Christian Lexcellent

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.

Characterization, thermomechanical behaviour and micromechanical-based constitutive model of shape-memory CuZnAl single crystals

Abstract CuZnAl shape memory alloys are grown as single crystals by the Bridgman technique with a final shape directly suitable for thermomechanical tests (cylinders with tapered heads: 25 mm gauge length, 4 mm in diameter). The four classical transformation temperatures are checked by Differential Scanning Calorimetry and resistivity. The orientation of crystal structure is investigated by X-ray diffraction. Isothermal pseudoelastic tensile tests show that the width of the hysteresis loops and the slope of the stress-strain curves during phase transformation increase as the applied stress rate increases. A micromechanical-based constitutive model allows us to describe this single crystal behaviour.

Analysis of the behavior of a Shape Memory Alloy beam under dynamical loading

Abstract Shape Memory Alloys (SMAs) are widely studied as new materials with potential for use in various passive or active vibration isolation systems. Up to now, few papers deal with a precise description of their proper dynamic behaviour. However, it is important to clearly understand the dissipation mechanisms in order to optimize the design of a structure. We present here a detailed characterization of a Cu–Al–Be beam. The stress induced phase transformation austenite → martensite produces a strongly nonlinear behaviour. The aim of this study is to confront experimental results to a rheological model of the beam. The experimental setup consists in a cantilever beam excited by a light electromagnetic actuator. The response is measured by an accelerometer fixed at the free end of the beam. Stepped sine measurements have been performed around the frequency of the first mode of the beam under different excitation levels. The obtained frequency response functions strongly depend on the global vibration amplitude. Then a specific finite element model has been designed, taking into account the geographic repartition of the two phases inside the beam. The simulations show a similar behaviour and allow the interpretation of the experimental observations.

Ni diffusion in near-equiatomic Ni-Ti and Ni-Ti(-Cu) alloys

Tracer volume diffusion of 63 Ni in Ni-50.07 at.% Ti binary and Ni-48.83 at.% Ti-9.29 at.% Cu ternary alloys have been measured between 783 and 1288 K. The temperature dependence of the diffusion coefficients can be well described by a straight Arrhenius function This illustrates that a diffusional anomaly (typical in some bcc metallic alloys and related to the well-known phonon softening), if there is any in this system, is negligible. On the other hand, the small activation energy (about half the value expected from simulations for the commonly accepted mechanism with thermally activated vacancies) shows that the mechanism of diffusion is probably mediated by structural vacancies.

Modeling of the strain rate effect, creep and relaxation of a Ni-Ti shape memory alloy under tension (compression) - torsional proportional loading in the pseudoelastic range

Several biaxial (tension (or compression)-torsion) proportional and non-proportional loading experiments are performed by Jesse Lim and McDowell (in 1998) for thin-wall tubes of a Ni-Ti shape memory alloy (SMA). In addition to the mechanical behavior, temperature was measured with thermocouples during the experiments, as for Cu-Zn-Al SMA polycrystal (Vacher and Lexcellent in 1991), and their Ni-Ti exhibits an asymmetrical behavior for tension-compression cycling. A model of pseudoelasticity in SMA performed by Raniecki and Lexcellent (in 1998) allowed one to take into account the asymmetry observed in tension-compression and the pure shear is extended in order to fit these biaxial proportional loadings. The change in temperature linked to the strain rate or during creep or relaxation is evaluated by the integration of the heat equation. Correlation between the results obtained and the predicted results is fairly good.

Effect of hydrostatic pressure on the martensitic transformation in near equiatomic Ti-Ni alloys

Abstract The effect of hydrostatic pressures on martensitic phase transformations in near-equiatomic Ti-Ni shape-memory alloys has been investigated up to 1.5 GPa. The relative volume changes δV c/V and the entropy changes δS c have also been determined at atmospheric pressure. Supposing that the volume derivative of the elastic energy differs from zero at the martensite start (and austenite finish) temperatures as well, that is e 0 ≠ 0, and that the Clausius-Clapeyron equation is valid for the equilibrium transformation temperature T 0, it was shown that the dissipative energy terms were approximately zero for the B2-R transition. For the R → B19′ transformation this energy was independent of pressure, while for the reverse transformation it had a defmite pressure dependence. The pressure derivatives of elastic energy e m (which corresponds to the martensite finish or austenite start) and e 0 are similar in both directions for the R-B19′ transformations, while for the B2-R transition it is negative for e 0.

Isothermal and anisothermal implementations of 2D shape memory alloy modeling for transient impact response calculation

A numerical implementation of the Raniecki Lexcellent (RL)?(Raniecki et al 1992 Arc. Mech. 44 261?284, Raniecki and Lexcellent 1994 Eur. J. Mech. A 13 21?50, Raniecki and Lexcellent 1998 Eur. J. Mech. A 17 185?205) models for shape memory alloys (SMA) coupled with the heat equation is presented in this paper, adapted to high strain rate loading. The objective is to predict the time response of a 2D SMA structure subjected to an impulse force and induced free vibration with a decreasing amplitude for isothermal and anisothermal conditions. The choice of material mechanical properties has been done in order to have phase transformations during the oscillations. The apparent damping and stiffness effects due to these phase changes is clearly identified when the results are compared with a linear model without induced martensite. The thermomechanical constitutive relation of the SMA has been fulfilled to be able to take into account the time reaction when the strain rate is very high. The full model has been implemented in a finite element code and tested on a 2D sample.

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.

Modelling Shape Memory Alloy Behaviour under Proportional Loading and Anisothermal Conditions

Based on the experimental identification of the phase transformation surface, a modelling of anisothermal behaviour of shape memory alloys (SMA) is proposed. Within the framework of the thermodynamics of irreversible processes, two internal variables are chosen: the stress-induced martensite volume fraction and the self-accommodating martensite volume fraction. A special attention is paid to take into account the asymmetry between tension and compression behaviours.