Thermal fatigue cracking may be observed in some components of nuclear power plants. The evaluation of mesocrack nucleation in the components subjected to thermomechanical loadings is very important to determine investigation periods and maintenance programmes. On the idea that damage is localized at the microscopic scale, a scale smaller than the mesoscopic scale of the Representative Volume Element (RVE), a three-dimensional model is proposed. It consists in a two scale analysis with quasi-brittle fatigue damage behaviour modelled by plasticity coupled with damage constitutive equations (with thermal stresses) at the microscale. Microscopic failure is assumed to coincide with the RVE failure when the damage at microscale reaches the critical value Dc. In order to compute the strains, the stresses and the damage history at the microscale a micromechanics based model of a weak micro-inclusion subjected to plasticity and damage embedded in an elastic meso-RVE is considered. A localization law, in the sense of the homogenization theories, is developed to link the two scales and a numerical scheme is proposed to integrate the constitutive equations.
AbstractThe purpose of this work is to develop an approach in crack initiation assessment in components operating at high temperature. Components of fossil power plants, such as rotors, casings or headers, have to face the combination of highly damaging processes due to the operating temperatures and the high numbers of start-ups and shut-downs. The processes involved are indeed creep, thermal fatigue and creep-fatigue interaction. Our approach is based on viscoplastic constitutive equations coupled with a three dimensional law of continuum damage evolution. These laws use a formalism based on the thermodynamic of irreversible processes. Our goal is to represent the crack initiation of structures loaded with low cycles thermal loads more accurately. To identify the different constitutive coefficients, we have chosen to study a P22 (2,25CrMo) steel from a retired header of one of our coal fired plants. The methodology of identification is given and the influence of each variable is discussed. We focus our interest on the creep-fatigue interaction which is a major parameter in the estimation of residual life of low used fossil power plant. A numerical simulation is given, which shows the necessity to use viscoplastic constitutive equations including a kinematic hardening, to take into account the non-linear aspect of the creep-fatigue interaction. Then, we detail an example of a finite elements calculation. The hypothesis and the particularities of this calculation are explained. The results are analysed and linked to experimental datas. We expect to apply soon those laws to various industrial materials and structures such as turbines (casings and rotors) or header to improve the residual life estimation of our plants.Keywords: non-linearcreep-fatigue interaction
