Hugues Duflo

Ultrasonic characterization of a fluid layer using a broadband transducer

A measurement method is proposed for the ultrasonic characterization of a fluid layer, corresponding to the resin transfer molding (RTM) manufacturing process. The ultrasonic velocity and attenuation of the silicone oil are measured in three samples having different viscosities. The measurement method is established on the basis of the attenuation of ultrasonic waves in fluids. A correction of the beam diffraction is implemented to improve measurement precision. A single element transducer with central frequency of 15 MHz is used. The tested fluids simulate the industrial resin used to manufacture composite materials. When injecting this resin, its viscosity increases until it reaches a critical state of polymerization. In this paper we focus on ultrasonic characterization of three fluids representing three intermediate cases of fluid resin during its injection before reaching the polymerization state.

Numerical description of the edge mode at the beveled extremity of a plate

This paper deals with the reflection of a two-dimensional harmonic Lamb wave at the beveled end of a plate. The existence of a resonant edge mode is described by a numerical model. It is proved that the edge mode is the resonance of different complex modes. The behavior of this mode as a function of the bevel angle is studied. Its amplitude decreases and its resonance frequency shifts as the bevel angle decreases from 90 to 85 deg. An unexpected strong variation of the repartition of the reflected energy is linked to this phenomenon.

Ultrasonic broadband characterization of a viscous liquid: Methods and perturbation factors

The perturbation factors involved in ultrasonic broadband characterization of viscous fluids are analyzed. Precisely, the normal incidence error and the thermal sensitivity of the properties have been identified as dominant parameters. Thus, the sensitivity of the ultrasonic parameters of attenuation and phase velocity were measured at room temperature in the MHz frequency range for two reference silicone oils, namely 47V50 and 47V350 (Rhodorsil). Several methods of characterization were carried out: time of flight, cross-correlation and spectral method. These ultrasonic parameters are measured at room temperature. For this family of silicone oil, the dispersion of the attenuation spectrum is modeled by a power law. The velocity dispersion is modeled by two dispersion models: the quasi-local and the temporal causal. The impact of the experimental reproducibility of the phase velocity and acoustic attenuation was measured in the MHz frequency range, using a set of ultrasonic transducers with different center frequencies. These measurements are used to identify the dispersion of the ultrasonic parameters as a function of the frequency. A first experimental and descriptive approach is developed to assess the reproducibility of the normal incidence between the acoustic beam and the viscoelastic material. Thus, the relative error on the measurements of velocity and attenuation are directly related to the angular deviation of the ultrasonic wave, as well as the sampling and signal-to-noise ratio. A second experimental and phenomenological approach deals with the effect of a temperature change, typical of a polymerization reaction. As a result, the sensitivity of the phase velocity of silicone oil 47V50 was evaluated around -2 ms(-1) K(-1).

Adhesion characterization and defect sizing of sandwich honeycomb composites

Defects may appear in composite structures during their life cycle. A 10MHz 128 elements phased array transducer was investigated to characterize join bonds and defects in sandwich honeycomb composite structures. An adequate focal law throughout the composite skin gives the ultrasonic dispersive properties of the composite skin and glue layer behind. The resulting B-scan cartographies allow characterizing locally the honeycomb adhesion. Experimental measurements are compared in good agreement with the Debye Series Method (DSM). In the processed C-scan image, flaws are detectable and measurable, localized both in the scanning plane and in the thickness of the composite skin.

5K-1 F.E.M. Simulation of Lamb Wave Propagation: Application to the Thermo-Oxidative Ageing of Carbon-Epoxy Plates

Thermo-oxidative ageing of carbon-epoxy plates has been done for up to 9000 hours at two temperatures (160 and 180degC). Non-destructive evaluation by means of non contact ultrasonic methods has been tested to characterize the aged plates. It is shown that the phase velocity of the A0 and S0 Lamb modes is decreasing when ageing duration increases. Micrographs reveal the presence of oxidation cracks propagating from the free surfaces of the plate. The plates thus become inhomogeneous in the thickness. The propagation of the Lamb waves, in the aged plates, is simulated using a finite element model of an inhomogeneous and anisotropic plate. The excitation of several modes in a frequency range (chirp excitation) or a single mode at a chosen frequency (burst windowed excitation) is applied. To simulate the ageing of carbon-epoxy composite plates (8 plies), the elastic parameters are decreased in the two external plies. The results are then compared with experiments. Using this FE model, we demonstrate that this phenomenon is related to an important decrease of the C55 elastic parameter of the plate

Immersed narrow plate study. Lamb wave identification

An experimental investigation of the interaction of an acoustic plane wave with a limited elastic plate is presented. This study shows that it is possible to observe Lamb waves on a duraluminium limited plate when the wavelengths of the generated Lamb waves are in the same order of magnitude as the target dimension. Impulsional experiments are performed and a laser vibrometer is used to measure the normal displacements on the plate, An experimental image of the vibrations is obtained by scanning the limited plate along a segment in the direction of the Lamb waves.

Scholte wave propagation and diffraction on a fluid-solid periodic rough surface

Experimental results concerning propagation and diffraction of a Scholte wave on periodic liquid-elastic solid interface are presented. We describe the behaviour of the Scholte wave when it meets the grating. Thus, to give a complete description of the phenomena observed, one must account for three effects: (1) Scholte conversion into Rayleigh waves, (2) diffraction along the grating in the liquid and in the solid, and (3) restoration of the Scholte wave at the end of the grating

Experimental determination of plate parameters with an air coupled instrument

The present work deals with an experimental determination of acoustical properties of a viscoelastic homogeneous plate using an air coupled equipment. Usually, these properties are determined separately through several measurements, while our paper provides a quick inverse method which permits to measure them simultaneously and requires only the knowledge of the sound velocity in air and its density. The experimental transmission through a face parallel plate, at normal incidence, is investigated and compared with the theoretically predicted one. Measurements are conducted using a chirp signal having 1.2 MHz central frequency with 800 kHz bandwidth and 100 μs duration. Two signals are being exploited : one only through air path and another one through the sample inserted between the transducers. The FFTs of these signals provide the complex transmission coefficient as a function of frequency. The identification of parameters is carried out using the real and imaginary parts of the transmission coefficient. Then, assuming the wave velocity and density of air are known, we deduce the four properties of the plate (thickness, density, longitudinal velocity and attenuation coefficient) as well as the attenuation coefficient of air. A variety of viscoelastic materials, whose impedances have weak values, has been studied such as Polyethylene, Plexiglas and carbon/epoxy composite. The physical properties for each plate are obtained with a good accuracy in the frequency range of investigation.

Simultaneous determination of physical and acoustical properties of a homogeneous plate in air

This work is concerned with measurements of transmission through an elastic homogenous plate using air-coupled transducers. An experimental technique for investigating the transmission is presented. This method permits to determine simultaneously the velocity and attenuation of ultrasonic waves in the material, its mass density and the thickness of the plate, through a comparison between theory and experiment in the frequency range of investigation.

Thermal aging characterization of composite plates and honeycomb sandwiches by electromechanical measurement

After identifying the parameters of the piezoelectric transducer, the mechanical impedance of the front medium is deduced. More particularly, the wave propagation velocity and attenuation are deduced in the inspected plate. By fitting the electrical impedance measurement results, the aging of composite materials is quantified, showing the effectiveness of this means of nondestructive evaluation. A specific measurement tool and protocol are proposed. Several estimators are identified on the basis of Gaussian fits of the resonances observed on the electrical impedance measurements. Those estimators are identified and the obtained results show that both the frequencies and widths of the resonances peaks vary according to the health of the plate, or aging duration. This method is applied successfully on non-perfect sandwich plates, porous, and with non-ideally flat surface. The sensitivity and limits of the most relevant estimators are discussed for the two studied plate families.