Frédéric Jenot

Ultrasonic characterization of residual stresses in steel rods using a laser line source and piezoelectric transducers

The surface residual stresses in steel rods made of the same material but with different heat treatments have been studied from Rayleigh wave velocity measurements using the laser ultrasonics technique and piezoelectric transducers. The Rayleigh wave velocities, both along the circumference and in the axis direction on the cylindrical surface of every rod, are measured and compared with one another in order to evaluate qualitatively the surface stress state for each rod. The experimental results clearly show that the Rayleigh wave velocities on the cylindrical surfaces of these rods are different, and the results obtained by laser ultrasound are in good agreement with those measured with piezoelectric transducers. From the measured velocities, different stress states have been identified for these rods according to the heat treatments they have undergone.

Corrosion thickness gauging in plates using Lamb wave group velocity measurements

The use of guided waves in ultrasonic inspection of plate and pipe structures is faster but more complicated than conventional bulk wave inspection. This is due to dispersion effects and multimode propagation. This paper describes an ultrasonic measurement method allowing us to inspect the corroded thickness of plates using Lamb waves. This method consists of measuring the group velocity of the S0 mode and then observing variations in velocity due to successive chemical attacks. From these data, it is possible to evaluate the progressively reduced thickness. Experimental and numerical results show that, depending on the chosen mode, the group velocity can be very sensitive to the reduction in thickness. Wavelet signal processing is suggested to extend this technique when several modes overlap.

Theoretical determination of Rayleigh wave acoustoelastic coefficients: comparison with experimental values

The characterization of stress states in materials is often necessary in some industrial application. The ultrasonic methods can be potentially convenient since stress states inside materials can be obtained even if materials are opaque. Nevertheless, the knowledge of acousto-elastic coefficients is generally necessary to estimate residual stresses by ultrasonic methods, but the experimental determination of these acousto-elastic coefficients can be difficult in some cases. In this paper, Rayleigh wave (RW) acousto-elastic coefficients of an orthotropic material are theoretically determined according to its characteristics, i.e. the density and the secondand third-order elastic constants. Then, these RW acousto-elastic coefficients are directly measured during an experimental stage and a comparison between calculated and measured coefficients is realized. This study allows on the one hand to check the theoretical development and on the other hand to show that it is possible to calculate acousto-elastic coefficients theoretically from intrinsic characteristics of the material rather than measuring them directly during a calibration phase which is sometimes long and difficult to realize.

Development of interdigital transducer sensors for non-destructive characterization of thin films using high frequency Rayleigh waves.

In this paper, Rayleigh waves were generated and studied over a broad frequency range (5-50 MHz) and from the dispersion phenomenon, two substrate on layer type-samples with thin layer thicknesses of 1 μm and 500 nm, respectively, were characterized. The originality in this paper is the use of surface acoustic wave interdigital transducers (IDT) to generate surface waves as well as the development of a measuring device enabling an accurate estimation of the phase velocity to be obtained, which is essential in order to characterize such thin layers. Considering the excitation frequencies (5-50 MHz) and therefore the widths necessary on the electrodes for these types of IDT sensors (20-200 μm), a lift-off procedure was chosen to deposit the electrodes on the lithium niobate (LiNbO(3)) piezoelectric substrates. The use of these IDT, first enabled problems of loss and attenuation linked to the high frequency of conventional sensors (wedge sensors) to be overcome and second to carry out quasi-monochromatic measurements in order to obtain an extremely accurate estimation of the phase velocity with rapid post-processing. An inverse method provided a very precise estimation of the thickness of the layers and the elastic constants of the substrate. The estimations of the thicknesses were then confirmed by measurements with a profilometer.

Interferometric detection of acoustic waves at air-solid interface applications to non-destructive testing

In this paper, some properties of acoustic waves at the air-solid interface are reviewed and laser-ultrasonics is used to optically excite and detect these waves. In comparison with the leaky Rayleigh wave, a large amplitude is observed for the acoustic disturbance which is composed of Scholte and lateral waves. However, it is apparent from theoretical results, taking source terms into account, that the normal displacement of these waves cannot be large in such an interface. Another explanation for the high-intensity fluctuation measured is the optical heterodyne detection of the refractive index variation induced in the fluid. This assumption is experimentally checked by probing the acoustic field parallel to the surface of the sample. The potentialities of these waves in non-destructive testing are also investigated. It is clearly shown that the transmitted, reflected, or diffracted acoustic fields provide useful information about the position or the size of structures intercepting the propagation path....

Laser generated guided waves and finite element modeling for the thickness gauging of thin layers

In this paper, nondestructive testing has been performed on a thin gold layer deposited on a 2 in. silicon wafer. Guided waves were generated and studied using a laser ultrasonic setup and a two-dimensional fast Fourier transform technique was employed to obtain the dispersion curves. A gold layer thickness of 1.33 microm has been determined with a +/-5% margin of error using the shape of the two first propagating modes, assuming for the substrate and the layer an uncertainty on the elastic parameters of +/-2.5%. A finite element model has been implemented to validate the data post-treatment and the experimental results. A good agreement between the numerical simulation, the analytical modeling and the experimentations has been observed. This method was considered suitable for thickness layer higher than 0.7 microm.

Characterization of micrometric and superficial residual stresses using high frequency surface acoustic waves generated by interdigital transducers.

Controlling thin film deposition of materials and property gradients is a major challenge for the implementation of applications in microelectronics or glassmaking. It is essential to control the level of residual stress and thus important to have the right tools to characterize this stress in terms of scale and nature of the deposits. In this context, dispersion of ultrasound surface waves caused by the presence of a residual micrometric surface stress was studied in an amorphous medium for different superficial fields of residual stress. The design and implementation of SAW-IDT MEMS sensors enabled quasi-monochromatic Rayleigh-type surface waves to be generated and the dispersion phenomenon to be studied over a wide range of frequencies. The thicknesses of the stressed cortical zones as well as the level of stress were estimated with good accuracy using an inverse method.

Influence of a superficial field of residual stress on the propagation of surface waves—Applied to the estimation of the depth of the superficial stressed zone

In this study, we were interested in the dispersion of surface waves caused by the presence of a micrometric field of residual stress on the surface of an amorphous medium. We have shown that in relation to surface waves, a stressed structure like this is comparable to a layer on substrate type structure. The design and implementation of SAW-IDT MEMS sensors enabled quasi-monochromatic Rayleigh-type surface waves to be generated and the dispersion phenomenon to be studied over a wide range of frequencies for different superficial fields of residual stress. The thicknesses of the stressed cortical zones were estimated with good accuracy using an inverse method.

High frequency ultrasonic detection of C-crack defects in silicon nitride bearing balls

A non-destructive testing method for silicon nitride bearing balls based on ultrasonic resonance spectroscopy is proposed here. Through the theoretical study of their elastic vibrations, it is possible to characterize the balls using a vibration mode that is similar to surface wave propagation. The study of the influence of C-crack defects on the resonances of Rayleigh modes is presented here. These C-cracks are typically formed by impacts between balls during finishing or handling. They are frequently found on the surface of silicon nitride bearing balls and these C-cracks decrease the rolling contact fatigue life considerably. This kind of defect is difficult to detect because the C-shaped surface crack is very small and narrow (500 microm x 5 microm), and its depth does not exceed 50 microm. The proposed methodology can both excite spheroidal vibrations in the ceramic balls and detect such vibrations over a large frequency range. In particular, high frequency vibrations are considered because these are similar to the surface waves propagating in the cortical zone of the ceramic balls and consequently they can be used to detect C-crack defects.

Effective elastic constants in acoustoelasticity

In the context of studies on ultrasonic wave propagation in stressed media (acoustoelasticity), a classic formalism can be considered (second order) by replacing the elastic constants with effective elastic constants (EECs) for which the influence of stress is taken into account. Up until now, this formalism presented some limitations since certain EEC split, which meant the authors have to consider the pseudo-orthotropy or pseudohexagonality to describe the symmetry of the materials with respect to ultrasonic wave propagation. In this letter, the possibility of avoiding these degenerations by carrying out the developments using the natural coordinates of the material is shown.