1. Introduction

Here we describe the implementation of the Lemaître nonlinear viscoelasticity model (keyword LEMAITRE), which can be reduced, for certain particular values of the parameters, to a Norton viscoelastoplastic behavior relationship.

A variant (depending on the fluence) of this Lemaître model has been added, in order to model fuel assemblies (keyword LEMAITRE_IRRA). The particularity of this model is that it includes an additional anelastic deformation: the enlargement deformation.

A viscoelastoplastic model with threshold has been added. It is a material whose behavior is purely elastic up to a threshold and then once this threshold is exceeded, the behavior relationship becomes a particular case of the Lemaître relationship (LEMA_SEUIL).

Finally, a viscoelastoplastic model with logarithmic flow of fluence has been implanted, it is accessible by the keyword VISC_IRRA_LOG.

For each of these models, it is assumed that the material is isotropic (except for the enlargement deformation, which is uniaxial). They can be used in 3D, in plane deformations (D_ PLAN) and in axisymmetry (AXIS).

In this note, the equations constituting the models and their implementation in Code_Aster are presented.