3D Printing of Small-Scale Soft Robots with Programmable Magnetization

status: publishe

Ultrasonic evaluation of stresses in orthotropic materials using Rayleigh waves

International audienc

Evaluation of Dispersion Modes for Layered Structures Using Focused Acoustic Waves

The evolution of Lamb wave dispersion has a great significance for the characterization of layered materials such as composite plates, bonded joints, and coating materials. The method traditionally used to evaluate locally the dispersion Lamb modes refers to the use of two wide-band transducer in “pitch-catch” disposition. By varying the emitter-receiver at the opposite incident-reflection angle ø, and scanning the emission tone-burst frequency f, one distinguishes the minima in the reflection coefficientR(f)of the test sample at each ¸, the dispersion relation of group velocity v g versus frequency f can be established by v g = v 0/sin ¸ where v 0 is the wave velocity in coupling liquid [1–3]. However, there are some inconveniences for this method. Firstly, it needs an ultrasonic goniometer to adjust two probes at a varying oblique incident angle, which is not practical for NDE utilization. Secondly, with finite-sized transducers where the incident beam is not really a plane wave, the angular and spectra resolution will be limited, and in addition there will be a dead angle for small incident angle. Thirdly, there exists the so called “non-specular reflection” occurring at Lamb critical angles and it causes perturbations for reflection wave detection [1,2]. To overcome these disadvantages, we employ a focused acoustic beam of large angular aperture at normal incidence to evaluate the reflection coefficient R(¸, f) for layered structures. This method demands only one probe and the problem of the non-specular reflection can be avoided as the wave reflection at the different angle is all captured at the same time. The principle is based on registration of Acoustic Material Signature or V (z)curve of the sample. An inversion algorithm of Fourier transform permits us to reconstruct R(¸) if the V (z) is measured in amplitude and in phase. By scanning f for each V (z) measurement, we obtain the reflection coefficient function R(¸,f). The Lamb wave modes is then be evaluated from the minima appearing in the magnitude of R(¸,f)data.</p

Determination of stresses in aluminium alloy using optical detection of Rayleigh waves

International audienc

Measurement of Reflectance Function for Layered Structures Using Focused Acoustic Waves

In the ultrasonic NDE of layered materials and structures, such as bonded joint, coating, and in particular the composite material, the surface or Lamb wave velocity or the reflection and transmission coefficient are measured, to determine for examples, the elastic constants, the anisotropy and the integrity of the materials, etc. A commonly used technique to determine locally the surface or Lamb wave velocity V g is based on the measurement of the reflection minima or the transmission maxima at oblique incidence of the test sample. It is supposed that at the critical incident angle θ c where the reflection coefficient appears the minima, the surface or Lamb waves are favorably generated and V g =V o/sinθ c where V 0 is the wave speed in coupling liquid. So, the determination of the reflection function is essential and important. In general, the acoustic reflection or transmission coefficient of a layered medium depends on the wave incident angle θ, the wave frequency ƒ and the orientation angle φ if the material is anisotropic. To obtain the whole information of this reflectance function R(θ,φ,ƒ), one needs to insonify the structure at varying incident and orientation angles and do the frequency spectroscopy using the wide-band transducer.</p

Focusing of Surface Acoustic Waves on Nonpiezoelectric Materials: Geometrical Approach and Application to NDT

Surface acoustic waves, whose energy remains confined to a small layer typically one acoustic wavelength thick, are used in various non destructive testing applications [1–3]. The detection of defects lying at a small depth inside the bulk of materials or evolving perpendicularly to their surface, which are not easily attainable by other methods, may then be performed. In non destructive testing domain, it is of great interest to use focused acoustic waves in order to increase sound intensity and to improve spatial resolution [3–6]. The detection of defects with their longer dimension perpendicular to the sample surface, like some cracks, need a focusing of surface acoustic waves. This operation allows to increase the sensitivity of the method. The traditional SAW generation systems may not be used for focusing Rayleigh waves on nonpiezoelectric substrates. For this purpose one must use geodesical lenses or thin layer coatings inserted along the SAW propagation path [7]. Recently some authors have worked on the focusing problem. They used a dispersed wide beam width giving a poor lateral resolution [8] or a PFDF transducer at low frequency [9,10]. A new ultrasonic system working at 30 MHz is reported here for the generation of focused surface acoustic waves on nonpiezoelectric materials. Using a simple geometrical model, the resolution and conversion efficiency of the system are estimated. Experimental results on ceramic samples with artificial defects are reported and the performances of the technique are analyzed.</p

Stress State Evaluation of Laminated Aluminum Alloy Sheets by Surface Acoustic Waves

The stress state characterization in materials, is more and more necessary in mechanical industries. Indeed, the performance of pieces under mechanical, thermal or chemical perturbations, is intimately related to their stresses distribution.</p

CAPTEUR ULTRASONORE HAUTE-FRÉQUENCE À ABSORBEUR ACTIF

L'utilisation des sondes ultrasonores haute fréquence (100MHz et plus) dans le domaine biomédical, permet la caractérisation et la visualisation de structures de plus en plus fines. Les capteurs haute fréquence habituellement utilisés sont constitués d'un transducteur piézoélectrique de faible épaisseur collé sur une ligne à retard. Compte tenu de la gamme de fréquence utilisée, en plus des problèmes d'atténuation et d'adaptation, les multiples réflexions dans la ligne à retard introduisent un bruit ultrasonore qui entraîne une diminution de la sensibilité du capteur. Pour pallier à ces inconvénients, nous avons réalisé une sonde ultrasonore (100MHz) sans ligne à retard émettant directement dans le milieu biologique. Pour augmenter la bande passante, donc la résolution axiale du capteur, nous avons réalisé un absorbeur actif constitué d'un autre élément piézoélectrique. Dans notre article, nous présentons une comparaison entre résultats théoriques et expérimentaux obtenus avec ce nouveau capteur.The characterization and visualization of smaller and smaller structures become nowadays feasible in the biomedical ultrasonics area, owing to the design and construction of very high frequency ultrasonic probes (above 100MHz). These probes consist in a very thin piezoelectric transducer cemented on to an a delay line. The classical problems of ultrasonic attenuation and electrical and mechanical impedance matching are encountered. Moreover, in the used frequency range, multiple reflections on the various boundaries induce an acoustic noise which reduces the probe sensitivity. To overcome this, an ultrasonic probe radiating directly into biological medium, without buffer delay line has been built. An active damping using a second piezoelectric transducer has been designed in order to increase the available frequency bandwidth and then the axial resolution of the probe. In this paper, the experimental and theoritical results for this new kind probe are computed

CARACTÉRISATION DU SANG PAR ULTRASONS HAUTES FRÉQUENCES

Nous nous sommes attachés dans ce travail, à déterminer expérimentalement l'atténuation des ultrasons dans le sang en fonction de ses constituants essentiels (plasma et globules rouges) dans une gamme de fréquences s'étendant de 50 à 400 MHz à la température de 20°C. L'originalité et l'intérêt d'une telle entreprise consistent en une prospection plus fine des caractéristiques acoustiques du sang étant donné que dans cette gamme de fréquences, les longueurs d'ondes deviennent comparables à la taille des diffuseurs (les globules rouges). Toutefois, le choix de l'atténuation en tant que moyen de caractérisation hautes fréquences n'est pas le plus judicieux, car il semblerait qu'il n'existe pas de corrélation remarquable entre la taille des diffuseurs et la longueur d'onde d'une part et l'atténuation mesurée avec un transducteur sensible à la phase d'autre part. Il sera nécessaire d'étudier l'évolution d'autres grandeurs telle que la répartition spatiale du diffusé en fonction de la fréquence. Des mesures de l'atténuation en fonction de l'hématocrite révèlent la même dépendance en fréquence des diverses courbes. L'atténuation montre une évolution croissante jusqu'à un hématocrite de 22.5%, se stabilise entre 22.5% et 36% et évolue de nouveau jusqu'à 45%. Il a également été mis en évidence que le plasma participe dans une proportion de 36% à l'atténuation dans le sang.Our interest here is in the experimental determination of ultrasonic attenuation in blood as a function of its main components (plasma and erythrocytes) in the frequency range between 50 and 400 MHz at the room temperature of 20°C. The originality of this work is that, with such high frequencies, the wavelengths become very close to the scatterers' (the erythrocytes) size, leading to a microscopic characterization of the tissue. However, the choice of attenuation as a mean for high frequency characterization is not the most judicious one because it seems that no remarkable correlation exists between the scatterers' size and the ultrasonic wavelengths on one hand and the ultrasonic attenuation measured with a phase sensitive transducer on another hand. It will also prove necessary to study the variations of other physical parameters as the retrodiffused acoustic field spatial distribution versus frequency. The relation between the attenuation and the hematocrit is also examined up to a hematocrit of 45 percent. According to our findings, the attenuation increases along with the hematocrit until 22.5 percent, remains constant between 22.5 and 36 percent and then increases together with the hematocrit. We also measured a nearly 36 percent contribution of plasma to blood attenuation