Thermomechanical couplings in shape memory alloy materials

In this work we address several theoretical and computational issues which are related to the thermomechanical modeling of shape memory alloy materials. More specifically, in this paper we revisit a non-isothermal version of the theory of large deformation generalized plasticity which is suitable for describing the multiple and complex mechanisms occurring in these materials during phase transformations. We also discuss the computational implementation of a generalized plasticity based constitutive model and we demonstrate the ability of the theory in simulating the basic patterns of the experimentally observed behavior by a set of representative numerical examples

FEM-Analysis of the One-Dimensional Coupled Thermomechanical Problem of TiNi SMA

In problem solving regarding SMA materials thermomechanical coupling has to be taken into account due to latent heat of martensitic transformation and heat production resulting from the SMA material internal dissipation processes. Recently there has been developed thermodynamic three dimensional RL model of SMA behaviour in pseudoelastic range taking into account relevant transformation kinetic laws of thermoelastic martensitic transformations. In the present paper finite element code with implemented RL model constitutive equations has been applied to one-dimensional model of SMA wire sample placed in testing machine. The heat exchange conditions in the form of heat conduction along the wire and heat convection into the surrounding air have been considered. The performed coupled thermomechanical calculations allowed for quantitative évaluation of the influence of the strain rate on the stress-strain curves. The obtained numerical results have been compared with available in the literature experimental data to show sufficiently good quantitative agreement

Hystérésis d'alliages thermoélastiques, étude comparative de quelques modèles de comportement des alliages à mémoire de forme

The hysteretic behaviour of shape memory alloys (SMA) needs a more and more thin analysis because of its importance for technological applications. The comparison between different approaches allows to explicite the specifity of every model (macroscopic approach, micro-macro level, local description, phenomenological approach) and their points of convergence. On one hand, a thermodynamic treatment with a free energy expression as a mixing rule of each phase (parent or austenite phase and martensite) by adding a coupling term: the configurational energy, allowes modelling of material hysteresis loops. On the other hand, a phenomenological treatment based on a local investigation of two single crystals with a visualisation of microscopic parameters allows to perceive the phase transition mechanisms (nucleation, growth). All the obtained results show the importance of entropy production (or of the definition of the configurational energy term) for the correct description of hysteresis loops (subloops or external).Du fait de son importance pour les applications industrielles, le comportement hystérétique des alliages thermoélastiques (alliages à mémoire de forme : AMF) nécessite une analyse de plus en plus approfondie. L'examen partiel des différentes approches permet de souligner la spécificité de chacun des modèles (approche macroscopique, échelle micro-macro, descriptions locales, approches phénoménologiques) et leurs points de convergence. D'une part, un traitement thermodynamique avec une énergie libre écrite comme une loi de mélange des énergies de l'austénite et de la martensite en y ajoutant un terme de couplage (une énergie de configuration) permet la modélisation des cycles d'hystérésis du matériau. D'autre part, l'étude locale expérimentale est limitée à deux monocristaux avec des traitements thermiques différents et permet ainsi d'appréhender les mécanismes de transition de phase (nucléation, croissance, friction des interfaces ...). L'ensemble des résultats obtenus souligne l'importance décisive de la production d'entropie (ou de la définition choisie de l'énergie de configuration) pour une description correcte de boucles d'hystérésis (interne ou externe)