Xavier Balandraud

A review of rotary actuators based on shape memory alloys

The development of rotary actuators is an important aspect of the engineering applications of shape memory alloys. This article reviews about a hundred references on this topic, and presents around eighty actuators driven by shape memory alloys. A classification is made according to the type of rotation (continuous or non-continuous, single or reversible direction). Different factors are then discussed, such as the characteristics of the shape memory alloy elements, the heating and cooling system for the shape memory alloy, the control of the actuator, and the output torque and stroke which can be attained. This article provides the first review focused on rotary actuators triggered by shape memory alloys, highlighting the specificities and potentialities of such actuators for new applications in the future.

FEM-Based Generation of Stiffness Maps

In robotics, static stiffness maps are used as tools for the performance analysis of robots employed in production tasks, such as pick-and-place or manufacturing. This paper evaluates the relevance of a numerical tool built from a commercial finite element package to generate stiffness maps for any type of robot (serial, parallel, hybrid or compliant). The key points are the spatial resolution, the precision, and the calculation time of a stiffness map. The method for obtaining the 36 static stiffness maps of a 3-D robotic structure in its operational space is presented. The mechanical model is based on a finite element calculation using beam elements for the links and spring elements for the joints. The approach is first applied to a rigid-body mechanism. Numerical results show that a good compromise can be obtained between spatial resolution, precision, and calculation time. Then, the method is applied to a compliant structure requiring processing in a large displacement framework for the relevant estimation of a stiffness map. The numerical tool opens new prospects for the design of robotic structures, in terms of both optimization and use of various material behaviors.

Characterization of the mechanical dissipation in shape-memory alloys during stress-induced phase transformation

The high reversibility of the martensitic transformation occurring in shape-memory alloys (SMAs) is at the origin of the mechanical performance of these materials. Infrared thermography is employed in this study to measure the mechanical dissipation produced by Cu–Zn–Al SMA specimens during stress-induced phase transformation. In practice, temperature change is first measured on the surface of a specimen subjected to cyclic mechanical loading at constant ambient temperature. The heat produced by the material can then be deduced from the temperature change by using the heat diffusion equation. The heat associated to mechanical irreversibility is expected to be very low compared to the other heat quantities produced by the material (such as the latent heat due to phase change and the heat due to thermoelastic coupling), so measuring this quantity requires special attention, as emphasized in this paper. The procedure which enables us to extract the mechanical dissipation from the measured heat source is first presented. The technique is then applied to two types of specimens: martensitic and austenitic. Different values of mechanical dissipation were measured, thus revealing different levels of mechanical irreversibility.

Curvature effect on the mechanical behaviour of a martensitic shape-memory-alloy wire for applications in civil engineering

The mechanical response of a bent shape memory alloy (SMA) wire is a key point for the understanding of the process of the creation of confining effects in a wrapped concrete cylinder for example. The objective of the present study is to model the phenomena involved in the bending of a martensitic SMA wire. The mechanism of martensite reorientation is considered in the model, which also takes into account the asymmetry between tension and compression. For validation purposes, experiments were performed on Ni?Ti wires: measurement of residual curvatures after bending release and tensile tests on pre-bent wires. In particular, the analysis shows a variation in axial stiffness as a function of the preliminary curvature. This result shows the necessity of modelling the distributions of the state variables within the wire cross-section for the simulation of confinement processes using SMA wires. It also opens prospects to potential application to the bending of SMA fibres in smart textiles.

Measurement of Mechanical Dissipation in SMAs by Infrared Thermography

The reversibility of the phase transformation in shape-memory alloys (SMAs) directly governs the mechanical response of these materials. Infrared thermography is used in this study to measure the mechanical dissipation produced by copper-based SMAs under cyclic loading at ambient temperature. Several specimens with different chemical compositions are tested. Mechanical dissipation which is produced by the material is deduced from the temperature change using a 0D version of the heat equation. Results obtained show that the chemical composition as well as the nature of the phase involved (martensite or austenite) influence the mechanical dissipation produced by the specimens during cyclic mechanical loading.

Compliant Rotary Actuator Driven by Shape Memory Alloy

This paper presents a compliant rotary actuator composed of a monolithic plastic structure and a shape-memory-alloy (SMA) wire. The plastic structure features a helical shape. The SMA wire can be permanently deformed at ambient temperature. It is first pre-strained in tension, then wrapped on the helical structure and fixed at its two ends. The SMA wire is finally heated by Joule effect, leading to a continuous rotation between the two ends of the plastic structure. The rotation angle is driven by varying the intensity of the electric current. A preliminary prototype has been built and first results are presented.

Application of Full-Field Measurements to Analyse the Thermo-Mechanical Response of a Three-Branch Rubber Specimen

The study deals with the characterization of the thermomechanical behavior of rubber. A test performed on a three-branch-shaped rubber specimen is used for this purpose. This heterogeneous test simultaneously induces the three types of stretch states classically considered to identify mechanical properties of rubber (uniaxial and equibiaxial tension, and pure shear), as well as the intermediary states. Recent works in which such heterogeneous tests are studied only consider the deformation field, but neither the corresponding thermal field nor the heat sources field are taken into account. The aim of the present study is to push forward the idea of heterogeneous tests by measuring both the displacement and thermal fields on the specimen. During the experiments, the displacement and thermal fields are measured using cameras. Measurements are then processed to associate a temperature and a strain level to each material point using a motion compensation procedure. The heat source fields are then derived from the temperature maps. Indeed heat source appears to be more relevant than temperature for characterizing the thermomechanical response of materials. Results obtained during the experiments will be presented in this paper. A discussion will also be initiated on the influence of the loading conditions on the heat source maps.

Thermomechanical characterization of leathers under tension using infrared thermography

Leather materials are subjected to various deformation states during their elaboration and their use as a final product. Although the mechanical response of leathers under tension has been studied in the literature for decades, scarce information is available on the nature of their elasticity and more generally on their thermomechanical behavior. In the present study, four leathers were tested under uniaxial loading conditions while temperature changes were measured at the specimen surface using infrared thermography. Two types of tests were performed at constant ambient temperature: monotonous displacement-controlled tests until failure and cyclic load–unload tests with increasing amplitudes. The heat sources at the origin of the temperature changes were also determined by using a version of the heat diffusion equation applicable to homogeneous tests. Results enabled us to discuss the nature of thermoelastic couplings in leathers. Intrinsic dissipation caused by mechanical irreversibility was also detected and quantified. Distinct responses are evidenced depending on the type of leather tested.

Applying Infrared Thermography and Heat Source Reconstruction for the Analysis of the Portevin-Le Chatelier Effect in an Aluminum Alloy

This study deals with the Portevin-Le Châtelier (PLC) effect in an aluminum alloy. Tensile tests are performed and the temperature variations are measured at the specimen surface during the loading by infrared thermography. In order to improve the spatio-temporal identification of the plastic instabilities, the heat sources are estimated by processing the temperature fields by using a two-dimensional version of the heat diffusion equation. Filtering is a key-point of the technique since the goal is calculate both temporal and spatial derivatives from noisy temperature fields. The idea is to convolve the temperature variation fields with a kernel chosen as a first and a second derivative of a Gaussian, in order to estimate the first-order temporal derivative and the second-order spatial derivative, respectively. The study focuses on the effect of the strain rate in terms of heat source band patterns associated with the PLC effect.

Applying a Gad Filter to Calculate Heat Sources from Noisy Temperature Fields

This study deals with a post-processing technique to reconstruct heat source fields from temperature fields measured by infrared thermography. A Gradient Anisotropic Diffusion (GAD) image filter is used to process the data. The technique is first described. Synthetic temperature fields corrupted by added noise are then considered to assess the robustness of the procedure and the filter is optimized in order to reconstruct at best the heat source fields. Results are compared with those obtained with an averaging filter and a Gaussian derivative filter. The second part of the study presents an application to experimental temperature fields. Obtained results illustrate the relevancy of the GAD filter to reliably extract heat sources in thermomechanics of materials.