Erik Blomme

Recent observations with air-coupled NDE in the frequency range of 650 kHz to 1.2 MHz

An air-coupled NDE-system has been used for the inspection and imaging of discontinuities and inhomogenities in different kinds of materials, such as coating variations on tissue, spot welds on steel, and air inclusions in metal plates and welds. The measurement system operates in either continuous or pulse mode at sound frequencies between 0.65 and 1.2 MHz. Air-coupled piezo-based transducers with matching layers are used to overcome the acoustic impedance gap with air.

Two-dimensional simulation of the single-sided air-coupled ultrasonic pitch-catch technique for non-destructive testing.

Non-contact air-coupled ultrasonic inspection of materials using single-sided access offers interesting possibilities for the development of in-line non-destructive testing (NDT) systems. This contribution reports observations and simulations obtained from a single-sided air-coupled pitch-catch configuration. The pitch-catch technique involves a set-up in which transmitter and receiver are located at the same side of the test object. Sound waves, reflected once or multiple times from the back-wall of the object or refracted by a discontinuity, are recorded and analyzed for visualization. The feasibility of the technique is demonstrated, experimentally, in the case of artificial defects in aluminium samples. Depending on the configuration one or more ultrasonic images of the defect can be observed, their number and relative position containing information about the location of the defect. The experiments are simulated using two distinctive methods. The first simulation is based on a ray tracing (shadow) approach, the second method uses a spectral solution implemented within COMSOL. Both simulation methods allow simple prediction of the response images in experimental conditions with supplementary levels of complexity, which will assist the development and optimization of online inspection techniques.

Intermediate and Bragg acousto-optic interaction in elastically anisotropic medium

The Raman-Nath, intermediate and Bragg regimes of acousto-optic interaction in an acoustically anisotropic medium are theoretically examined in the paper. The general model of the interaction is extended over the case of an elastically anisotropic medium. Basic results of computer modeling of diffraction processes taking place in crystals are presented in the paper. The interaction of light and ultrasound is investigated in media with arbitrary elastic anisotropy in a wide range of the Klein-Cook and Raman-Nath parameters. The influence of the acoustic anisotropy on the process of light diffraction, especially on the light intensity in diffraction maxima, is also discussed.

Air-coupled ultrasonic assessment of wood veneer.

Air-coupled ultrasound (ACU) provides a tool to evaluate wood samples of small or moderate thickness (<30 mm) thereby avoiding direct contact or liquid coupling. Results of through-transmission ACU measurements on wood veneer samples and related products are reported with respect to a wide variety of quality aspects. Fluctuations in the averaged received signal levels appear to be correlated to the presence of natural or machine-induced thickness and density variations, flaws and grain damage, errors produced by the manufacturing process, insufficient bonding on a substrate, etc. In addition it is seen that the variability of the signal levels enables to distinguish between quarter and crown areas.

Plane‐wave analysis of the near field of light diffracted by ultrasound

Although the phenomenon of light diffraction by ultrasound has been studied very extensively during the last 40 years, almost all investigations were concentrated on the individual far field (Fraunhofer) diffraction orders. In the present paper, the basic theory is developed for studying the near field (Fresnel region) of light diffracted by an arbitrary plane ultrasonic wave and the fundamental periodicity properties are stated. The general plane-wave theory of Raman-Nath has been taken as a starting point. From the analysis, the near field of the diffracted light is seen to be highly sensitive to variations of the ultrasonic amplitude and this feature provides a useful technique for observing weak ultrasonic waves. In particular, for the specific case of Raman-Nath-type diffraction, a procedure is presented allowing the reconstruction of the time waveform of the ultrasonic wave from the diffracted light intensity signal.

Air-coupled ultrasonic evaluation of coated textiles

The feasibility of on-line monitoring variations in the weight of a coating applied on a textile carrier by means of air-coupled ultrasound is demonstrated experimentally. To that purpose, coating layers of weight varying between 25 and 95 g/m/sup 2/ were applied on cotton carriers of different weight and the acoustic attenuation of transmitted ultrasound of 0.7 MHz was measured.

A dozen Bragg effects in tellurium dioxide single crystal

The paper is devoted to the theoretical and experimental investigation of Bragg diffraction of light by shear acoustic waves in tellurium dioxide single crystal. Various cases of Bragg anisotropic interaction accompanied by a change of optical modes in the material are examined. Unknown types of isotropic diffraction of light by shear acoustic waves in paratellurite are investigated as well. Frequency dependencies of Bragg angles for different interaction types in the crystal are calculated and measured thus confirming the basic statement that not only the traditional anisotropic but also previously unknown isotropic regimes of AO interaction may be simultaneously observed in the birefringent crystal.

Amplitude modulation of diffracted light waves caused by adjacent ultrasonic beams of frequency ratio 1:n

Abstract A study has been made of the influence of the phase-difference of two ultrasonic beams, a fundamental and its n th harmonic, on the intensities or amplitudes of diffracted lightwaves. After showing the similarity between superposed and adjacent ultrasound in the Raman-Nath region ρ ⪡ 1, simplified expressions for the intensities have been set up to draw conclusions concerning the symmetry of the diffracted pattern, the amplitude modulation, and the numerical calculation of the factor α n , from the intensities in which α n represents the ratio of the ultrasonic pressures. All results are illustrated in the case of a fundamental and its second or third harmonic.

Acoustically induced light polarization effects in isotropic media

Isotropic media which become anisotropic in the presence of an acoustic wave are demonstrated to act as optical polarizors. In contrast to traditional polarizing plates or rotators, the final state of polarization of light passing through a crystal with ultrasonically induced anisotropy, can smoothly be controlled by varying the acoustic parameters. The main diffraction and polarization effects are visualized in the case of fused quartz and dense flint.

Diffraction of light by phase-shifted adjacent ultrasound beams; ρ ⪢ 1

Abstract The system of difference-differential equations for the amplitudes of a diffracted light-wave caused by two adjacent ultrasonic beams, a fundamental followed by its nth harmonic, is solved for values of ρ outside the Raman-Nath region, and simplified expressions are deduced for the intensities up to order n. Conclusions are drawn concerning the influence of the phase-shift δ on the diffraction spectrum, and the calculation of the factor αn from the intensities (αn represents the ratio of the ultrasonic pressures). All results are applied to the case of a fundamental and its third or fourth harmonic.