M. Ouaftouh

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.

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

In this paper, a non-destructive method for the determination of residual stress profiles as a function of depth in laminated aluminium alloy sheets is presented. An ultrasonic method using Rayleigh waves propagating along the sides of the sheet is proposed. The determination of residual stresses is based on the measurement of the relative variation of the ultrasonic wave velocity versus the depth. An experimental device, using the acousto-optic interaction, has been developed to measure the velocity of the Rayleigh wave. Several residual stress profiles obtained by this technique are shown. The latter are compared to other stress profiles obtained by other methods: layer removal method and acoustic detection using wedge transducer.

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.

Ultrasonic evaluation of residual stresses in flat glass tempering: Comparing experimental investigation and numerical modeling

In order to control residual stress distribution in glass, techniques based on the phenomenon of photoelasticity are efficient, though subject to the inherent limitations of all optical techniques. To mitigate these limitations, we exploit the phenomenon of acoustoelasticity to estimate residual stress distribution, using surface acoustic waves. Experimental results are obtained for a 6mm thick soda-lime silicate flat glass plate that had been subjected to nonuniform thermal tempering and whose stress distribution is calculated using finite element modeling. The estimated stress distributions provided by our ultrasonic method compare quite well with the results from the modeling, from both the qualitative and quantitative points of view.In order to control residual stress distribution in glass, techniques based on the phenomenon of photoelasticity are efficient, though subject to the inherent limitations of all optical techniques. To mitigate these limitations, we exploit the phenomenon of acoustoelasticity to estimate residual stress distribution, using surface acoustic waves. Experimental results are obtained for a 6mm thick soda-lime silicate flat glass plate that had been subjected to nonuniform thermal tempering and whose stress distribution is calculated using finite element modeling. The estimated stress distributions provided by our ultrasonic method compare quite well with the results from the modeling, from both the qualitative and quantitative points of view.

Influence of natural and initial acoustoelastic coefficients on residual stress evaluation: Theory and experiment

The velocities of the ultrasonic bulk waves propagating in a prestressed material with orthorhombic symmetry are formulated in two frames of reference. From these velocities, acoustoelastic coefficients are theoretically determined with the natural and initial coordinates. For each acoustoelastic coefficient, the differences between its expressions according to the two systems of coordinates are then theoretically calculated and experimentally determined. From these results, the error on the residual stress evaluation due to the use of natural acoustoelastic coefficients rather than those defined with the initial coordinates is estimated.

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....

Ultrasonic spectroscopy of composite materials

Abstract Composite materials are more and more used in high technology industries owing to their unique mechanical properties. For such heterogeneous and quasi-periodic materials, classical non-destructive evaluation (NDE) methods prove very unsatisfactory, and even often inapplicable. Our aim here is to investigate the particular case of ultrasonic NDE of fibre-reinforced composite materials. When using the classical A-scan echography method, it is difficult to interpret time diagrams. This is mainly due to the heterogeneous nature of the materials, the complexity of the diagrams being especially increased by the occurrence of distributed defects (like porosities). Using spectral analysis techniques (ultrasonic spectroscopy), information may be extracted from these intricate time diagrams, In particular, the occurrence of a selective absorption frequency in both the transmitted and reflected energy spectra is evident. This absorption phenomenon is clearly related to the material structure and periodicity and advantage may then be taken of its characteristics (centre frequency, bandwidth and relative depth of the absorption dip in the spectrum) to characterize the material and identify the presence of defects of porosities.

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.