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ABSTRACT: Dynamic acoustoelastic testing is applied to weakly pre-loaded unconsolidated water-saturated glass beads. The gravitational acceleration produces, on the probed beads, a static stress of order 130 Pa, thus the granular medium is close to the jamming transition. A low-frequency (LF) acoustic wave gently disturbs the medium, inducing successively slight expansion and compaction of the granular packing expected to modulate the number of contacts between beads. Ultrasound (US) pulses are emitted simultaneously to dynamically detect the induced modification of the granular skeleton. US propagation velocity and attenuation both increase when the LF pressure increases. The quadratic nonlinear elastic parameter β, related to the pressure dependence of US propagation velocity, was measured in the range 60-530 if water-saturated glass beads are considered as an effective medium. A dynamic modification of US scattering induced by beads is proposed to modulate US attenuation. Complex hysteretic behaviors and tension-compression asymmetry are also observed and analyzed by time-domain and spectral analyses. Furthermore acoustic nonlinearities are measured in cases of quasi-static and dynamic variations of the LF wave amplitude, providing quantitatively similar acoustic nonlinearities but qualitatively different.
The Journal of the Acoustical Society of America 12/2010; 128(6):3344-54. · 1.55 Impact Factor
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ABSTRACT: The transient analysis of piezoelectric transducers is often performed using finite-element or finite-difference time-domain methods, which efficiently calculate the vibration of the structure but whose numerical dispersion prevents the modeling of waves propagating over large distances. A second analytical or numerical simulation is therefore often required to calculate the pressure field in the propagating medium (typically water) to deduce many important characteristics of the transducer, such as spatial resolutions and side lobe levels. This is why a hybrid algorithm was developed, combining finite- difference and pseudo-spectral methods in the case of 2-D configurations to simulate accurately both the generation of acoustic waves by the piezoelectric transducer and their propagation in the surrounding media using a single model. The algorithm was redefined in this study to take all three dimensions into account and to model single-element transducers, which usually present axisymmetrical geometry. This method was validated through comparison of its results with those of finite-element software, and was used to simulate the behavior of planar and lens-focused transducers. A high-frequency (30 MHz) transducer based on a screen-printed piezoelectric thick film was fabricated and characterized. The numerical results of the hybrid algorithm were found to be in good agreement with the experimental measurements of displacements at the surface of the transducer and of pressure radiated in water in front of the transducer.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 05/2010; 57(5):1188-99. · 1.80 Impact Factor
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ABSTRACT: Elastography applications require the use of efficient models to simulate the propagation of shear waves in soft media such as human tissues. These models are needed to improve understanding of the measured displacement field, to reconstruct the viscoelasticity of heterogeneous tissues, and to test inversion algorithms. This paper reports a numerical model based on a pseudospectral time domain method developed to simulate shear and compression wave propagation in an axisymmetric heterogeneous viscoelastic medium. This model was adapted to the study of soft tissues where the ratio between the compression and the shear wave velocity was about a thousand and validated in the homogeneous situation by comparison with an analytical model based on elastodynamic Green's functions. Displacements obtained experimentally using transient elastography are presented, compared with simulation results, and discussed.
The Journal of the Acoustical Society of America 10/2009; 126(4):2108-16. · 1.55 Impact Factor
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ABSTRACT: In a recent publication [E. Filoux, S. Callé, D. Certon, M. Lethiecq, F. Levassort, Modeling of piezoelectric transducers with combined pseudospectral and finite-difference methods, J. Acoust. Soc. Am. 123 (6) (2008) 4165-4173], a new finite-difference/pseudospectral time-domain (FD-PSTD) algorithm was presented and used to model the generation of acoustic waves by a piezoelectric resonator and their propagation in the structure and the surrounding water. In this paper, the model has been extended to simulate the two-dimensional behaviour of a complete single-element transducer, composed of the resonator, a backing and a front matching layer. This further version of the model takes into account the mechanical loss in materials, and enables the calculation of electrical impedance, which is a characteristic of high interest to optimize the performance of ultrasonic transducers. The impedance curves of a PZT [URL: http://www.ferroperm-piezo.com (last viewed 04/2008); B. Jaffe, R.S. Roth, S. Marzullo, Piezoelectric properties of lead zirconate-lead titanate solid-solution ceramics, J. Appl. Phys. 25 (1954) 809-810] plate-based high-frequency transducer, with a 50 MHz thickness resonant frequency, were compared to those of a KLM model [R. Krimholtz, D.A. Leedom, G.L. Matthei, New equivalent circuit for elementary piezoelectric transducers, Electron. Lett. 6 (1970) 398-399] in the one-dimensional case. The acoustical properties were also found to be in good agreement with those obtained using the finite element (FE) method of ATILA software in two-dimensional configuration.
Ultrasonics 07/2009; 49(8):611-4. · 1.84 Impact Factor
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ABSTRACT: Remote dynamic acoustoelastic testing is proposed to provide a noninvasive and regional measurement of elastic and dissipative acoustic nonlinearities in fluids and solids. The probed medium is dynamically stressed by a low-frequency sinusoidal variation in the hydrostatic pressure in the surrounding fluid. Simultaneously, ultrasound pulses propagate undergoing time of flight and energy modulations, associated with elastic and dissipative nonlinearities, respectively. Acoustic nonlinearities in tension phases can be distinguished from those measured in compression phases. Instantaneous modulations of ultrasound time of flight and energy are analyzed as functions of the instantaneous low-frequency acoustic pressure, similar to a quasistatic acoustoelastic testing.
Applied Physics Letters 01/2009; 94(1):011905-011905-3. · 3.84 Impact Factor
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ABSTRACT: During the last few years, the combination of Time Reversal (TR) acoustics with Nonlinear Elastic Wave Spectroscopy (NEWS) techniques has been used as a new tool for local investigation of nonlinearity in complex media. TR process is used to retrofocus and reconstruct coherent signals in order to locally generate nonlinear stressed area. It is necessary to evaluate the exact signature of the nonlinear response i.e. classical nonlinearity, local or global nonlinearity or non‐classical nonlinearity generated by cracks. This study deals with the investigation of the amplitude dependence of the nonlinear component versus the fundamental excitation level with numerical and experimental analyzes. Parametric interaction of TR synchronized acoustic waves (f1 = 490 kHz and f2 = 860 kHz) has been chosen as the NEWS technique for local interrogation of a solid steel sample with and without cracks. Pulse inversion filtering has been used in order to extract nonlinear signature at frequencies fL = f2−f1 kHz and fH = f2+f1 kHz. A pseudo‐spectral 3D time domain algorithm developed for the NEWS‐TR method has been adapted to this amplitude dependence study. As done for experiments, the parametric component amplitude dependence is observed for various excitation parameters and comparison with the Westervelt theory is discussed.
AIP Conference Proceedings. 06/2008; 1022(1):521-524.
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ABSTRACT: To study "non-classical" acoustical nonlinearities in cracked materials, we developed a nonlinear (NL) wave coupling technique. Propagation velocity and amplitude of short high-frequency (HF, 600 kHz) bursts are modulated as result of nonlinear interaction with a low-frequency (LF, 3 kHz) wave. Time Of Flight Modulation (TOFM) is indeed related to both elasticity and density variations. Because high acoustic nonlinearities in glass beads have been reported, we applied our technique to glass beads with different diameters (hundreds micrometers). A small container is filled up with glass beads saturated with water and placed below the LF source. The large HF to LF ratio (200) and the small sample size (6 cm) compared to the LF wavelength in water (50 cm) allow: 1) to measure instantaneous Time Of Flight Modulation (TOFM) and attenuation as functions of the instantaneous LF pressure, 2) to consider the LF pressure field a quasi-static hydrostatic variation of the ambient pressure in the medium. The LF pressure amplitude in water is approximately 10 kPa. Different TOFM and NL attenuation behaviors in tension and in compression, as well as different hysteresis patterns, are observed. The influence of the LF pressure amplitude is discussed.
The Journal of the Acoustical Society of America 06/2008; 123(5):3289. · 1.55 Impact Factor
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ABSTRACT: Nonlinear elastic wave spectroscopy (NEWS) techniques have been developed to study the anomalously high level of "nonclassical" nonlinearity of cracked materials. Within AERONEWS project (http:www.kuleuven-kortrijk.beaeronews), more recent innovative nondestructive NEWS techniques have been proposed to detect microinhomogeneities like cracks. It has been notably shown that NEWS and time reversal (TR) techniques can be combined to precisely localize cracks with a high sensitivity. NEWS techniques can be used either as a post-treatment of TR used as a tool for strong localized stress generation (TR-NEWS), or as a pre-treatment of TR used as a tool for defect (nonlinear source) identification (NEWS-TR). A 3D multiscale pseudo-spectral time domain (PSTD) code has been developed to simulate nonlinear acoustic propagation in heterogeneous nonlinear hysteretic solids. The hysteretic nonlinearity is introduced owing to a PM space model, based on a multiscale approach, extended in 3D owning to Kelvin notations. Using this algorithm, the influence of various parameters such as the defect position or the filtering method is particularly described for NEWS-TR and TR-NEWS methods. Finally, the sensitivity and feasibility of both methods are discussed.
The Journal of the Acoustical Society of America 06/2008; 123(5):3832. · 1.55 Impact Factor
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ABSTRACT: The numerical simulation of acoustic waves propagating in inhomogeneous media is often achieved using pseudospectral (PS) algorithms, which require few nodes per wavelength to converge, while complex piezoelectric structures are simulated with finite-difference (FD) or finite-element (FE) methods. A combination of the PS and FD algorithms, retaining their advantages, is presented in order to simulate the behavior of various piezoelectric transducers used in ultrasonic imaging with one single model. The theory is exposed and the algorithm is applied to simulate PZT resonators flooded into water. Perfectly matched layers are developed to absorb the mechanical waves at the borders of the computational domain, and space-shifted grids are used to reduce Gibbs phenomenon. The electrical impedance and various physical parameters (displacements, electric potentials) are calculated. Different high frequency transducer configurations have been modeled. In the case of a simple two-dimensionnal plate, described in Cartesian coordinates, the results are satisfactorily compared to those obtained with a commercial FE software. Then, simulations of an axisymmetrical single-element transducer are favorably compared to FE simulations and experimental measurements. The hybrid algorithm is also used to calculate the large radiation pattern of an annular array with little time-processing, which illustrates the efficiency of the method.
The Journal of the Acoustical Society of America 06/2008; 123(5):3787. · 1.55 Impact Factor
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ABSTRACT: Bone tissue contains microcracks which may affect its mechanical properties as well as the whole trabecular structure. The relationship between crack density and bone strength is nevertheless poorly understood. Efficient nonlinear (NL) ultrasound methods have been widely developed for nondestructive testing and geophysical applications to detect microdamage. Moreover it has been reported that elastic nonlinearities increase with induced damage. We propose to monitor trabecular bone microdamage using a NL wave coupling technique. Ultrasonic short bursts times of flight (TOF) are modulated as result of NL interaction with a low-frequency (LF) wave in the medium. TOF modulation (TOFM), or propagation velocity variations, are directly related to NL elasticity variations. This technique allows measuring the instantaneous TOFM as a function of the LF pressure. It is thus possible to analyze separately elasticity variations in tension and in compression, and to distinguish the tension to compression phase from the compression to tension phase (hysteresis). In several trabecular bone samples, different TOFM amplitudes in tension and in compression are observed, probably due to microdamage. For increasing damage levels progressively induced by quasi-static compression testing, linear and nonlinear ultrasound parameters are compared to biomechanical parameters.
The Journal of the Acoustical Society of America 06/2008; 123(5):3633. · 1.55 Impact Factor
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ABSTRACT: One way to characterize metallic materials in the presence of defects like dislocation networks is to measure their large dynamic nonlinear elastic response. In this numerical study, a new method combining the nonlinear elastic wave spectroscopy (NEWS) method with a time reversal (TR) process is proposed. This method, called NEWS-TR, uses nonlinear analysis as a pretreatment of time reversal and then consists of retrofocusing only nonlinear components on the defect position. A two-dimensional pseudospectral time domain algorithm is developed here to validate the NEWS-TR method as a potential technique for damage location. Hysteretic nonlinear behavior of the materials being studied is introduced using the Preisach-Mayergoyz model. Moreover, in order to extend this solver in two dimensions, the Kelvin notation is used to modify the elastic coefficient tensor. Simulations performed on a metallic sample show the feasibility and value of the NEWS-TR methodology for microdamage imaging. Retrofocusing quality depends on different parameters such as the filtering method used to keep only nonlinear components and the nonlinear effect measured. In harmonic generation, pulse inversion filtering seems to be a more appropriate filtering method than classical harmonic filtering for most defect positions, mainly because of its ability to filter all fundamental components.
The Journal of the Acoustical Society of America 01/2008; 122(6):3220-9. · 1.55 Impact Factor
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ABSTRACT: One of the stress sources that can be used in dynamic elastography imaging methods is the acoustic radiation force. However, displacements of the medium induced by this stress field are generally not fully understood in terms of spatial distribution and temporal evolution. A model has been developed based on the elastodynamic Green's function describing the different acoustic waves generated by focused ultrasound. The function is composed of three terms: two far-field terms, which correspond to a purely longitudinal compression wave and a purely transverse shear wave, and a coupling near-field term which has a longitudinal component and a transverse component. For propagation distances in the shear wavelength range, the predominant term is the near field term. The displacement duration corresponds to the propagation duration of the shear wave between the farthest source point and the observation point. This time therefore depends on the source size and the local shear modulus of the tissue. Evolution of the displacement/time curve profile, which is directly linked to spatial and temporal source profiles, is computed at different radial distances, for different durations of force applications and different shear elastic coefficients. Experimental results performed with an optical interferometric method in a homogeneous tissue-mimicking phantom agreed with the theoretical profiles.
The Journal of the Acoustical Society of America 12/2005; 118(5):2829-40. · 1.55 Impact Factor
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ABSTRACT: The image of a tissue deformability is obtained with the vibro-acoustography imaging method using mechanical low frequency (LF) excitation. This ultrasound excitation is locally created by means of a focused annular array emitting two primary beams at two close frequencies, f1 and f2 (f2 = f1 + fLF). The LF acoustic emission resulting from the vibration of the medium is detected by a sensitive hydrophone and then used to form the image. A theoretical model of oscillations in soft biological tissue induced by focused ultrasound is described. In the case of vibro-acoustography, oscillations originate from shear waves induced by an alternative radiation force. This non-invasive imaging method was demonstrated in this study to be suitable for tissue imaging, with x and y transverse resolutions less than 300 μm. A vibro-acoustic image of a calcaneus bone slice is presented and compared with images of the same bone realised with other bone imaging methods.
Acta Acustica united with Acustica 10/2003; 89(6):936-941. · 0.57 Impact Factor
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ABSTRACT: A tissue deformability image is obtained with the vibroacoustography imaging method using mechanical low-frequency (LF) excitation. This ultrasonic excitation is created locally by means of a focused annular array emitting two primary beams at two close frequencies, f(1) and f(2) (f(2) = f(1) + f(LF)). The LF acoustic emission resulting from the vibration of the medium is detected by a sensitive hydrophone and then used to form the image. This noninvasive imaging method was demonstrated in this study to be suitable for bone imaging, with x and y transverse resolutions less than 300 micro m. Two bone sites susceptible to demineralization were tested: the calcaneus and the neck of the femur. The vibroacoustic method provides valuable ultrasonic images regarding the structure and the elastic properties of bone tissue. Correlation was made between vibroacoustic bone images, performed in vitro, and images acquired by other imaging methods (i.e., bone ultrasound attenuation and x-ray computerized tomography (CT)). Moreover, the amplitudes of vibroacoustic signals radiating from phosphocalcic ceramic samples (bone substitute) of different porosity were evaluated. The good correlation between these results and the description of our images and the quality of vibroacoustic images indicate that bone decalcification could be detected using vibroacoustography.
Ultrasound in Medicine & Biology 04/2003; 29(3):465-72. · 2.29 Impact Factor
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ABSTRACT: Dynamic elastography using ultrasound radiation force is an imaging technique of biological tissues elastic properties. In a mechanical point of view, biological tissues are supposed isotropic, so their properties are independent of the reference axis. In these mediums, the tensor of elastic constants can be expressed as a function of two independent constants : the elastic bulk modulus K (which is linked to the compression wave propagation ) and the elastic shear modulus μ (which is linked to the shear wave propagation). The development of some cancers can result in weak variations of the bulk elastic modulus, but can considerably modify the shear elastic modulus. Measurement of m can then help for the diagnosis of this type of tissue pathology. A judicious mean to measure this parameter is the use of a non-linear effect called ultrasound radiation force. This force is proportional to the attenuation and the intensity of the ultrasound beam emitted by the imaging system. This stress source essentially generates a shear wave that propagates with a velocity proportional to the shear modulus and with a purely transverse polarisation in the far-field (far from the stress source ). Measurement of the medium displacements induced by shear wave propagation can allow to calculate the shear modulus of the medium (inverse problem resolution). We performed these measurements from the radio-frequency (RF) lines obtained with an imaging ultrasound transducer. This work describes precisely the signal processing realized on the RF lines. This processing is based on the use of a delay estimation method to measure temporal delays between RF lines during the shear wave propagation. Influence of different parameters (length of the analyse window, Signal to Noise Ratio of RF lines, sampling frequency, ultrasound transducer characteristics...) on the measurement precision has been studied. We present displacement curves as a function of time obtained after optimisation of processing parameters. Experimental results have been favourably compared to a physical model and allowed us to calculate the shear modulus of the medium. L'élastographie dynamique par force de radiation ultrasonore est une technique d'imagerie des propriétés élastiques des tissus biologiques. D'un point de vue mécanique, nous supposons que ces milieux sont isotropes c'est-à-dire que leurs propriétés sont indépendantes du choix des axes de référence. Le tenseur élastique qui définit les constantes physiques de ce milieu s'exprime en fonction de deux constantes indépendantes, le module d'élasticité volumique K (qui intervient lors de la propagation des ondes de compression), et le module d'élasticité de cisaillement μ (qui intervient lors de la propagation des ondes de cisaillement). L'apparition de certains type de cancers entraîne de faibles variations du module d'élasticité volumique K, mais peut modifier considérablement le module d'élasticité de cisaillement μ. La mesure de ce paramètre μ peut ainsi aider au diagnostic de ce type de pathologie des tissus. Un moyen judicieux de mesurer ce paramètre est d'utiliser un effet non linéaire de force de radiation ultrasonore. Cette force est proportionnelle à l'atténuation et à l'intensité des ultrasons émis dans le tissu par le système d'imagerie. Cette source de contrainte génère principalement une onde de cisaillement qui se propage avec une vitesse de phase proportionnelle au module de cisaillement et une polarisation purement transversale en champ lointain (loin de la source de contrainte). La mesure des déplacements du milieu, induits par la propagation de cette onde, peut permettre par résolution du problème inverse de remonter au module de cisaillement. Nous avons réalisé ces mesures à partir des lignes radiofréquences (RF) obtenues par un transducteur d'imagerie ultrasonore. Ce travail décrit précisément le traitement que nous avons réalisé sur les lignes RF. Ce traitement est basé sur l'utilisation d'une méthode d'estimation des retards temporels entre les lignes radiofréquences obtenues pendant la propagation de l'onde de cisaillement. L'influence de différents paramètres (taille de la fenêtre glissante d'analyse, rapport signal sur bruit des lignes RF, fréquence d'échantillonnage, caractéristiques du transducteur ultrasonore...) sur la précision de mesure des déplacements a été étudiée. Nous présentons les courbes des déplacements en fonction du temps obtenus après optimisation des paramètres de traitement. Ces résultats expérimentaux ont été favorablement comparés à un modèle physique et nous ont permis de remonter au module de cisaillement du milieu.
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ABSTRACT: Three methods to measure qualitative and quantitative aspects of bone non-linearity were investigated in the context of diagnosis of bone “biomechanical health”: i.e. harmonic generation, parametric emission and parametric reception using phase modulation. Trabecular bone exhibited hysteretic non-linear behavior due to microcracks in bone tissue, and parametric reception using phase modulation seemed to be the best configuration for in vivo application. However, the relationship between level of non-linearity and crack density needs to be validated by histological analysis.
International Journal of Non-Linear Mechanics.
