Phase transformation yield surface of anisotropic shape memory alloys

International audienceTwo theoretical investigations, i.e. a phenomenological macroscopic model and a "micro-macro" model are developed for modelling the experimental surfaces of initiation of phase transformation in shape memory alloys. A possible initial anisotropy of the materials is taken into account

Thermomechanical modelling of a NiTi SMA sample submitted to displacement-controlled tensile test

AbstractShape Memory Alloys (SMAs) undergo an austenite–martensite solid–solid phase transformation which confers its pseudo-elastic and shape memory behaviours. Phase transformation can be induced either by stress or temperature changes. That indicates a strong thermo-mechanical coupling. Tensile test is one of the most popular mechanical test, allowing an easy observation of this coupling: transformation bands appear and enlarge giving rise to a large amount of heat and strain localisation. We demonstrate that the number of transformation bands is strongly associated with the strain rate. Recent progress in full field measurement techniques have provided accurate observations and consequently a better understanding of strain and heat generation and diffusion in SMAs. These experiments bring us to suggest the creation of a new one-dimensional thermomechanical modelling of the pseudo-elastic behaviour. It is used to simulate the heat rise, strain localisation and thermal evolution of the NiTi SMA sample submitted to tensile loading

Surfaces seuil de début de transformation : modèle cristallin pour un alliage à mémoire de forme

Les AMF ont des propriétés spécifiques dues à une transformation de phase solide-solide réversible pouvant être induite par la contrainte ou la température. Pour les AMF de type Ni49,75%at-Ti, nous proposons un modèle polycristallin multiaxial basé sur les géométries locales des deux phases (austénite et martensite) et de la comparaison des énergies par variante. Ce modèle est comparé à des essais multiaxiaux en termes de surfaces seuil de début de transformation et de déformation équivalente

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

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

Caractérisation thermo-mécanique et modélisation des bandes de transformation dans un alliage à mémoire de forme

Dans les Alliages à Mémoire  de Forme, la transformation martensitique est à l’origine de la pseudoélasticité. C’est ce phénomène, qui se localise sous forme de bandes, que l’on se propose d’observer, de caractériser, et de modéliser. Pour cela, les champs de déplacement par corrélation d’image seront observes puis comparés aux prédictions de différents modèles de localisation. Ensuite, l’analyse des résultats de la thermographie infrarouge permettra d’estimer les champs de sources de chaleur

Modélisation polycristalline du comportement d’un Alliage `a M´emoire de Forme (AMF) de type Ni-Ti sous sollicitations multiaxiales

Les propriétés des AMF sont dues à une transformation de phase solide solide réversible induite par la contrainte ou la température. Pour les AMF de type Ni49,75%at-Ti, c’est une transformation isochore qui, d’une phase cubique (austénite), forme une phase monoclinique (martensite) avec 24 orientations possibles (variantes). A partir des géométries locales et des énergies par variante, nous proposons un modèle polycristallin multiaxial qui rend compte du fort couplage thermomécanique lors de la transformation de phase. Des essais quasi statiques permettent de le valider

Thermomechanical modelling of a NiTi SMA sample submitted to displacement-controlled tensile test

Shape Memory Alloys (SMAs) undergo an austenite–martensite solid–solid phase transformation which confers its pseudo-elastic and shape memory behaviours. Phase transformation can be induced either by stress or temperature changes. That indicates a strong thermo-mechanical coupling. Tensile test is one of the most popular mechanical test, allowing an easy observation of this coupling: transformation bands appear and enlarge giving rise to a large amount of heat and strain localisation. We demonstrate that the number of transformation bands is strongly associated with the strain rate. Recent progress in full field measurement techniques have provided accurate observations and consequently a better understanding of strain and heat generation and diffusion in SMAs. These experiments bring us to suggest the creation of a new one-dimensional thermomechanical modelling of the pseudo-elastic behaviour. It is used to simulate the heat rise, strain localisation and thermal evolution of the NiTi SMA sample submitted to tensile loading

Multiaxial Shape Memory Effect and Superelasticity

WOSInternational audienceThe specific behaviour of shape memory alloys (SMA) is due to a martensitic transformation. This transformation consists mainly in a shear without volume change and is activated either by stress or temperature. The superelastic behaviour and the one-way shape memory effect are both due to the partition between austenite and martensite. The superelastic effect is obtained for fully austenitic SMA: loaded up to 5% strain, a sample recovers its initial shape after unloading with a hysteretic loop. The oneway shape memory effect is obtained when a martensitic SMA, plastically deformed, recovers its initial shape by simple heating. Superelasticity and one-way shape memory effect are useful for several three-dimensional applications. Despite all these phenomena are well known and modelled in 1D, the 3D behaviour, and especially the one-way shape memory effect, remains quite unexplored. Actually, the development of complex 3D applications requires time-consuming iterations and expensive prototypes. Predictive phenomenological models are consequently crucial objectives for the design and dimensioning of SMA structures. Therefore, a series of 2D proportional and non-proportional, isothermal and non-isothermal tests have been performed. This database will be used to build a phenomenological model within the framework of irreversible processes

Thermo-mechanical description of phase transformation in Ni-Ti Shape Memory Alloy

The pseudo-elasticity of Shape Memory Alloys is due to a change in volumetric fraction between the high temperature phase (Austenite) and the low temperature phase (Martensite) under a mechanical loading. When a tensile loading is considered, transformation bands occur leading to strong localization of the deformation and a strong local heating. The modeling of this strongly coupled phenomenon is discussed for a polycrystalline specimen in a multiaxial mechanical framework. Three different scales are considered: the variant scale (or phase scale), the single-crystal scale and the polycrystalline scale. The free energy of each variant is first computed from the loading and the geometrical lattice transformations associated to each variant. The volumetric fraction of each phase is then defined at the grain scale as function of their free energy. A simple averaging operation allows to estimate the deformation at the grain scale. The polycrystalline scale is not considered at present