Inconsistency of uhyper and umat in Abaqus for compressible hyperelastic materials

Abstract

In this article, we revisited Bažant's comments on the implementation of hyperelastic material models in commercial finite element software. We would like to clarify that our assertions only apply if the material models are implemented as hypoelastic, i.e. by incremental stress updates, in common interfaces (including, in particular, umat in Abaqus). This assumption was not made sufficiently clear in the article. If, on the other hand, the stress calculations are implemented using the umat interface with absolute (or "total") stress updates, as is also assumed in the uhyper interface, there is no difference in the internal processes or the results between the umat and the uhyper implementation. This applies to highly compressible formulations as well, where the Kirchhoff and Cauchy stress tensors are clearly distinguished.

The article does not sufficiently distinguish between the behaviour of Abaqus for incremental and absolute stress updates