N. Mercier

Modeling of transient deformation of piezoelectric ceramics (US NDE)

A method based on a finite-element analysis that permits the calculation and the visualization of deformations of ultrasonic transducers, in different configurations is presented. The method uses a Fourier series synthesis giving deformation of the free surface and electrical admittance for wide frequency bands by the use of finite-element procedures previously developed at Universite de Technologie de Compiegne (UTC). The method was originally developed for the monochromatic case. The authors report on the adaptation and validation of the method for the multifrequency excitation case, which is a more realistic approach used in the field of ultrasonic NDE. Several geometries were tested. In spite of the limitations of the method, due to the calculation requirements, the results show the role of the radial displacement behavior usually neglected in monodimensional analysis.<<ETX>>

Characterization of flaws using the poles and zeros of the transfer function of the corresponding ultrasonic system

Abstract This investigation deals with a technique of flaw characterization which exploits the spectral phase of the echographic signal as a principal parameter. A method has been developed to determine the distribution of the poles and zeros of the transfer function of the echographic system in the complex plane. It seems to be easier to exploit this distribution rather than dealing directly with the spectral amplitude and phase. The results obtained on several artificial flaws are presented.

Phase Information in Ultrasonic Spectroscopy

In the domain of flaw characterisation using Ultrasonic Spectroscopy we have developed a method which complements the information given by the amplitude of the echographic signal by the analysis of the phase of this signal as a function of frequency. First of all it was necessary to estiblish a criterion t o overcome the effects of the phase variations due to the propagation of the ultrasonic wave between the transducer and the flaw. The existence of supplementary information in the phase signal which is not available in the amplitude signal takes the form of pure dephasers in the corresponding transfer function. We present the results obtained on several artificial flaws (simple targets of different dimensions and shapes) and particularly the configuration of the zeros of the transfer function in the s plane.

Dispersion Curves Analysis for Bonded Plates at Low Fd

Today the use of adhesive bonds is more developed in many industries and more specifically in Space Industries. Its development is limited by the possibility to make a non-destructive evaluation of the quality of the bond. Up to now, the inspection of the cohesion and the thickness of the glue are well controlled. With regard to the adhesion, the problem is not yet solved.

INDICATION OF THE PHENOMENON OF ULTRASOUND DISPERSION IN COMPOSITES BY A TIME FREQUENCY REPRESENTATION

Time frequency representation methods are well adapted to the study of time varying signals and dispersion phenomena. A time frequency analysis is presented for the echographic signal obtained on a concentrated porosity and for the transmitted signal obtained at oblique incidence through a defect free composite.

OPTIMISATION OF ULTRASOUND ADHESIVE BOND TESTING METHODS USING NUMERICAL MODELING

This paper presents a method which permits the optimum conditions to test adhesive bonds by ultrasound to be evaluated using a numerical model. The model calculates the propagation of ultrasonic waves in a multilayer medium for various boundary conditions (perfect and imperfect) which simulate a good or bad adhesive bond. It is used to establish the dispersion curves of Lamb waves for the multilayer medium. The parameters for which there is a difference between good and bad conditions permit the bond to be evaluated. A practical application of the method and experimental results is presented.

DETERMINATION OF TRANSIENT DEFORMATIONS ON THE ACTIVE SURFACE OF N.D.E. ULTRASONIC ELEMENTS

A method based on a finite element analysis is presented which permits the calculation and the visualization of deformations of ultrasonic transducers in different configurations. The method uses a Fourier series synthesis giving deformation of the free surface and electrical admittance for wide frequency bands by the use of finite element procedures previously developped at Universite de Technologie de Compiegne (UTC) for the monochromatic case. The work reported in this paper deals with the adaptation and validation of the method for the multi-frequency excitation case, which is a more realistic approach used in the field of ultrasonic NDE.

Visualization of transient behaviour of piezoelectric ceramics by means of finite elements method

A method based on a finite-element analysis that permits the calculation and the visualization of deformations of ultrasonic transducers in different configurations is presented. The method uses a Fourier-series synthesis giving deformation of the free surface and electrical admittance for wide frequency bands by the use of finite element procedures previously developed. These methods were originally developed in the monochromatic case: one of the difficulties of this work was the adaptation and the validation of the methods in the multifrequency excitation cases, more realistic in the area of nondestructive examination (NDE) using ultrasonic methods. Several geometries were then tested, and an experimental confirmation of the results was achieved.<<ETX>>

DEPTH RESOLUTION BY MEANS OF ULTRASONIC SIGNALS PROCESSING TECHNIQUES

SUMMARY In order to solve the problem of detection of small flaws near the surface of the piece to be controlled, several methods have been developped at U.T.C. We present here one of these methods which exploits a relatively delicate parameter, the spectral phase of the echographic signal. Then we compare the results obtained with the ones given by two other methods : a spectral substitution method and a method which consists to excite the transducer with an optimal signal giving the shortest possible echographic response.