Céline Bouby

A constitutive model for Fe-based shape memory alloy considering martensitic transformation and plastic sliding coupling: Application to a finite element structural analysis

In this article, we propose a finite element numerical tool adapted to a Fe-based shape memory alloy structural analysis, based on a developed constitutive model that describes the effect of phase transformation, plastic sliding, and their interactions on the thermomechanical behavior. This model was derived from an assumed expression of the Gibbs free energy taking into account nonlinear interaction quantities related to inter- and intragranular incompatibilities as well as mechanical and chemical quantities. Two scalar internal variables were considered to describe the phase transformation and plastic sliding effects. The hysteretic and specific behavior patterns of Fe-based shape memory alloy during reverse transformation were studied by assuming a dissipation expression. The proposed model effectively describes the complex thermomechanical loading paths. The numerical tool derived from the implicit resolution of the nonlinear partial derivative constitutive equations was implemented into the Abaqus® finite element code via the User MATerial (UMAT) subroutine. After tests to verify the model for homogeneous and heterogeneous thermomechanical loadings, an example of Fe-based shape memory alloy application was studied, which corresponds to a tightening system made up of fishplates for crane rails. The results we obtained were compared to experimental ones.

Modeling of hydrogen effect on the superelastic behavior of Ni-Ti shape memory alloy wires

Superelastic NiTi wires are widely used in orthodontic treatments, but sometimes fracture can be observed after few months of use in buccal cavity and attributed to the degradation of NiTi mechanical properties due to hydrogen absorption. In this paper, a modeling approach is proposed in order to describe the effect of hydrogen diffusion on the transformation properties of NiTi SMAs. In order to experimentally predict such effects, cathodic hydrogen charging was performed at a current density of 10 for 6h, 24h, 48h and 72h in 0.9% NaCl aqueous solution at room temperature. Tensile tests were carried out shortly after hydrogen charging. The obtained stress-strain curves showed an increase of yield transformation stresses for forward and reverse martensitic transformations and a decrease of maximum transformation strain. Using Ficks second law, the transformation temperatures variation can be expressed as a function of the mean concentration of absorbed hydrogen and then taked into account in the SMA constitutive model developed by Chemisky et al (2011). The numerical results are compared to the experimental ones to calibrate the proposed method. Simulations showed that hydrogen diffusion induces a shifting of transfomation temperatures, a decreasing of maximum transformation strain and an increasing of yield transfomation stresses.

A Multiscale Analysis on the Superelasticity Behavior of Architected Shape Memory Alloy Materials

In this paper, the superelasticity effects of architected shape memory alloys (SMAs) are focused on by using a multiscale approach. Firstly, a parametric analysis at the cellular level with a series of representative volume elements (RVEs) is carried out to predict the relations between the void fraction, the total stiffness, the hysteresis effect and the mass of the SMAs. The superelasticity effects of the architected SMAs are modeled by the thermomechanical constitutive model proposed by Chemisky et al. 2011. Secondly, the structural responses of the architected SMAs are studied by the multilevel finite element method (FE2), which uses the effective constitutive behavior of the RVE to represent the behavior of the macroscopic structure. This approach can truly couple the responses of both the RVE level and structural level by the real-time information interactions between two levels. Through a three point bending test, it is observed that the structure inherits the strong nonlinear responses—both the hysteresis effect and the superelasticity—of the architected SMAs at the cellular level. Furthermore, the influence of the void fraction at the RVE level to the materials’ structural responses can be more specifically and directly described, instead of using an RVE to predict at the microscopic level. Thus, this work could be referred to for optimizing the stiffness, the hysteresis effect and the mass of architected SMA structures and extended for possible advanced applications.

Porous Shape Memory Alloy: 3D Reconstitution and Numerical Simulation of Superelastic Behavior

As a new class of metallic foam materials have attracted increasing interest in different fields of engineering. They are particularly versatile because of their interesting mechanical and physical properties: relative low density makes it possible to obtain a high stiffness/weight radio, existence of cavities results in the abilities of energy absorption and of damping, and also gives them thermal and acoustic insulation properties. As a well-known material for reversible inelastic deformation, shape memory alloys (SMA) have been paid attention on over last few years. They possess two important properties: superelasticity and shape memory effect. Cellular structures in SMAs are particularly interesting for their potential to provide superelasticity and shape memory effect in a lightweight material. In this work, 3D foam CAD structure of NiTi material is reconstituted using ellipsoid cell units. A “taking” and “placing” algorithm based on uniform distribution and normal distribution is adopted for the reconstitution process of Representative Volume Element (RVE). In the RVE, dimensions, positions, and orientations are all random. A constitutive model for shape memory alloy including phase transformation, martensitic reorientation and twins accommodation is used to simulate by the finite element analysis the superelastic behavior of the SMA foam. In order to show the efficiency of the proposed methodology, some applications are presented to simulate the compression of shape memory alloy foam. The effects of porosity, size, orientation, and ratio of long and axes short of the unit cell on the superelastic behavior of porous material are discussed.

Development of a 2-D Interfacial Dynamic Model for Detwinning in HTSMAs

This article presents the development of a 2D model for HTSMA which focus on interfaces motion in the martensitic state. We consider the topology of a 2D representative “volume” constituted of 4 martensite variants. Interfacial force is derived using Eshelby’s energy momentum tensor ans compared with a critical threshold which depends on interfacial lenght. A criterion for annihiling compatible interfaces is incorporated in this framework. The model is firtly applied to the description of detwinning mechanism in a grain. First results are presented.Copyright