The neo-Hookeian solid

The predictions of the theory for a solid that obeys equation (6.26) are now considered for various types of deformation. 

The strain energy t/ per unit of original unstrained area is thus 

7 Equibiaxial extension. The stresses a, and t2 applied to an original unit square of material of thickness t cause it to deform to the dimensions shown. 

When X exceeds about 2 this force F becomes essentially constant and rather like a surface tension. 

The results shown in flg. 6.8 for the same rubber as that used for the simple uniaxial extension considered earlier (flg. 6.6) show that the fit between experiment and theory is good for low extension ratios with the same value of G as was used for the simple extension. At higher extension ratios the experimental force is higher than predicted. 

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