Christophe Bouvet

Mechanical Behavior of a Cu-Al-Be Shape Memory Alloy Under Multiaxial Proportional and Nonproportional Loadings

This paper is concerned with the mechanical behavior of a Cu-Al-Be Shape Memory Alloy (SMA). A large experimental database, made of tension-internal pressure tests and biaxial compressive tests, is reported. Particular attention is paid to the behavior of the material under multiaxial proportional and nonproportional loadings. Moreover, these two types of complementary tests allow for the determination, on the one hand, of the shape of the initial surface of transformation onset and, on the other hand, of the initial direction of the transformation strain rate. A generalized macroscopic J 2 -J 3 criterion to describe the transformation onset is proposed and identified.

Experimental and numerical determinations of the initial surface of phase transformation under biaxial loading in some polycrystalline shape-memory alloys

Biaxial proportional loading such as tension (compression)–internal pressure and bi-compression tests are performed on a Cu-Zn-Al and Cu-Al-Be shape memory polycrystals. These tests lead to the experimental determination of the initial surface of phase transformation (austenite→martensite) in the principal stress space (σ1,σ2). A first “micro–macro” modeling is performed as follows. Lattice measurements of the cubic austenite and the monoclinic martensite cells are used to determine the “nature” of the phase transformation, i.e. an exact interface between the parent phase and an untwinned martensite variant. The yield surface is obtained by a simple (Sachs constant stress) averaging procedure assuming random texture. A second modeling, performed in the context of the thermodynamics of irreversible processes, consists of a phenomenological approach at the scale of the polycrystal. These two models fit the experimental phase transformation surface well.

Analysis of mechanical behavior and in-situ observations of Cu-Al-Be SMA under biaxial compressive tests by using DIC

A Digital Image Correlation technique is used to develop and validate a new biaxial compression set-up. Some experimental test have been performed under proportional and nonproportional loading conditions with the experimental set-up. During these test, a particular attention is paid to the appearance and the disappearance of martensite plates during the loading path. By using a long distance microscope with a CCD camera, we show the importance of the mechanical loading path shape on the martensite formation.

What about the Yield Transformation Surface Determination (Austenite → Martensite) with The Measurement of Austenite and Martensite Lattice Parameters for some Shape Memory Alloys?

Like in the plasticity theory, the prediction of phase transformation yield surfaces constitutes a key point in the modeling of polycrystalline shape memory alloys thermomechanical behavior. Generally in some micro-macro integration, the nature of the interface between austenite and twinned or untwinned martensite under stress free state and the choice of correspondance variants (CV) or habit plane variant (HPV) are determining for the explicit expression of the yield criterion. If the prediction of some copper-based alloys (interface between austenite and one single variant of martensite) and the Cu-Al-Ni for cubic to orthorhombic phase transformation (interface between austenite and twinned martensite) is fairly good, the prediction is not efficient for the important case of Ti-Ni (interface between austenite and twinned martensite with stress free state). The usual hypothesis consisting in neglecting the effect of stress on the interface geometrical configuration must be revisited.

Testing and Modeling A Cu-Al-Be Shape Memory Alloy Behavior under Complex Stress Loading

On one hand, bi-compression tests and tension (compression)-internal pressure tests on a Cu-Al-Be shape memory alloy (SMA) polycrystals permit the determination of the yield surface of phase transformation initiation (Austenite → Martensite). On the second hand, the lattice measurements of the L21 austenite and the monoclinic martensite cells for this alloy permit to determine the “nature” of the phase transformation i.e. an exact interface between the parent phase and an untwinned martensite variant. Hence, a micro-macro integration allows to predict the yield curve of the same polycrystalline alloys. At last, some tools are given for a general phenomenological modeling at the macroscopic scale, in the frame of the thermodynamic of irreversible process of isotropic pseudoelasticity in SMA. These two different investigations fit well the experimental results.