Microstructure des matériaux par microscopie acoustique. Mesure locale non destructive de paramètres mécaniques (Application à l’étude de l’endommagement de l’acier inoxydable 304L)

La microscopie acoustique permet de localiser à la surface ou à l’intérieur d’un matériau des différences de propriétés mécaniques. Cette information reste qualitative sur les images acoustiques obtenues par le balayage parallèlement à la surface de l’échantillon d’un capteur qui focalise un faisceau d’ondes ultrasonores haute fréquence. Néanmoins, la comparaison d’images optique et acoustique montre la meilleure sensibilité de la Microscopie acoustique à la présence de micro-fissures même lorsque ces défauts sont invisibles optiquement car enterrés sous la surface du matériau. La résolution spatiale obtenue peut atteindre le micron. Les images acoustiques ainsi prélevées révèlent la structure des matériaux étudiés en détectant des variations de propriétés mécaniques et d’orientation cristallographique. Des « Signatures acoustiques » permettent de mesurer la vitesse et l’atténuation des ondes ultrasonores à la surface du matériau sur une région dont le diamètre à la surface du matériau peut être aussi faible qu’une centaine de microns et sur une profondeur de quelques microns. L'utilisation de la théorie de l’élasticité linéaire permet alors de calculer le module d’Young et le coefficient de Poisson à partir des vitesses mesurées et de la connaissance de la masse volumique du matériau.Cette méthode de mesure de paramètres mécaniques est appliquée dans ce travail à la détection du degré d’endommagement d’un acier 304L suite a l’écrouissage du matériau par laminage a froid et par traction. L’effet de divers traitements thermiques sur la propagation de ces ondes et les propriétés mécaniques est aussi étudié

Improvement of the minimal characterisation size available by acoustic microscopy for mechanical parameters evaluation

An ultrasonic method using a large bandwidth transducer with a spherical lens and based on acoustic waves separation near the focal region is presented. The aim of this technique is to reduce the investigation size for non destructive mechanical properties evaluation. Compared to traditional acoustic microscopy (acoustic signature) the size of the analysed zone on the sample has been highly reduced. For instance, this technique has been applied on an aluminium sample with an acoustic frequency of 15 MHz. The Rayleigh wave velocity has been measured individually on grains smaller than one millimetre. Such local measurements would have required an acoustic lens working at higher frequency. All the efficiency of our experimental method and numerical signal processing has been proved by conclusive experiments on different materials such as glass, steel, silicon and uranium dioxide at different frequencies. This new method has also been tested at 100 MHz and we have demonstrated that its resolution was similar to performances of higher frequency acoustic microscopy working around 500 MHz. Furthermore our study shows that with this microdefocusing method, it is possible to assess directly from the same acquisition data Rayleigh, longitudinal and transverse velocities and consequently the elastic properties

Emulsion characterisation by focused ultrasonic waves

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Ultrasonic assessment of coatings dispersion curves by microdefocusing

In 2001 we have published a paper [1] in which an ultrasonic method using a large bandwidth transducer with a spherical lens and based on acoustic waves separation near the focal region was presented. We have shown that compared to traditional acoustic microscopy (acoustic signature) the size of the zone analysed on bulk samples was highly reduced. This method is now extended to thin films on bulk substrates. Experimental dispersion curves for thin DLC (Diamond Like Carbon) films on steel are presented. The ultrasonic velocity of leaky Sezawa mode is assessed on a large bandwidth even in zones where the transducer is not very efficient. We show that the signal processing used enlarges the frequency domain explored. Such an element is essential for inverse problem treatment and coating elastic modulus calculation. Once again we show that the length of defocusing can by highly reduced. Hence, the zone analysed on the sample is smaller

Adaptation of a High Frequency Ultrasonic Transducer to the Measurement of Water Temperature in a Nuclear Reactor

AbstractMost high flux reactors possess for research purposes fuel elements composed of plates. Their relative distance is a crucial parameter, particularly concerning the irradiation history. For the High Flux Reactor (RHF) of the Institute Laue-Langevin (ILL), the measurement of this distance with a microscopic resolution becomes extremely challenging. To address this issue, a specific ultrasonic transducer, presented in a first paper, has been designed and manufactured to be inserted into the 1.8mm width channel existing between curved fuel plates. It was set on a blade yielding a total device thickness of 1mm. To achieve the expected resolution, the system is excited with frequencies up to 70MHz and integrated into a set of high frequency acquisition instruments. Thanks to a specific signal processing, this device allows the distance measurement through the evaluation of the ultrasonic wave time of fight. One of the crucial points is then the evaluation of the local water temperature inside the water channel. To obtain a precise estimation of this parameter, the ultrasonic sensor is used as a thermometer thanks to the analysis of the spectral components of the acoustic signal propagating inside the sensor multilayered structure. The feasibility of distance measurement was proved during the December 2013 experiment in the RHF fuel element of the ILL. Some of the results will be presented as well as some experimental constraints identified to improve the accuracy of the measurement in future works

Signature V(z) en microscopie acoustique : influence de la non-uniformité de l'illumination du capteur

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Efficiency of Lamb modes in the spatial and frequency acoustic signatures for a thin layer by the resonance theory

To determine the mechanical properties of materials in thin layers by the measure of Lamb waves velocities, the acoustic microscopy can be used in two different manners: by the spatial acoustic signature V(z), and, less widespread, by the frequency acoustic signature V(f). However, in this study, we show that for each of these signatures V(z) or V(f), there are complementary domains of use in which the detection and the efficiency of Lamb modes are optimized. A specific formalism permits to foresee each of these domains where the efficiency is optimized