[en] If a metal is submitted to cyclic loading under a non-zero mean stress, plastic strain may accumulate, generally in the direction of mean stress, over the cycles: this is ratchetting.The background of the Ph.D. Thesis was the improvement of the description of this phenomenon un metallic structures. Towards this end, we have chosen to focus our attention on the phenomenological macroscopic models that describe material behaviour in structural computations. As suggested in several articles, one way of improvement was to take into account yield surface distortion, a phenomenon many times observed experimentally but neglected up to now for simplicity. In order to introduce properly this new element into the modelization, we have used a dialogue between a micro-macro model (more relevant from a physical point of view but also more CPU time consumer) and the macroscopic model to build. We have taken advantage of the information given by the micro-macro model to build a macroscopic model that can describe multiaxial ratchetting as well as the yield surface distortions predicted by the micro-macro model, over a large base of 'virtual tests'. Then, the model thus developed has been successfully identified and validated on several complex experimental results obtained on the volume element of a real material. A final validation of the model on a non homogeneous (structural) test is under investigation. (author)

[en] A new finite element method is proposed to model the evolution of cracks in a 3D lattice under mechanical and thermal fatigue loading. This step-by-step method, based on the separation of initiation and propagation phenomenon, is tuned by fitted entrance parameters which command fatigue laws. It is founded on linear elasticity assumptions, and the crack's evolution is taken into account by a so-called element deletion method. After the algorithm of the model is presented, two examples of mechanical and thermal fatigue are considered. The evolutions of different crack network characteristic parameters as a function of the number of cycles is proposed and match, at least qualitatively, the general trends observed experimentally. (authors)

[en] For nuclear reactor components, uniaxial isothermal fatigue curves are used to estimate the crack initiation under thermal fatigue. However, such approach would be not sufficient in some cases where cracking was observed. To investigate differences between uniaxial and thermal fatigue damage, tests have been carried out at CEA using the thermal fatigue devices SPLASH and FAT3D: a bi-dimensional (2-D) loading status is obtained in SPLASH, whereas a tri-dimensional (3-D) loading status is obtained in FAT3D. All the analysed tests clearly show that crack initiation in thermal fatigue is faster than in uniaxial isothermal fatigue conditions: for identical levels of strain, the number of cycles required to achieve crack initiation is significantly lower. The enhanced damaging effect probably results from a pure mechanical origin: a nearly perfect biaxial state corresponds to an increased hydrostatic stress. Consequently, multiaxial fatigue criteria must be applied. The Zamrik's strain criterion and the energy criterion proposed by Ecole Polytechnique provide the best estimations. In that framework, the proposed new method coupling both RCC-MR strain estimations and Zamrik's criterion appears to be more promising for the designer. (orig.)

[en] The application of Miner's rule using a loading issued from a mock-up of a RHR (removal heat system) of PWR plant, made of 304 steel gives a very important non-conservative fatigue lifetime in strain control when strain fatigue curve is used. A large number of test in strain and stress control are performed in different laboratories. Two modeling of literature Smith-Watson-Topper (SWT) and Fatemi-Socie (FS) have been used to simulate these tests. Much better responses than Miner's rule are obtained. However these models need an elastic-plastic constitutive law which is difficult to propose in the presence of high cycle secondary hardening observed in austenitic stainless steels. So a conservative model for fatigue damage accumulation under variable amplitude loading is proposed for austenitic stainless steels (AISI 304, 316) in strain control, which does not need a constitutive law. Linear damage accumulation is used, while, sequence effect is taken into account using the elastic-plastic memory effect through cyclic strain stress curves with pre-hardening. This modeling is based on the fact that for stainless steels, pre-hardening is detrimental for fatigue life in strain control while it is beneficial in stress control. In the case of materials that do not demonstrate load sequence memory the modeling is identical to Miner rule. In the presence of low mean stress, the modeling is approved based on a large number of tests. Moreover the modeling permits to explain the larger detrimental effect of a tension mean stress in strain control tests than in stress control tests. To extend the modeling to higher values of mean stress it is proposed to divide mean stress effect into maximal and 'real' mean stress effects. Extending this work to the case of significant mean stress is ongoing. (authors)

[en] An extensive study of the uniaxial cyclic material behavior of an AISI 304L austenitic stainless steel is proposed in the high cycle regime and in constant amplitude loadings. More particularly, the effect on the material behavior and lifetime of a mean axial stress is evaluated imposing either a stress or a strain amplitude. Almost no effect is observed on the stress fatigue curve while a reduction of about 30% is obtained on the strain fatigue curve in the endurance regime. It appears that a stress amplitude fatigue parameter or a Smith-Watson-Topper parameter predict accurately such differences based on the material behavior at maximum cyclic softening. (authors)

[en] The plastic behavior of irradiated Reactor Pressure Vessel (RPV) steels is described by constitutive equations capturing the temperature and strain rate sensitivities. The flow stress is decomposed into its fundamental components associated with the microstructure features peculiar to RPV steels, such as carbides, dislocation network and deformation confinement inside grains. Dislocations are assumed to move on the {110} and {112} crystallographic planes and a simplified interaction matrix is proposed. The predicted yield stress is obtained without adjustable parameters and found in close agreement with a large number of experimental results over a large temperature range. Finally, the contribution of radiation defects is accounted for using atomistic and dislocation dynamics results. The effect of solute cluster is analyzed in details in terms of the cluster size and density and strength. Results are discussed and compared with an experimental database on neutron-irradiated RPV steels.

[en] Highlights: → Intergranular stress distributions in a bainitic steel. → Comparison of local mean stress field with neutron diffraction results. → Application of the local stress distribution in a brittle fracture model. - Abstract: The evaluation of the reliability of pressure vessels in nuclear plants relies on the evaluation of failure probability models. Micromechanical approaches are of great interest to refine their description, to better understand the underlying mechanisms leading to failure, and finally to improve the prediction of these models. The main purpose of this paper is to introduce the stress heterogeneities arising within the polycrystal in a probabilistic modeling of brittle fracture. Stress heterogeneities are evaluated from Finite-Element simulations performed on a large number of Statistical Volume Elements. Results are validated both on the measured averaged behavior and on the averaged stresses measured by neutron diffraction in five specific orientations. A probabilistic model for brittle fracture is then presented accounting for the carbide distribution and the stress distribution evaluated previously inside an elementary volume V₀. Results are compared to a 'Beremin type' approach, assuming a homogeneous stress state inside V₀.

[en] Fully reversed uniaxial tests performed under total strain and stress control on 304 stainless steels specimens show that, under strain control the fatigue damage for High-Low (H-L) cycling is more significant than that using Miner's rule, but under stress control opposite results are obtained. This has been attributed to opposite effects of pre-hardening under strain and stress control. Classical non linear damage accumulation models are not able to take into account this difference in sequence effect. Smith-Watson-Topper (SWT) and Fatemi-Socie (FS) criterion combined to linear damage accumulation can take into account this difference in sequence effect through the presence of maximum stress. However these models require an elastic-plastic constitutive law which is difficult to propose due to the presence of high cycle secondary hardening observed on 304 stainless steel. A conservative model for damage accumulation under variable amplitude strain control loading is thus proposed, which does not require a constitutive law. Linear damage accumulation is used, while sequence effect is taken into account using the elastic-plastic memory effect through cyclic strain-stress curves (CSSC) with pre-hardening. This modeling classifies metallic alloys in two groups for damage accumulation, with a stable (independent to pre-hardening) CSSC as for aluminum alloys and with an unstable (dependent to pre-hardening) one as for austenitic stainless steels. For the former case the modeling is identical to Miner's rule. The modeling is approved based on a large number of tests on 304 stainless steel and is compared with SWT and FS models. In presence of mean stress the modeling permits in a qualitative way to explain the fact that tensile mean stresses in constant amplitude strain control tests are more detrimental than for constant amplitude stress control tests. Moreover it is shown that the SWT model is not always able to predict accurately the fatigue life in presence of a mean stress. Finally, it is concluded that for a 304 stainless steel, in order to take into account the mean stress in fatigue life, the mean stress effect has to be decomposed into two parts: maximum and 'intrinsic' mean stress effects. (authors)

[en] Based on recent dislocation dynamics simulations investigations, a set of constitutive equations and model parameters for the description of plasticity of body-centered cubic materials is proposed. Assuming the flow stress to be controlled at low temperatures by the mobility of screw dislocations and by forest interactions at high temperatures, this model allows for the prediction of the mechanical behavior in monotonic loading over a large range of temperatures and strain rates. The consideration of the difference in mobility between screw and non-screw dislocations is found to affect strain hardening in a complex manner. The constitutive equations are implemented in a finite-element method to simulate tensile tests on iron single crystal at different temperatures. The use of finite transformation formalism enables the computation of crystal rotations which affect slip system activities. The calculated critical resolved shear stress and crystal rotations are in good agreement with existing experimental results. (authors)

[en] Thermal shocks are applied to a 304L austenitic stainless steel plate with a pulsed laser. A stroboscopic reconstruction is used for infrared (IR) and visible light camera measurements. The displacement fields are measured with a digital image correlation (DIC) technique. Different IR devices are used to measure the temperature variations (i.e. medium wave camera and short wave pyrometry). Several ways of determining the emissivity or absorptivity are discussed. The complete 3D thermal loading is numerically determined by minimising the difference between experimental measurements and finite element analyses of thermal fields. An elastoplastic model is then used to compute mechanical fields that are compared with DIC measurements. (authors)