Local damage detection by nonlinear coda wave interferometry combined with time reversal.

Coda wave interferometry (CWI) is a sensitive ultrasound method for the detection of weak and local changes in complex inhomogeneous media. In a nonlinear modification of the method discussed here, a high-frequency probe coda is compared to its replica obtained in the presence of low-frequency pumping. If, after the filtering-out of low frequencies, the coda signals are different, this is attributed to nonlinear pump-probe interaction induced by contact acoustical nonlinearity in the damaged zone. Actually, the CWI methods are used for global inspection of complex media, such as for example, concrete structures. In this work, a step forward is made; it consists in combining the CWI with the time-reversal (TR) technique in order to allow one to focus the pump wave on a selected area in the structure and to detect and localize a flaw. Time-reverse pump is possible only in pulsed mode due to the spatio-temporal wave compression. By this reason, the particularities of coda wave mixing in conventionally used continuous and pulsed pump mode are considered. It has been experimentally observed that an aftereffect of a pulsed pump provides a nonlinear interaction between pump and probe waves of a sufficient overall level for defect detection with TR. Finally, it was shown that a TR focusing even with the minimal available quality i.e., with only one transducer produces a sufficient contrast allowing to distinguish intact and damaged zones with nonlinear CWI.

Influence of acousto-optic interactions on the determination of the diffracted field by an array obtained from displacement measurements.

This paper deals with the influence of acousto-optic interactions on the displacement measurements performed over transducer array and their effects on the predicted diffraction field. Changes on the temporal/spatial responses and the plane wave decomposition of the displacement are discussed. Modifications made on the directivity pattern are shown. A theoretical analysis of acousto-optic phenomenon, based on the plane wave decomposition of radiated field by the array is developed. Theoretical and experimental results are compared, showing first that waves with phase velocity near the one of the fluid are greatly amplified. Second, the interaction of laser beam with edge wave produced by the vertical size of elements induces a parasitic temporal pulse on the x-t diagram and so an interference pattern in the omega-k diagram. Corrections are proposed to eliminate errors induced by acousto-optic interactions and validated by comparing predicted diffraction field with measurements.

Contact phase modulation method for acoustic nonlinear parameter measurement in solid.

In this work, a new method to measure in contact the nonlinearity parameter beta of solid plates is presented. A high frequency (HF) tone-burst signal of 20 MHz is inserted in the material by a contact-transducer (with a suitable coupling). A low frequency (LF) pulse (2.5 MHz) is applied to the other face, in the opposite direction, so that the nonlinear interaction of the two waves takes place during the back propagation toward the HF transducer. This collinear interaction creates a phase modulation of the HF tone-burst which is proportional to the beta coefficient and the particle velocity of the LF wave. To determine this particle velocity, in time domain, an extended self-reciprocity calibration of the contact LF transducer is used. A numeric phase demodulation is then performed, giving the beta coefficient of the sample. The proposed method is validated by nonlinearity parameter measurements in Fused Silica. The nonlinear parameter of Fused Silica measured is found to be in good agreement with the literature, and specially the negative sign of this parameter.

Presence of nonlinear interference effects as a source of low frequency excitation force in vibro-acoustography.

The vibro-acoustography imaging method consists of forming an image of the deformability of a tissue submitted to a low frequency fLF stress field. This sound field can be created locally by means of a focused annular array emitting two primary beams driven at two close frequencies fa and fb = fa + fLF. In the existing literature, the origin of this stress field has been identified as the low frequency radiation pressure of the two primary beams. However, this work intends to show that another contribution to this internal stress is the low frequency field distributed in the object volume and created by the nonlinear interferences of the two primary beams. This nonlinear field was calculated in the case of multiple ring annular arrays and compared with the qLF beam experimentally measured in a water tank. The agreement between the theoretical and experimental curves provides information on the possibility that this nonlinear effect takes place in vibro-acoustography.

Time-domain modeling of nonlinear distortion of pulsed finite amplitude sound beams.

This work aims to validate a time domain numerical model for the nonlinear propagation of a short pulse of finite amplitude sound beam propagation in a tissue-mimicking liquid. The complete evolution equation is simply derived by a superposition of elementary operators corresponding to the 'one effect equation'. Diffraction LD, absorption and dispersion LAD, and nonlinear distortion LNL effects are treated independently using a first order operator-splitting algorithm. Using the method of fractional steps, the normal particle velocity and the acoustical pressure are calculated plane by plane, at each point of a two-dimensional spatial grid, from the surface of the plane circular transducer to a specified distance. The LA operator is a time convolution between the particle velocity and the causal attenuation filter built after the Kramers-Kroning relations. The LNL operator is a time-based transformation obtained by following an implicit Poisson analytic solution. The LD operator is the usual Rayleigh integral. We present a comparison between theoretical and experimental temporal pressure waveform and axial pressure curves for fundamental (2.25 MHz), second, third and fourth harmonics, obtained after spectral analysis.

Characterization of airborne transducers by optical tomography

This paper describes the application of an acousto-optic method to the measurement of airborne ultrasound. The method consists of a heterodyne interferometric probing of the pressure emitted by the transducer combined with a tomographic algorithm. The heterodyne interferometer measures the optical phase shift of the probe laser beam, proportional to the acoustic pressure integrated along the light path. A number of projections of the sound field, e.g. a set of ray integrals obtained along parallel paths, are made in moving the transducer to be tested. The main advantage of the method is its very high sensitivity in air (2 x 10(-4) Pa Hz-1/2), combined with a large bandwidth. Using the same principle as X-ray tomography the ultrasonic pressure in a plane perpendicular to the transducer axis can be reconstructed. Several ultrasonic fields emitted by wide-band home made electrostatic transducers, with operating frequencies between 200 and 700 kHz, have been measured. The sensitivities compared favorably with those of commercial airborne transducers.