Modeling of a new SMA micro-actuator for active endoscopy applications.

International audienceShape memory alloys (SMA) are good candidates to actuate endoscope heads but the cooling problem must be solved particularly in confined situations. For these reasons, a new SMA micro-actuator specially designed for active endoscopy applications has been developed in our laboratory. This work is a new step in the approach of using integrated thermoelectric cooling with SMA actuators. In fact, the Peltier effect is very attractive in such a case because this reversible phenomenon reduces the overheating of the external environment and provides forced cooling that decreases the response time. In this paper the actuator design and its working principle are presented. A fine modeling of the coupled mechanical and thermal behaviors gives a better understanding of the physical phenomenon involved in the actuator. Finally an experimental prototype has been developed and tested in order to verify the model predictions

Equivalent transformation strain and its relation with martensite volume fraction for isotropic and anisotropic shape memory alloys.

International audienceThe present paper deals with the superelastic behavior of both isotropic and anisotropic shape memory alloys (SMA). Recently, a macroscopic model, which permits to simulate the superelasticity of SMA under complex multi-axial loading, has been proposed by Bouvet et al. [Bouvet, C., Calloch, S., Lexcellent, C., 2004. A phenomenological model for pseudoelasticity of shape memory alloys under multi-axial proportional and non-proportional loadings. Eur. J. Mech. A Solids 23, 37-61]. In this model, a conjecture concerning the proportionality of the equivalent transformation strain with the martensite volume fraction has been adopted. The main goal of this study is to show the validity of this conjecture when the stress state is multi-axial. In a first part, the case of isotropic SMA is considered. An equivalent stress and an equivalent transformation strain are introduced. In the second section, the case of anisotropic SMA is considered. The previous equivalent stress and equivalent transformation strain are generalized to take into account the anisotropy of the material. The relation between the equivalent transformation strain and the martensite volume fraction is discussed by using, on one hand, experimental results under proportional tension-torsion loadings and, on the other hand, a polycrystalline model

Magnetic shape memory alloy and actuator design.

International audienceIn the field of micromechatronics, microrobotics and specially microfactories, active materials are used in most cases. They permit high resolution and distributed actuation. In this area, Magnetic Shape Memory Alloys (MSMA) are possible candidates. If a lot of studies deal with MSMA, only few applications use them until now. MSMA are attractive active materials because they have large strain (about 10%) as the classical shape memory alloys (SMA), but can provide a 100 times shorter time response. The main disadvantages of MSMA based actuators are the brittleness of the single-crystal material, the difficulty to apply the strong magnetic field required to obtain sufficient strain and the nonlinear behaviour. We propose in this paper a novel MSMA based actuator changing the disadvantage of the hysteretic behaviour into an advantage. This device is a push-pull actuator: two pieces of MSMA material act in an opposite way. The magnetic fields are created by coils and concentrated by ferromagnetic circuits. In order to move the central part of the actuator, a current pulse in the first coil is generated. The hysteretic behaviour of the material permits to keep a stable position when no current is applied. A current pulse in the second coil permits to displace the central part in the opposite direction. The stable position depends on the magnitude and the time duration of the current pulses and an infinity of stable positions can be reached. The use of current pulses permits also a reduction of the coil heating (Joule effect losses) and a reduction of the magnetic circuit size. The performances and characteristics of MSMA are between these of classical SMA and these of piezo-electric materials. A thermo-magneto-mechanical model of our actuator is currently in development in order to design an efficient control law welladapted to the specific MSMA properties

Relation between the martensite volume fraction and the equivalent transformation strain in shape memory alloys

International audienceThis study concerns the pseudoelasticity of shape memory alloys. A series of tests under multiaxial loadings and a micro-macro model are used to show the validity of a conjecture concerning the relation between the volume fraction of martensite and the equivalent transformation strain

Corrélation entre le fluage et la diffusion des solutions solides fortement concentrées. Cas envisagé : les phases β Ag-50% at Zn Ag-50% Cd - Cu-43-53% at Zn - Cu-14,5% Sn Cu-25% at Al

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Corrélation entre le fluage et la diffusion des solutions solides fortement concentrées. Cas envisagé : les phases β Ag-50% at Zn Ag-50% Cd - Cu-43-53% at Zn - Cu-14,5% Sn Cu-25% at Al

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Micromechanical modelling for tension–compression pseudoelastic behavior of AuCd single crystals

On one hand, the experimental crystallographic data of Chang and Read and the theoretical ones of Lieberman et al. permit a physical approach of the martensitic transformation of Au–47.5% at. Cd. On the other hand, an extension to the AuCd crystallographic system of the simple micromechanical model, developed by Lexcellent et al. for CuZnAl single crystals, allows an efficient modelling of the tension-compression and pure tension tests, performed on single crystals by Nakanishi et al. in the pseudoelastic range

Fluage de la phase β de l'Ag-Zn équiatomique

Study of the β phase of equiatomic Ag-Zn. The utilisation of Dorn equation demands the measurement of stationary creep rates versus temperature and stress σ and the one of Coulomb modulus G. The measured activation energy which takes place in this equation is compared with that of the diffusion coefficient as calculated in the two extreme cases : the stationary and the homogeneous ones.Etude du fluage de la phase β équiatom ique Ag-Zn. L'utilisation de l'équation de Dorn nécessite la mesure des vitesses de fluage stationnaire en fonction de la température T et de la contrainte σ et celle du module de Coulomb G. On compare l'énergie d'activation mesurée intervenant dans cette équation avec celle du coefficient de diffusion prévu par les cas extrêmes : régime stationnaire et régime homogène

SOME REMARKS ON THE MODELLING OF THE THERMOMECHANICAL BEHAVIOUR OF SHAPE MEMORY ALLOYS

Our aim is to examine from the continuum thermodynamic point of view, some models predicting the pseudoelastic behaviour of shape memory alloys. We investigate the choice of states variables, the structure of free energy and complementary laws. A special emphasis is made on generalized standard models. In the state of art, there is a lot of progress to do for modelling the behaviour of shape memory alloys

Une étude expérimentale et théorique du comportement en fluage biaxial de l'aluminium A5 (99,5 % en poids) aux températures intermédiaires (300 °C à 450 °C)

Des mesures en fluage stationnaire du paramètre d'écrouissage h st, de restauration rst et de la contrainte interne Xstij sont effectués aux températures intermédiaires (300 ≤ T ≤ 450 °C). La dépendance de r et h avec la contrainte interne est identique dans l'état stationnaire à celle mesurée aux premiers instants du fluage (cf. mesures sur l'A99 de Nakashima et al., 1987). Cela permet l'extension en température et pour des états transitoires du pseudo-potentiel de dissipation Ω(σij, Xij, T) déjà identifié en stationnaire