Sylvain Calloch

Model-Free Control of Shape Memory Alloys Antagonistic Actuators ⋆

This paper deals with a first application of the new framework of model-free control to the promising technology of shape memory alloys actuators. In particular antagonistic shape memory alloys actuator. These devices are known to be difficult to model in a control perspective. Simulations results are exposed and some preliminary experimental results illustrates the paper.

Comparative analysis of torsional and bending behavior through finite-element models of 5 Ni–Ti endodontic instruments

OBJECTIVES The objectives of this study were to compare numerically the bending and torsional mechanical behavior of 5 endodontic rotary Ni-Ti instruments with equivalent size and various designs for tapers, pitch, and cutting blades.First, the geometries of Hero (20/0.06), HeroShaper (20/0.06), ProFile (20/0.06), Mtwo (20/0.06), and ProTaper F1 were generated by finite element code. Then, the 2 most representative clinical loadings, i.e., bending and torsion, were studied with an ad hoc model for the superelasticity of Ni-Ti. Bending was generated by tip deflection and torsion by a constant twist-angle of the tip. RESULTS Mechanical behavior of these 5 endodontic rotary Ni-Ti instruments could be evaluated and compared. Protaper F1 presented the greatest level of bending stress and torque. Hero and HeroShaper were more rigid than ProFile and Mtwo. CONCLUSIONS This numerical comparison evaluated the effects of the geometrical parameters on the instrumental mechanical behavior. The 5 endodontic instruments, investigated in the present study, do not have the same bending and torsional mechanical behavior. Each clinician must be aware of these behavior differences so as to use the adequate file according to the clinical situation and to the manufacturers recommendations.

F.E. computation of a triaxial specimen using a polycrystalline model

Abstract The crystallographic approach provides an improved framework with respect to the classical macroscopic models to predict the stress-strain behavior of polycrystalline material. The model consists in a set of equations to represent the phase behavior, and a concentration rule. The present paper shows a numerical implementation of such a model, written in the framework of viscoplasticity with a threshold, in a parallel version of the F.E. code ZeBuLoN. Heavy computations can then be made. The system is used to simulate the mechanical response of a biaxial specimen developed at Laboratoire de Mecanique et Technologie de Cachan. This type of specimen is specially designed to support three-dimensional non-proportional loading paths, which are badly represented by classical models. The response obtained with the polycrystalline model is shown, and compared with the experimental data at the global scale. Local information concerning the stress and strain heterogeneity and the slip system activity are also available from the computation.

An improved model of 3-dimensional finite element analysis of mechanical behavior of endodontic instruments

OBJECTIVES The aim of this study was to develop a numeric method to study the mechanical behavior of an endodontic instrument during different loading paths and to demonstrate the importance of the behavior model in the finite element results. STUDY DESIGN A numeric study of an endodontic instrument was carried out. At first, the geometry was meshed with a finite element code. Then, 3 among the most representative loadings in clinical use, i.e., bending, torsion, and nonproportional bending-torsion, were studied. Each of them was simulated by setting 3 different behaviors: elasticity, elastoplasticity, and an ad hoc model for the superelasticity. RESULTS The simulations with nonproportional bending-torsion loading showed that the mechanical behavior of Ni-Ti shape memory alloy was strongly affected by change in the loading direction. Elastic and elastoplastic models were unable to consider this feature of Ni-Ti behavior. Only a superelastic model taking into account the effects of nonproportional loading proved to respect this crucial point. CONCLUSION To realize valid simulations of the mechanical behavior of Ni-Ti instruments during different mechanical loading paths, it is necessary to use an ad hoc mechanical behavior model.

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.

Contribution of accurate thermal measurements to the characterisation of thermomechanical properties of rubber-like materials

Abstract Since the pioneer works of Gough and Joule, the thermal characterisation of elastomers under mechanical loading has been investigated by numerous research teams. This is not surprising as the thermal signature of rubber is very useful data to investigate the dissipation mechanisms as well as the thermodynamical variables and couplings. In former recent studies dealing with fatigue investigations, an experimental protocol was developed. This protocol imposes cyclic loading to hourglass shaped samples, takes into account the large displacements and permits dissociation between the intrinsic dissipation, responsible for the mean temperature variation (called heat build-up in the literature) and thermomechanical couplings responsible for the temperature variation around this mean value during one cycle. Up to now, the mean temperature has been investigated in order to feed an energetic fatigue criterion. The aim of the present study is to investigate the thermomechanical couplings and the ability of thermal measurements to exhibit some specific thermomechanical properties observed for rubberlike materials. The materials studied are natural rubber and styrene butabiène rubber compounds filled with several amounts of carbons blacks. The experimental data clearly exhibit interesting features such as the thermoelastic inversion point and difference in the temperature signal between mechanical loading and unloading. This rich database is analysed and correlated to other results from the literature. The main results obtained are dealing with the ability of accurate measurements to characterise the thermodynamic couplings and to detect the stress induced crystallisation.

A New Shape Memory Alloy-Based Damping Device Dedicated to Civil Engineering Cables

Most of civil engineering cable structures are subjected to potential damages mainly due to dynamic oscillations induced by wind, rain or traffic. If vibration amplitudes of bridge cables for example are too high, it may cause a fatigue phenomenon. Recently, researches had been conducted dealing with the use of damping devices in order to reduce vibration amplitudes of cables. Thin shape memory alloy (SMA) NiTi (Nickel-Titanium) wires were used as a simplified damping device on a realistic full scale 50 m long cable specimen in Ifsttar (Nantes - France) laboratory facility, and its efficiency was shown. It has been done using finite element simulations, as well as experimental test methods. The aim of this work is to link the wire material behavior with the local damping induced along the cable qualitatively. Indeed, thermomechanical energy dissipation of the NiTi-based wires enables their damping power. The hysteretic behavior in NiTi-based alloys demonstrates a consequent dissipation because of an exothermic martensitic transformation and then an endothermic reverse transformation.

A cyclic model for superelastic shape memory alloys

This study concerns the superelasticity of Shape Memory Alloys (SMA) under cyclic loading. A particular attention is paid to the evolution of residual strain with number of cycles (like ratcheting in cyclic plasticity of classical metals). To study the phenomenology of the cyclic behavior and to identify the origin of the developed residual strain a series of cyclic uniaxial tensile tests on copper based alloys wires has been realized. A macroscopic model describing the cyclic behavior of superelastic SMA has been proposed. The originalities of the model are, on the one hand, the definition of a particular elasticity domain when the material is in a two phased state and, on the other hand, an ad hoc kinetic of transformation strain taking into account a residual strain evolution. The proposed model has been identified using our experimental data base and has been used to simulate various cyclic multiaxial loadings.

Relation between martensite volume fraction and equivalent transformation strain in an anisotropic CuAIBe shape memory alloy

This study concerns the pseudoelasticity of Shape Memory Alloys (SMA). A series of tests under tension-compression-torsion multiaxial loadings is used to show the validity of a conjecture concerning the relation between the volume fraction of martensite and the equivalent transformation strain. It is shown that the proportionality between an ad doc equivalent transformation strain and the volume fraction of martensite is confirmed under multiaxial proportional and nonproportional loadings.

Experiments of transformation-induced plasticity under multiaxial loadings for a 16MND5 low-carbon steel

This paper is concerned with the transformation plasticity of a 16MND5 low carbon steel. We present an experimental set-up to perform thermo-mechanical loads under tension-compression- torsion. Some results in the case of constant and non constant applied stresses are shown.