On the Determination of Strain Energy Functions of Rubbers

Methodology of determining coefficients of material strain energy functions of rubbers from experimental measurements of homogeneous deformations is examined within the scope of the 3rd order deformation theory [3]. Closed-form solutions of most frequently employed material testing deformations - simple tensile, pure shear and equal bi-axial deformations - are examined to reveal the shortcomings of using any one deformation in characterizing the strain energy function, In order to obtain a more representative material strain energy function, employment of more than one homogenous deformation is recommended along with improved material sampling. In situations, however, where data from simple-tensile deformation alone are available a set of constraints on the coefficients have been derived that improves the extrapolation characteristics of the fitted function.