Two types of compressible isotropic neo-Hookean material models

In this contribution, we present a systematic study of the performance of two known types of compressible generalization of the incompressible neo-Hookean material model. The first type of generalization is based on the development of vol-iso neo-Hookean models and involves the additive decomposition of the elastic energy into volumetric and isochoric parts. The second simpler type of generalization is based on the development of mixed neo-Hookean models that do not use this decomposition. Theoretical studies of model performance and simulations of some homogeneous deformations have shown that when using volumetric functions $(J^q+J^{-q}-2)/(2q^2)$ ($J$ is the volume ratio, and $q\in \mathbb{R}$ is a parameter, $q\geq 0$) from the Hartmann-Neff family [Hartmann and Neff, Int. J. Solids Structures, 40: 2767-2791 (2003)] with parameter $q\geq 2$ (the preferred value is $q=5$), mixed and vol-iso models show similar performance in applications and have physically reasonable responses in extreme states, which is convenient for theoretical studies. However, contrary to vol-iso models, mixed models allow the use of a wider set of volumetric functions with physically reasonable responses in extreme states. A further feature of mixed models is simpler expressions for stresses and tangent stiffness tensors.