K. Van Den Abeele

Nonlinear acoustic time reversal imaging using the scaling subtraction method

Lab experiments have shown that the imaging of nonlinear scatterers using time reversal acoustics can be a very promising tool for early stage damage detection. The potential applications are however limited by the need for an extremely accurate acquisition system. In order to let nonlinear features emerge from the background noise it is necessary to enhance the signal-to-noise ratio as much as possible. A comprehensive analysis to determine the nonlinear components in a recorded time signal, an alternative to those usually adopted (e.g. fast Fourier), is proposed here. The method is based on the nonlinear physical properties of the solution of the wave equation and takes advantage of the deficient system response scalability with the excitation amplitude. In this contribution, we outline the adopted procedure and apply it to a nonlinear time reversal imaging simulation to highlight the advantages with respect to traditional imaging based on a fast Fourier analysis of the recorded signals.

On the use of a chaotic cavity transducer in nonlinear elastic imaging

We discuss the advantages of “chaotic cavity transducer focusing” to enhance the localization of microdamage in conjunction with nonlinear elastic wave spectroscopy methods. Chaotic cavity transducer focusing is defined as the hardware-software combination of a piezoelectric ceramic glued on a cavity of chaotic shape on the one hand with the reciprocal time reversal (or inverse filter) technique on the other hand. Additional optimization through the use of sweeps and inverse filtering techniques are discussed as well. The technique is applied to image a crack in a steel sample.

A spectroscopic study of the chromatic properties of GafChromic™EBT3 films

PURPOSE This work provides an interpretation of the chromatic properties of GafChromicEBT3 films based on the chemical nature of the polydiacetylene (PDA) molecules formed upon interaction with ionizing radiation. The EBT3 films become optically less transparent with increasing radiation dose as a result of the radiation-induced polymerization of diacetylene monomers. In contrast to empirical quantification of the chromatic properties, less attention has been given to the underlying molecular mechanism that induces the strong decrease in transparency. METHODS Unlaminated GafChromicEBT3 films were irradiated with a 6 MV photon beam to dose levels up to 20 Gy. The optical absorption properties of the films were investigated using visible (vis) spectroscopy. The presence of PDA molecules in the active layer of the EBT3 films was investigated using Raman spectroscopy, which probes the vibrational modes of the molecules in the layer. The vibrational modes assigned to PDAs were used in a theoretical vis-absorption model to fit our experimental vis-absorption spectra. From the fit parameters, one can assess the relative contribution of different PDA conformations and the length distribution of PDAs in the film. RESULTS Vis-spectroscopy shows that the optical density increases with dose in the full region of the visible spectrum. The Raman spectrum is dominated by two vibrational modes, most notably by the ν(C≡C) and the ν(C=C) stretching modes of the PDA backbone. By fitting the vis-absorption model to experimental spectra, it is found that the active layer contains two distinct PDA conformations with different absorption properties and reaction kinetics. Furthermore, the mean PDA conjugation length is found to be 2-3 orders of magnitude smaller than the crystals PDAs are embedded in. CONCLUSIONS Vis- and Raman spectroscopy provided more insight into the molecular nature of the radiochromic properties of EBT3 films through the identification of the excited states of PDA and the presence of two PDA conformations. The improved knowledge on the molecular composition of EBT3s active layer provides a framework for future fundamental modeling of the dose-response.

Direct optical characterization of ultrasonic waves using Raman-Nath diffraction of convergent light

Abstract An acousto-optic method for visualizing ultrasonic pulses based on Raman-Nath diffraction of a convergent light beam is presented and its basic features are experimentally confirmed. The laser light intensity of the far field diffraction orders becomes dependent on time, with a period of 1/f∗p (where f∗p denotes the repetition frequency of fundamental tone of the diffracting pulse). The spectrum of the modulated light measured by a photodetector in one specific direction behind the acousto-optic cell contains all the relevant information about the ultrasonic wave. Experimental verification has been obtained for continuous and pulsed ultrasound in a TeO2 crystal.

Main-sequence broadening in the double clusterh and Ξ persei

Precise photometric observations of stars in the double cluster h and Ξ Persei reveal a large spread in the colours and magnitudes of the upper Main-Sequence; half of the stars are variables that are Be stars or related stars. The reported age difference between both clusters is found to be spurious. Rotation apparently affects both the intrinsic and the observed colours of the early-type stars in h and Ξ Persei. This result questions the validity of photometric calibrations that heavily rely on h and Ξ Persei or similar clusters.

Experimental Detection of Structural Damage in Natural Building Stones Using Nonlinear Wave Modulation Spectroscopy

Intact and damaged samples of natural building stones used in restoration projects throughout Europe have been examined using both linear and nonlinear acoustic methods. Specific attention is drawn to the examination of near‐surface deterioration. The samples are locally excited with a high frequency sinusoidal wave (order of 100 kHz) and in some cases a coupling with a low frequency signal is produced by an impact. Linear wavespeed and attenuation measurements are supplemented with nonlinear acoustic measurements investigating the creation of harmonics and intermodulation frequencies. Consistent observations were obtained on various samples. Undamaged areas are essentially linear in their response, while the damaged zones become highly nonlinear.

Strong interaction of arbitrary fields of sound and light: Application to higher‐order Bragg imaging

Small objects positioned in a high‐frequency ultrasonic beam can be imaged by Bragg diffraction of light. The first order contains one image. Using a light beam with a considerable convergence angle and reducing the ultrasonic frequency, one observes that the second diffraction order contains two adjoining images, the third order three, etc., and that the positive orders are the mirror images of the negative ones. These experimental observations are explained by the present theory and general expressions for the angular distribution of the light in the different diffraction orders are presented in the form of a series expansion. Evidence for the multiple images in the higher diffraction orders is found by analyzing the first term in this expansion. The center‐to‐center separation of the images within the higher orders is found to be proportional to the ultrasonic frequency and the interaction width.

Probing of ultrasonic pulses by multidirectional light

Abstract An acousto-optical reconstruction method for acoustic signals using multidirectional light diffraction by finite amplitude ultrasonic pulses is presented. When crossing the ultrasonic field, the far field diffracted laser light intensity of an incident convergent lightbeam becomes modulated in time. It is found that for special conditions, concerning direction of observation, ultrasonic frequency, power level and interaction length, the modulated light intensity is almost an exact copy of the diffracting acoustic pulse. Reconstruction can be completed by applying a fast Fourier transform (FFT) routine. Examples are provided and applications of this optical probing technique are suggested.

Acoustooptic Reflection Coefficient for Bounded Beams on Plates Using Inhomogeneous Wave Description

Basic work in the study of reflection of bounded beams on liquid-solid interfaces was done by Schoch [1], showing a displacement of the beam when the angle of incidence corresponds to what is known as the Rayleigh angle. As this simple model did not account completely for different observed phenomena, Bertoni and Tamir [2] developed a more successful analytical model to describe nonspecular reflectivity for liquid-solid at this critical angle of incidence. Pitts, Plona and Mayer [3] have extended Bertoni and Tamir’s method to describe nonspecular reflectivity for a bounded beam incident at Lamb-mode angles on a plate immersed in a liquid. Although these results are qualitatively consistent with experimental observations, the method yields to considerable analytical problems when applied to non-Gaussian beam profiles and is only applicable when the resonant conditions prevail.

Nonlinear and Hysteretic Constitutive Models for Wave Propagation in Solid Media with Cracks and Contacts

This chapter is devoted to theoretical concepts and models for wave propagation, vibrations, or other elastic deformations in solids containing internal contacts (cracks, delaminations, etc.). A direct problem of solid mechanics is solved by building up a solution for elastic fields in materials with known geometry and properties. This study is oriented to nondestructive testing and therefore focuses on the case where the material contains few cracks of known configuration, in contrast to microcracked solids in which a statistical ensemble of a large number of internal contacts is present. Our approach is based on finite element simulations and a frictional contact model assuming generic semi-analytical solutions. These solutions account for surface roughness, friction, and the evolution of stick and slip zones in the contact area. Finally, load–displacement relationships valid for arbitrary loading histories are produced which are used as boundary conditions imposed at internal boundaries (cracks) in the material. As a result, we have developed a numerical toolbox capable of modeling elastic waves and vibrations in damaged samples or structures. The access to all elastic fields together with their nonlinear components makes nondestructive testing fully transparent and offers an opportunity of purposeful optimization of the experimental techniques.