Rupture différée dans l'acier austénitique 301LN

L'objectif de ce travail est l'étude multi-échelles du phénomène de rupture différée associé à la transformation martensitique dans l'acier austénitique instable 301LN. L'observation et l'étude de la rupture différée sont réalisées sur des emboutis cylindriques à fond plat. L'identification des éléments microstructuraux à l'origine des fissures est réalisée par observation au Microscope Electronique à Balayage (MEB) des zones de rupture. La distribution des contraintes résiduelles dans chaque phase, a été déterminée par Diffraction des Rayons X (DRX), et associée à l'évolution de la fraction volumique de martensite pour différentes conditions d'emboutissage. Nous établissons un lien direct entre l'apparition de la rupture différée, l'état de contrainte dans la martensite, et les fractions de martensite formée dans les godets emboutis

Identification of Model Parameter for the Simulation of SMA Structures Using Full Field Measurements

With the design of new devices with complex geometry and to take advantage of their large recoverable strains, shape memory alloys components (SMA) are increasingly subjected to multiaxial loadings. The development process of SMA devices requires the prediction of their thermomechanical response, where the calibration of the material parameters for the numerical model is an important step. In this work, the parameters of a phenomenological model are extracted from multiaxial and heterogeneous tests carried out on specimens with the same thermomechanical loading history. Finite element analysis enables the computation of numerical strain fields using a thermodynamical constitutive model for shape memory alloys previously implemented in a finite element code. The strain fields computed numerically are compared with experimental ones obtained by DIC to find the model parameters which best matches experimental measurements using a newly developed parallelized mixed genetic/gradient-based optimization algorithm. These numerical simulations are carried out in parallel in a supercomputer to reduce the time necessary to identify the set of identified parameters. The major features of this new algorithm is its ability to identify material parameters of the thermomechanical behavior of shape memory alloys from full-field measurements for various loading conditions (different temperatures, multiaxial behavior, heterogeneous test configurations). It is demonstrated that model parameters for the simulation of SMA structures are thus obtained based on a reduced number of heterogeneous tests at different temperatures.NSF International Institute of Multifunctional Materials for Energy Conversion (IIMEC), award #084108

Homogenization methods applied to model the thermomechanical behavior of shape memory alloys

Modem scale transition methods are used to establish the thermomechanical behavior of shape memory alloys. Physical mechanisms responsible for the observed phenomena are considered and two differents models are used to deal with intra and intergranular effects. Discrete internal variables description is used to determine constitutive equations for single crystal and a self-consistent method is used to derive the polycrystalline response. Overall behavior so computed shows excellent agreement with experimental measurements

ELEMENTS OF STRUCTURE CALCULATION FOR SHAPE MEMORY DEVICE

In order to dimension the mechanical elements made of shape memory alloys (SMA), a method based on phenomenological relation between stress and strain has been developed. This method and applications to pseudoelastic helical spring problems are described in this paper

Micromechanical Modelling of Superelasticity in Shape Memory Alloys

Micromechanical methods developed to describe the thermomechanical behavior of solids are applied to phase transition related problem. Results obtained are compared with those obtained using a macroscopic phenomenological approach. This micromechanical analysis is based on a kinematical description of the physical strain mechanisms and a definition of a local thermodynamical potential. Volume fractions of the different variants of martensite are chosen as internal variables to describe the evolution of the microstructural state of the material. This analysis determines local constitutive equations for the behavior. Global relationships are obtained using a self consistent scheme. This approach gives results in good agreement with experimental observations performed on Cu-based Shape Memory alloys

THERMOMECHANICAL CONSTITUTIVE EQUATIONS FOR SHAPE MEMORY ALLOYS

Shape Memory Alloys present a large variety of behaviour in function of the thermomechanical loading pathes and the microstructural states of the material. These responses are due to different physical mechanisms of deformation which are associated to the thermoelastic martensitic transformation : - oriented growth of the martensitic plates by the applied stress in Superelasticity ; - mobility of the interfaces between the variants of martensitic in the so-called Shape Memory Effect ; - capability of the internal stress field produced in the material by oriented defects left by some previous transformation sequences, to influence the growing of the martensite in the Two-Way Shape Memory Effect. The determination of the constitutive equations for the mechanical behaviour of these alloys must take into account these particular mechanisms of deformation. For each physical mechanism it is necessary, at first, to make a kinematical study of the strain associated to it. After this step, an energy balance between the driving and the resistive forces is established in each case from the analysis of the Gibb's free energy of the transformation or by using the Eshelby formalism of energy momentum. Phenomenological flow rules are then determined from the classical concept of normality rule. In this contribution, the micromechanical aspects of the phase transformation mechanisms are presented both from the statical and the kinematical point of view. Special attention is given to the internal stress state associated with variant and grain interactions