1. Introduction

Applications in the flame thermal sector and certain nuclear technologies (GENIV, AGR) require the ability to predict the behavior of creep materials both in terms of viscoplastic deformation but also in terms of creep damage [bib1].

On the other hand, from a « material » point of view, creep mechanisms must be taken into account in the most physical way possible for the law of behavior to be valid both in the field of creep-dislocation (high stresses) and in the field of creep-diffusion (low stresses). In the area of lowest stresses, a drop in ductility is generally observed for tempered martenistic steels containing between 9 and 12% chromium. This drop in ductility can be modelled via an isotropic damage variable causing the material to break before significant plastic deformations have had time to develop. In these situations, the maximum deformation criteria cannot be applied, as can the laws of damage evolution strongly linked to deformation such as the law of LEMAITRE.

More generally, creep cavities appear in many families of metallic materials and can be observed on site by performing extractive replicas on the surface of the components under investigation. These cavities, associated with a reduction in the effective surface area resistant to forces in the material, can be directly correlated to damage of the Kachanov type.

In response to these modeling needs, while remaining within a simple phenomenological mechanical framework, a Hayhurst-type law of behavior (with reference to its viscoplastic flow) was proposed in [bib2] and applied to creep calculations on a P92 steel commonly used in modern flame-fired power plant components.

This model, implemented in Code_Aster, is a viscoplastic behavior model with isotropic double work hardening, viscosity in hyperbolic sine law and coupled with Kachanov damage.

It should be noted that laws HAYHURST and VENDOCHAB are both viscoplastic laws with isotropic damage, however the law of HAYHURST has its own advantages detailed later in this document.

Note Bene:

Reference [bib3] describes the application of this model to calculations on welded joints in P92 steel, and in reference [bib4] we will find preliminary work to extend this model to take into account the fatigue-creep interaction on this same material.