Didier Laux

Correction of diffraction effects in sound velocity and absorption measurements

The mean pressure over a receptor placed at a small distance from a circular source is calculated in order to estimate the effect of diffraction when the medium is transparent or slightly absorbing. The deviation relative to the propagation without diffraction is written in the form of a simple integral of the radial wave vector. The source and receiver are considered of the same size, but the expression for different diameters is given. Approximate analytical expressions are derived when the diffraction effect is small, i.e., when the source is much larger than the wavelength. The analytical expressions are used to show that diffraction corrections can easily be performed in a standard pulsed echo experiment.

Ultrasonic study of UO2: effects of porosity and grain size on ultrasonic attenuation and velocities

Abstract Ultrasonic techniques applied to nuclear fuel characterisation are developed in our group since 1996. Before applying our methods to irradiated fuel, we are searching sensitive parameters which could give interesting information. That is the reason why only results concerning non-irradiated UO2 are presented. This paper mainly deals with the investigation of a relevant acoustic parameter: the attenuation. Indeed, the ultrasonic attenuation in UO2 as a function of the operating ultrasonic frequency has been measured on samples with various microstructures: variable fraction volume porosity (1–6%) and grain size (10– 90 μm ). Using a 15 MHz operating frequency, no attenuation has been observed. With frequencies around 40 MHz, we show that the measured ultrasonic attenuation is only sensitive to grain size (no effect of porosity has been observed). On the contrary, the ultrasonic velocities (which are very sensitive to porosity) are not affected by the sizes of the grains. These reversed and non-correlated effects constitute an interesting tool for UO2 study because two aspects of the microstructure can be studied separately with ultrasonic waves.

Ultrasonic properties of water/sorbitol solutions.

Ultrasonic longitudinal velocity and attenuation were measured for aqueous solutions of sorbitol at approximately 5 MHz. For pure sorbitol, the ultrasonic velocity reached 3200 m s(-1), consequently leading to a high acoustical impedance (around 5 x 10(6) Rayleigh) and good matching between the ultrasonic transducers and material samples.

Wood elastic characterization from a single sample by resonant ultrasound spectroscopy

The goal of this paper is to propose an experimental method allowing the identification of the complete elastic tensor of anisotropic biological materials such as wood using only one sample. To do so, two complementary methods are used. First, the wood eigen-directions are defined from a sample of spherical shape that is then cut into a cube in a way to perform resonant ultrasound spectroscopy (RUS). The method is successfully applied on a reference beech sample with known orthotropic directions. A comparison of the identified elastic constants with those from the literature and some inferred from ultrasonic transmission measurements is given.

High frequency acoustic microscopy for the determination of porosity and Young's modulus in high burnup uranium dioxide nuclear fuel

During irradiation UO2 nuclear fuel experiences the development of a non-uniform distribution of porosity which contributes to establish varying mechanical properties along the radius of the pellet. Radial variations of the porosity and of elastic properties in high burnup UO2 pellet can be investigated via high frequency acoustic microscopy. Ultrasound waves are generated by a piezoeletric transducer and focused on the sample, after having travelled through a coupling liquid. The elastic properties of the material are related to the velocity of the generated Rayleigh surface wave (VR). A 67 MWd/kgU UO2 pellet was characterized using the acoustic microscope installed in the hot cells of the Institute of Transuranium Elements: 90 MHz frequency was applied, methanol was used as coupling liquid and VR was measured at different radial positions. By comparing the porosity values obtained via acoustic microscopy with those determined using ceramographic image analysis a good agreement was found, especially in the areas close to the centre. In addition Youngs modulus was calculated and its radial profile was correlated to the corresponding burnup profile.

Enhanced multiple ultrasonic shear reflection method for the determination of high frequency viscoelastic properties

In this work, we propose a study dedicated to the influence of the delay line nature in transverse ultrasonic sensors, dedicated to dynamic high frequency elastic moduli of viscoelastic materials estimation. In literature, these shear ultrasonic rheometers are using delay lines in glass or quartz and normal or oblique incidence of ultrasonic rays. The oblique incidence is used in order to improve the sensitivity of the measurements. We theoretically demonstrate in this work that the use of delay lines in polymers is recommended to improve the sensitivity. Due to modifications, performed on a 10 MHz commercial ultrasonic sensor, we experimentally show on glycerin (which is a Newtonian material) that it is possible to multiply by a factor 10 the sensitivity; compared to delay lines in quartz using a normal incidence of rays. Hence, we overpass the accuracy of the oblique incidence approach with a simpler experimental setup.

Elastic characterization of wood by Resonant Ultrasound Spectroscopy (RUS): a comprehensive study

The main principle of Resonant Ultrasound Spectroscopy (RUS) measurement method is to excite a sample and to deduce its elastic constants from its free mechanical resonant frequencies. The goal of this paper is to propose an application of RUS in the case of wood cubic samples by: (1) using frequencies and mode shapes (or vibration patterns) of the free resonant modes in an iterative numerical procedure to solve the inverse problem for identifying components of the stiffness tensor of the sample’s material, (2) finding the limits and optimizing the robustness of the identification procedure in the case of wood and (3) applying it to a large density range of wood samples. Specific continuous waves have been used as excitation signal in order to experimentally determine the free resonant frequencies and mode shapes of the sample in a faster way by means of Scanning Doppler Vibrometer measurements. Afterward, the stiffness tensor was derived by solving iteratively an inverse problem. The gain of using the mode shapes in the inverse identification procedure is demonstrated to be particularly necessary for wood, especially for pairing each measured frequency with its corresponding theoretically predicted one, as viscoelastic damping causes the resonant peaks to overlap and/or disappear. A sensitivity analysis of each elastic constant on the measured resonant frequencies has thus been performed. It shows that, in its current state of development, not all of the elastic constants can be identified robustly and a modified identification procedure is thus proposed. This modified procedure has been applied successfully to wood samples with a large density range, including softwood and hardwood, and particularly non-homogeneous wood species or with specific anatomical features.

Role of red cells and plasma composition on blood sessile droplet evaporation

The morphology of dried blood droplets derives from the deposition of red cells, the main components of their solute phase. Up to now, evaporation-induced convective flows were supposed to be at the base of red cell distribution in blood samples. Here, we present a direct visualization by videomicroscopy of the internal dynamics in desiccating blood droplets, focusing on the role of cell concentration and plasma composition. We show that in diluted suspensions, the convection is promoted by the rich molecular composition of plasma, whereas it is replaced by an outward red blood cell displacement front at higher hematocrits. We also evaluate by ultrasounds the effect of red cell deposition on the temporal evolution of sample rigidity and adhesiveness.

Ultrasonic assessment of olive oil mixtures properties

Mainly produced in the Mediterranean countries, olive oil excels by its nutritional and medicinal benefits. However, olive oil available in the market is not always authentic. It can be altered by inadequate storage conditions or if it is mixed to other kinds of oils. The objective of this work is to use ultrasonic methods for a possible quick detection of olive oil adulteration from the characterization of its ultrasonic properties. In this context, ultrasonic longitudinal propagation velocity and attenuation were measured for pure olive oil and for its mixtures with soya oil at different proportions, at 20 °C and using a 5 MHz ultrasonic transducer. Then, the longitudinal viscosity of the different oil samples has been deduced on the whole bandwidth of the transducer (3-7 MHz) thanks to frequential analysis of the rough data. Thanks to the measurement of the shear viscosity with a Couette viscosimeter, we also deduced the bulk viscosity. Propagation velocity starts from 1458 m.s-1 for pure olive oil and increases with percentage of soya oil in the mixture up to 1464 m.s-1. The corresponding longitudinal viscosity decreases with percentage of soya oil from 161.4 mPa.s, for pure olive oil, to 136.7 mPa.s, for pure Soya oil. As a function of frequency, longitudinal and bulk viscosities decrease, for all proportions of soya oil, indicating that ultrasonic relaxation may take place. Thanks to the high level of accuracy of the method proposed in this communication, the results show the ability of ultrasonic methods to be an efficient tool, which allows distinguishing mixtures of oils and thus detecting adulterated olive oil.

Fabrication, characterization and test of acoustic sensors for detection of pollutants in aquatic environments

In this paper, we present an ultrasonic transducer for the detection of hydrocarbon pollutants with a given concentration threshold. The device is made by screen-printing using piezoelectric PZT (Lead Zirconate Titanate). The piezo device is used as an ultrasound emitter and receiver that allow the measurement of sound velocity in a liquid medium, the value of the sound velocity being modified by the presence of pollutants.