a neo-Hookean solid in uniaxial extension is 3G.
Simple shear 
For the case of w:simple shear we will have:
where is shear deformation. Thus neo-Hookean solid shows linear dependence of shear stresses upon shear deformation and quadratic w:first difference of normal stresses.
The most important generalisation of Neo-Hookean solid is w:Mooney-Rivlin solid.
- C. W. Macosko Rheology: principles, measurement and applications, VCH Publishers, 1994, ISBN 1-56081-579-5