Coexistence and compatibility of martensite reorientation and phase transformation in high-frequency magnetic-field-induced deformation of Ni-Mn-Ga single crystal

High-frequency magnetic-field-induced Martensite Reorientation (MR) is one of the most important advantages of Ferromagnetic Shape Memory Alloys (FSMAs), but its stability is threatened by dissipation heat accumulation (“self-heating”) of cyclic frictional twin boundary motion, which can cause temperature-induced Phase Transformation (PT) and reduce the output strain amplitude significantly. In this paper, the interaction of the temperature-induced PT and the magnetic-field-induced MR during high-frequency magnetic actuation on FSMA is studied with in-situ observations of local-strain evolution in conjunction with microstructure compatibility analysis. Based on the nominal strain and temperature responses and the corresponding local-strain maps, it is revealed that, when the temperature-induced PT takes place during the high-frequency field-induced MR, the specimen is divided into three zones: non-active austenite zone (with a constant deformation), active martensite zone (with cyclic deformations of MR) and buffering needle zone (interfacial zone) with a fine-needle-twin structure which plays an important role in maintaining the compatibility between austenite and martensite zones with different cyclic deformations during the dynamic loading. A novel mechanism is revealed that, under the magnetic actuation with changing ambient airflow, the “self-heating” temperature-driven phase boundary motion and the magnetic-field-driven twin boundary motion can coexist, because the specimen needs to self-organize the different phases/variants to satisfy all the thermo-magneto-mechanical boundary conditions. Taking advantage of this mechanism, the volume fractions of austenite and martensite zones can be adjusted with changing ambient airflow velocity, which provides an effective way to tune the nominal output strain amplitude (from 1% to 6% in the current study) while the working temperature is kept almost constant (around Ms and Mf)

Thermal effects on high-frequency magnetic-field-induced martensite reorientation in ferromagnetic shape memory alloys: An experimental and theoretical investigation

Ferromagnetic Shape Memory Alloys (FSMAs) exhibit large strains by the magnetic-field-induced martensite reorientation. But, due to the high-frequency field-induced cyclic frictional martensite twin boundary motion in FSMAs, the dissipation heat can cause a large temperature rise. Thus, the output strain amplitude of FSMAs would decrease significantly if the temperature increases to be high enough to trigger the Martensite-Austenite phase transformation. Such thermal effects on the dynamic responses of FSMAs are unclear in literature because most existing dynamic experiments were performed only for a short-time period (a few seconds) to avoid the temperature rise. In this paper, systematic long-time experiments (>100 s) on a Ni-Mn-Ga single crystal are conducted at various levels of magnetic field frequency, initial compressive stress and ambient airflow velocity. It is found that, during the long-time actuation, the specimen temperature increases and then saturates at a certain level (stable temperature) while the strain oscillation evolves to a stable cycle; both the stable temperature and the stable strain amplitude depend on the frequency, the stress level and the heat exchange condition (i.e., ambient airflow velocity). Particularly, when the specimen temperature reaches a critical level to partially transform the martensite to the austenite, the output strain amplitude reduces suddenly because of less martensite reorientation. Changing the ambient heat-exchange condition (by the airflow) can modify the specimen temperature evolution to avoid the phase transformation, but it also changes the behaviors of the martensite reorientation that is sensitive to temperature. Eventually, the output strain amplitude depends on the airflow velocity non-monotonically, i.e., there exists a critical heat exchange condition to achieve the maximum stable strain amplitude. Based on the systematic experiments and a simplified one-dimensional heat-transfer model, the critical condition can be determined. The new experimental phenomena of the thermal effects can be well understood and described by the heat-transfer model. Further, instead of avoiding the temperature rise and the phase transformation, we propose to take advantage of the interaction between the temperature-induced phase transformation and the magnetic-field-induced martensite reorientation to develop a special “isothermal” FSMA actuator with a tunable output strain amplitude and a constant working temperature. This paper provides systematic experimental data and theoretical analysis for understanding the thermo-magneto-mechanical coupling in FSMAs and developing reliable high-frequency long-time running FSMA-actuators

Dynamique non linéaire d'un oscillateur à mémoire de forme

Nous étudions les réponses forcées d'un oscillateur reproduisant le comportement pseudo-élastique d'un alliage à mémoire de forme. Le modèle est dérivé d'une loi de comportement tridimensionnelle prenant en compte les couplages entre la thermique, la mécanique et les changments de phase solide-solide du matériau. Les réponses forcées montrent un comportement assouplissant dès que la transformation martensitique est activée, ainsi que l'existence de zones chaotiques. Nous présenterons aussi des comparaisons calcul/essai réalisées sur des fils en torsion

Sur la modélisation du changement de phase solide : application aux matériaux à mémoire de forme et à l'endommagement fragile partiel

La modélisation de certains matériaux solides nécessite la prise en compte des effets de changement de phase. Dans ce travail, un modèle d'un matériau solide présentant ce phénomène est proposé. Le changement de phase y est décrit par des variables internes qui représentent les proportions de phase. A partir de leur définition et du fait que ces variables ne sont pas indépendantes de la déformation macroscopique, le modèle considéré rentre dans le cadre des matériaux standards généralisés avec des variables d'état liées. Une extension de la méthode des deux potentiels en présence des liaisons internes est proposée. Par souci de clarté, la présentation est donnée suivant le formalisme des multiplicateurs de Lagrange. Les équations d'état sont écrites en fonction du lagrangien associé. L'évolution des variables internes irréversibles est obtenue par les lois complémentaires. La possibilité de considérer le changement de phase réversible ou irréversible est discutée d'une manière générale. Le problème de la stabilité du matériau est aussi examiné et met en lumière l'important rôle de l'énergie d'interaction entre les phases. Comme applications, le modèle en question est d'abord utilisé pour décrire le comportement des matériaux à mémoire de forme, ensuite il est appliqué comme un modèle d'endommagement fragile partiel quand l'endommagement est interprété comme étant un changement de phase irréversible. Enfin, un programme de calcul par éléments finis est écrit afin de simuler ce comportement

Sur le comportement magnéto-mécanique des alliages à mémoire de forme magnétiques

Ferromagnetic Shape Memory Alloys (FSMA) are promising candidates for sensors and actuators for their high-frequency response and large reversible strain. The aim of this dissertation is the analysis of the magneto-mechanical behaviors of FSMA. In this aim, we study, both experimentally and theoretically, the martensite reorientation in FSMA. Firstly, a 2D/3D magneto-mechanical energy analysis is proposed and incorporated into phase diagrams for a graphic study of path-dependent martensite reorientation in FSMA under 3D loadings. Criteria and material requirements for obtaining reversible strain in cyclic loadings are derived. The energy analysis predicts that FSMA in 2D/3D configurations (multi-axial stresses) has much more advantages than in 1D configuration, e.g., higher output stress and more application flexibility. Secondly, to validate the predictions of the energy analysis, 2D experiments are performed on FSMA and results reveal that the intrinsic dissipation and the transformation strain due to martensite reorientation are constant in all tested 2D stress states. Moreover, preliminary results validate that the output stress of FSMA in 2D configuration (magnetic field with biaxial stresses) is larger than in 1D configuration, and the output stress can be increased by increasing the auxiliary stress. Finally, to predict the magneto-mechanical behaviors of FSMA in general multi-axial loadings, a 3D constitutive model is developed within the framework of thermodynamics of irreversible processes. All the martensite variants are considered and the temperature effect is also taken into account. Model simulations agree well with all the existing 1D/2D experiments. The model is further incorporated into finite element analysis for studying the non-linear bending behaviors of FSMA beams. The sample-geometry effect and the material anisotropic effect are systematically investigated.PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Approche globale pour l'analyse à la fatigue des Alliages à Mémoire de Forme

We develop a comprehensive approach for fatigue analysis of SMAs in three steps: first, the determination of the constitutive law allows the computation of the stabilized thermomechanical state of the structure. In order to better predict this state, tensile-compressive asymmetry and strong thermomechanical coupling are introduced into the ZM models. Second, the stabilized state is computed using an extension of the Direct Cyclic Method, which is shown to provide significant saving in computation time compared to the conventional incremental method. Third, the fatigue lifetime of the structure is determined with an energy-based criterion accounting for the influence of the hydrostatic pressure. The dependence of the fatigue lifetime on temperature and loading frequency is discussed. The prospects of this work include the validation of the constitutive laws and fatigue criterion for the case of non proportional loadings.Nous développons, en trois étapes, une approche globale de calcul à la fatigue des Alliages à Mémoire de Forme. La détermination de la loi de comportement permet le calcul de l'état thermomécanique stabilisé de la structure. Afin d'obtenir une meilleure prédiction de cette réponse, la dissymétrie entre traction et compression et le couplage fort thermomécanique sont ajoutés aux modèles ZM. Ensuite, le calcul numérique de l'état stabilisé est réalisé grâce à une généralisation de la Méthode Cyclique Directe, permettant un gain de temps de calcul considérable par rapport à la méthode incrémentale. Enfin, la durée de vie en fatigue est déterminée par un critère de fatigue énergétique qui tient compte de l'effet de la pression hydrostatique. La dépendance de la durée de vie vis-à-vis de la température et de la fréquence de chargement est discutée. Les perspectives concernent la validation des lois de comportement et du critère de fatigue pour des chargements non proportionnels.PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Modélisation du comportement dynamique des matérieux à mémoire de forme

PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Evaluation of fatigue-ratcheting damage of a pressurized elbow undergoing damage seismic inputs

We present a simplified method to calculate the plastic ratchet of elbow-shaped pipes submitted to seismic loading and an internal pressure. This method is simplified in the sense that the value of the ratchet is obtained without the use of finite element method (FEM) calculations. Here we derive a formula and use it to evaluate the fatigue-ratcheting damage of an elbow. This approach is applicable to complex plastic response appropriately described by non-linear kinematics hardening, which is more realistic for stainless steel such as 316-L

Numerical Simulation of Pseudoelastic Shape Memory Alloys using the Large Time Increment Method

International audienceThe paper presents a numerical implementation of the large time increment (LATIN) me- thod for the simulation of shape memory alloys (SMAs) in the pseudoelastic range. The method was initially proposed as an alternative to the conventional incremental approach for the integration of nonlinear constitutive models. It is adapted here for the simulation of pseudoelastic SMA behavior using the Zaki-Moumni (ZM) model and is shown to be especially useful in situations where the phase transformation process presents little or lack of hardening. In these situations, a slight stress variation in a load increment can result in large variations of strain and local state variables, which may lead to difficulties in numerical convergence. In contrast to the conventional incremental method, the LATIN method solve the global equilibrium and local consistency conditions sequentially for the entire loading path. The achieved solution must satisfy the conditions of static and kinematic admissibility and consistency simultaneously after several iterations. 3D numerical implementation is accomplished using an implicit algorithm and is then used for finite element simulation using the software Abaqus. Numerical results are contrasted to those obtained using step-by-step incremental integration