Georges Cailletaud

Distribution of normal stress at grain boundaries in multicrystals: application to an intergranular damage modeling

Under transient power conditions in pressurized water reactor, zircaloy-4 fuel claddings are possibly submitted to stress corrosion cracking by volatile fission products. The localization of stress and strain in the inner surface of the cladding and the local aspects of the damage phenomena incite to consider a modeling at the granular scale. At this scale, the behavior of multicrystals is described by a crystal plasticity model including the local orientation of each grain and the Zy-4 slip-system families. Representative microstructures are meshed (2D and 3D) in order to evaluate intergranular but also intragranular heterogenities of the stress and strain fields. Large strain heterogenities appear due to deformation incompatibilities between grains, which induce over-stresses at the grain boundaries. 3D computations of multicrystalline aggregates are performed in order to compute the distribution of the normal stresses at the grain boundaries with respect to the angle between the load direction and the normal to the grain boundary. Effects of neighborhood is evaluated. In addition, an intergranular damage model is proposed. The formulation of this model is based on a decomposition of the strength at grain boundaries into normal and shear components. Finally, results on 2D aggregates are presented and show examples of anisotropic damage patterns.

Capabilities of the Multi-mechanism Model in the Prediction of the Cyclic Behavior of Various Classes of Metals

The paper deals with an evaluation of the multi-mechanism (MM) approach capabilities in the prediction of the cyclic behavior of different classes of metallic materials. For this objective, the tests detailed in (Taleb, Int J Plast 43:1–19, 2013a) have been simulated here by the MM model. In these tests, six alloys were considered: two ferritic steels (35NCD16 and XC18), two austenitic stainless steels (304L and 316L), one “extruded” aluminum alloy (2017A) and one copper-zinc alloy (CuZn27). The specimens have been subjected to proportional and non-proportional stress as well as the combination of stress and strain control at room temperature. The identification of the material parameters has been carried out using exclusively strain controlled experiments under proportional and non-proportional loading paths performed in the present study for each material. The model may describe a large number of phenomena with twenty five parameters in total but, it appears that for a given material under the adopted conditions, the activation of all parameters may be not necessary. Our attention was focused mainly on the capabilities to predict correctly the cyclic accumulation of the inelastic strain including the shape of the hysteresis loops. The comparison between test responses and their predictions by the MM model are generally satisfactory with relatively small number of material parameters (between eight and thirteen according to the material). One can also highlight the capability of the MM model to describe a transient ratcheting without activation of the dynamic recovery term in the kinematic variables. Finally, the MM model deserves improvement for a better description of the cyclic behavior of anisotropic materials.

Multi-mechanism modeling of inelastic material behavior

This book focuses on a particular class of models (namely Multi-Mechanism models) and their applications to extensive experimental data base related to different kind of materials. These models (i) are able to describe the main mechanical effects in plasticity, creep, creep/plasticity interaction, ratcheting extra-hardening under non-proportional loading (ii) provide local information (such us local stress/strain fields, damage, ….). A particular attention is paid to the identification process of material parameters. Moreover, finite element implementation of the Multi-Mechanism models is detailed.