Sylvain Leclercq

The two way shape memory effect of shape memory alloys: an experimental study and a phenomenological model

Abstract The paper presents a macroscopic description for the simulation of the two way shape memory effect (TWSME) of shape memory alloys (SMA), and is an extension of a previous work dealing with the global thermomechanical behavior of SMA. The model is set in the framework of the thermodynamics of irreversible processes. Two internal variables are taken into account : the volume fraction of self-accommodating (pure thermal effect) and oriented (stress-induced) product phase. A term dedicated to the modeling of the trained SMA is added to the specific free energy derived for the non-trained SMA. This term φ ed , depends on the thermomechanical procedure of training submitted to the sample. In the present case, it accounts for the stress tensor σ ed applied during thermal cycling. The parameters of the model have been identified for a Cu–Zn–Al SMA, and the simulated results show good agreement with experiments.

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.