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Shape memory alloys (SMA) undergo a solid-solid phase transformation called martensitic transformation, involving a "high temperature" phase, austenite, and a "low temperature" phase, martensite. The crystalline microstructure of the alloy alters mechanisms of transformations, and its pseudo-elastic behavior. The reduction of the grains to nanometric size shows new and unknown thermal and mechanical characteristics. Knowledge of thermomechanical behavior is however essential for the validation of multiaxial models to democratize the use of these materials. This work presents the development of innovative biaxial dynamic tests for the study of the pseudo-elastic mechanical behavior of a nickel titanium under biaxial dynamic compression. A single projectile, launched on a complex system of 45 ^ { circ} triangles, bars and transmitters, creates a biaxial solicitation of the sample. This mechanism ensures that the loads are synchronized along the two axes. Thanks to its architecture, the system makes possible many cases of multiaxial dynamic stress, unexplored until now. The forces and displacements across the sample are calculated independently, along both axes, from strain gauges glued on bars. The use of a thermal and optical camera allows strain and heating sources fields to be identified. The stress field is estimated by combining the information from the strain gauges placed on the bars and a finite element analysis of the specimen. The deformation appears homogeneous in the biaxial loading region of interest where a significant rise in temperature due to the phase change latent heat is observed. On the other hand the dynamic testing allows an equivalent dynamic stress/strain curve under biaxial and quasi-adiabatic conditions to be plotted. Experiments are finally compared to the results of finite difference axisymmetric model where the constitutive law is given by a fully coupled stochastic multiscale model. ; La microstructure cristalline de l'alliage joue un rôle vis-à-vis des mécanismes de transformations, et de son comportement pseudo-élastique. La réduction des grains vers une taille nanométrique offre notamment des caractéristiques thermiques et mécaniques bien différentes et peu connues. La démocratisation de l'utilisation de ces matériaux passe par la validation de modèles complets et donc par la connaissance du comportement thermomécanique multiaxial dynamique. Ce travail propose la mise en place d'essais dynamiques biaxiaux innovants pour l'étude du comportement mécanique pseudo-élastique d'un nickel titane sous compression dynamique biaxiale. Un seul et unique projectile, lancé sur un système complexe de triangles à 45^{circ}, barres et renvois d'angles, permet de solliciter l'échantillon de manière biaxiale. Le mécanisme permet notamment d'assurer la synchronisation des chargements selon les deux axes. Grâce à son architecture, le banc d'essais rend possibles des cas de sollicitations dynamiques multiaxiales inexplorés jusqu'à présent. Le grand nombre de points de mesure par jauges de déformations collées sur les barres permet de calculer les efforts et déplacements aux limites de l'échantillon et ce indépendamment suivant les deux axes de sollicitation. L'utilisation d'une caméra thermique et d'une caméra optique permet d'identifier les champs de déformations et de sources de chaleur. Le champ de contrainte est estimé en combinant les informations des jauges de déformations et une analyse par éléments finis de l'échantillon. La déformation apparaît homogène dans la région de chargement biaxiale, où une augmentation significative de la température due à la chaleur latente de changement de phase est observée. Les essais dynamiques permettent d'autre part d'établir une courbe contrainte / déformation dynamique équivalente dans des conditions biaxiales et quasi-adiabatiques. Les expériences sont finalement comparées aux résultats du modèle axisymétrique en différences finies où la loi de comportement est donnée par un modèle multi-échelle stochastique entièrement couplé.

Shape memory alloys (SMA) undergo a solid-solid phase transformation called martensitic transformation, involving a "high temperature" phase, austenite, and a "low temperature" phase, martensite. The crystalline microstructure of the alloy alters mechanisms of transformations, and its pseudo-elastic behavior. The reduction of the grains to nanometric size shows new and unknown thermal and mechanical characteristics. Knowledge of thermomechanical behavior is however essential for the validation of multiaxial models to democratize the use of these materials. This work presents the development of innovative biaxial dynamic tests for the study of the pseudo-elastic mechanical behavior of a nickel titanium under biaxial dynamic compression. A single projectile, launched on a complex system of 45 ^ { circ} triangles, bars and transmitters, creates a biaxial solicitation of the sample. This mechanism ensures that the loads are synchronized along the two axes. Thanks to its architecture, the system makes possible many cases of multiaxial dynamic stress, unexplored until now. The forces and displacements across the sample are calculated independently, along both axes, from strain gauges glued on bars. The use of a thermal and optical camera allows strain and heating sources fields to be identified. The stress field is estimated by combining the information from the strain gauges placed on the bars and a finite element analysis of the specimen. The deformation appears homogeneous in the biaxial loading region of interest where a significant rise in temperature due to the phase change latent heat is observed. On the other hand the dynamic testing allows an equivalent dynamic stress/strain curve under biaxial and quasi-adiabatic conditions to be plotted. Experiments are finally compared to the results of finite difference axisymmetric model where the constitutive law is given by a fully coupled stochastic multiscale model. ; La microstructure cristalline de l'alliage joue un rôle vis-à-vis des mécanismes de transformations, ...

Shape memory alloys (SMA) undergo a solid-solid phase transformation called martensitic transformation, involving a "high temperature" phase, austenite, and a "low temperature" phase, martensite. The crystalline microstructure of the alloy alters mechanisms of transformations, and its pseudo-elastic behavior. The reduction of the grains to nanometric size shows new and unknown thermal and mechanical characteristics. Knowledge of thermomechanical behavior is however essential for the validation of multiaxial models to democratize the use of these materials. This work presents the development of innovative biaxial dynamic tests for the study of the pseudo-elastic mechanical behavior of a nickel titanium under biaxial dynamic compression. A single projectile, launched on a complex system of 45 ^ { circ} triangles, bars and transmitters, creates a biaxial solicitation of the sample. This mechanism ensures that the loads are synchronized along the two axes. Thanks to its architecture, the system makes possible many cases of multiaxial dynamic stress, unexplored until now. The forces and displacements across the sample are calculated independently, along both axes, from strain gauges glued on bars. The use of a thermal and optical camera allows strain and heating sources fields to be identified. The stress field is estimated by combining the information from the strain gauges placed on the bars and a finite element analysis of the specimen. The deformation appears homogeneous in the biaxial loading region of interest where a significant rise in temperature due to the phase change latent heat is observed. On the other hand the dynamic testing allows an equivalent dynamic stress/strain curve under biaxial and quasi-adiabatic conditions to be plotted. Experiments are finally compared to the results of finite difference axisymmetric model where the constitutive law is given by a fully coupled stochastic multiscale model. ; La microstructure cristalline de l'alliage joue un rôle vis-à-vis des mécanismes de transformations, et de son comportement pseudo-élastique. La réduction des grains vers une taille nanométrique offre notamment des caractéristiques thermiques et mécaniques bien différentes et peu connues. La démocratisation de l'utilisation de ces matériaux passe par la validation de modèles complets et donc par la connaissance du comportement thermomécanique multiaxial dynamique. Ce travail propose la mise en place d'essais dynamiques biaxiaux innovants pour l'étude du comportement mécanique pseudo-élastique d'un nickel titane sous compression dynamique biaxiale. Un seul et unique projectile, lancé sur un système complexe de triangles à 45^{circ}, barres et renvois d'angles, permet de solliciter l'échantillon de manière biaxiale. Le mécanisme permet notamment d'assurer la synchronisation des chargements selon les deux axes. Grâce à son architecture, le banc d'essais rend possibles des cas de sollicitations dynamiques multiaxiales inexplorés jusqu'à présent. Le grand nombre de points de mesure par jauges de déformations collées sur les barres permet de calculer les efforts et déplacements aux limites de l'échantillon et ce indépendamment suivant les deux axes de sollicitation. L'utilisation d'une caméra thermique et d'une caméra optique permet d'identifier les champs de déformations et de sources de chaleur. Le champ de contrainte est estimé en combinant les informations des jauges de déformations et une analyse par éléments finis de l'échantillon. La déformation apparaît homogène dans la région de chargement biaxiale, où une augmentation significative de la température due à la chaleur latente de changement de phase est observée. Les essais dynamiques permettent d'autre part d'établir une courbe contrainte / déformation dynamique équivalente dans des conditions biaxiales et quasi-adiabatiques. Les expériences sont finalement comparées aux résultats du modèle axisymétrique en différences finies où la loi de comportement est donnée par un modèle multi-échelle stochastique entièrement couplé.

International audience ; Shape memory alloys (SMA) undergo a solid-solid phase transformation called martensitic transformation , involving a "high temperature" phase, austenite and a "low temperature" phase, martensite. This transformation can be activated by thermal loading (heating or cooling) or mechanical loading (stress) and explains for example the pseudo-elastic phenomenon where high reversible deformation (>6%) can be reached during a tensile loading. Although the uniaxial dynamic pseudoelastic behavior of SMA is relatively well documented today, this behavior under multiaxial stress remains unknown. Such knowledge is however essential for the validation of multiaxial models to democratize the use of these materials. The stress-strain pseudo-elastic behavior of a Nickel-Titanium under equi-biaxial dynamic compression is addressed in this work. It is measured thanks to a new homemade equi-biaxial impact testing setup using split Hopkinson bar. The use of thermal and optical camera allows strain and heating sources fields to be identified. The stress field is estimated by combining the strain gauges information placed on a coaxial transmitted bar and a transmitted tube and a finite element analysis of the specimen. The deformation appears homogeneous in the biaxial loading region of interest where a significant rise in temperature due to the phase change latent heat is observed. The dynamic testing allows on the other hand an equivalent dynamic stress/strain curve under biaxial and quasi-adiabatic conditions to be plotted. Experiments are finally compared to the results of finite difference axisymmetric model where the constitutive law is given by a fully coupled stochastic multiscale model.

International audience ; Shape memory alloys (SMA) undergo a solid-solid phase transformation called martensitic transformation , involving a "high temperature" phase, austenite and a "low temperature" phase, martensite. This transformation can be activated by thermal loading (heating or cooling) or mechanical loading (stress) and explains for example the pseudo-elastic phenomenon where high reversible deformation (>6%) can be reached during a tensile loading. Although the uniaxial dynamic pseudoelastic behavior of SMA is relatively well documented today, this behavior under multiaxial stress remains unknown. Such knowledge is however essential for the validation of multiaxial models to democratize the use of these materials. The stress-strain pseudo-elastic behavior of a Nickel-Titanium under equi-biaxial dynamic compression is addressed in this work. It is measured thanks to a new homemade equi-biaxial impact testing setup using split Hopkinson bar. The use of thermal and optical camera allows strain and heating sources fields to be identified. The stress field is estimated by combining the strain gauges information placed on a coaxial transmitted bar and a transmitted tube and a finite element analysis of the specimen. The deformation appears homogeneous in the biaxial loading region of interest where a significant rise in temperature due to the phase change latent heat is observed. The dynamic testing allows on the other hand an equivalent dynamic stress/strain curve under biaxial and quasi-adiabatic conditions to be plotted. Experiments are finally compared to the results of finite difference axisymmetric model where the constitutive law is given by a fully coupled stochastic multiscale model.

International audience ; Shape memory alloys (SMA) undergo a solid-solid phase transformation called martensitic transformation , involving a "high temperature" phase, austenite and a "low temperature" phase, martensite. This transformation can be activated by thermal loading (heating or cooling) or mechanical loading (stress) and explains for example the pseudo-elastic phenomenon where high reversible deformation (>6%) can be reached during a tensile loading. Although the uniaxial dynamic pseudoelastic behavior of SMA is relatively well documented today, this behavior under multiaxial stress remains unknown. Such knowledge is however essential for the validation of multiaxial models to democratize the use of these materials. The stress-strain pseudo-elastic behavior of a Nickel-Titanium under equi-biaxial dynamic compression is addressed in this work. It is measured thanks to a new homemade equi-biaxial impact testing setup using split Hopkinson bar. The use of thermal and optical camera allows strain and heating sources fields to be identified. The stress field is estimated by combining the strain gauges information placed on a coaxial transmitted bar and a transmitted tube and a finite element analysis of the specimen. The deformation appears homogeneous in the biaxial loading region of interest where a significant rise in temperature due to the phase change latent heat is observed. The dynamic testing allows on the other hand an equivalent dynamic stress/strain curve under biaxial and quasi-adiabatic conditions to be plotted. Experiments are finally compared to the results of finite difference axisymmetric model where the constitutive law is given by a fully coupled stochastic multiscale model.