Serge Dos Santos

A two-dimensional pseudospectral model for time reversal and nonlinear elastic wave spectroscopy

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.

Chirp‐coded excitation applied with advanced pulse inversion for nonlinear acoustics in complex steel samples

Nonlinear acoustics for nondestructive testing is still an emergent technique in complex materials with localized cracks because linear effects can be cancelled using specific symmetries for the pre‐ and post‐signal processing. The local nonlinear signature is accessible even in complex aeronautic structure by using the symmetry properties: i. e. time reversal invariance and pulse inversion associate to chirp‐coded excitation. Excitation Symmetry Analysis Method [Dos Santos et al., JNLM, 2007] which is an extension of the pulse inverted method for extraction of third order nonlinearity is applied experimentally for the measurement of relative level of nonlinearity on a airplane steel bracket, on which visible crack was created by fatigue tests. This pre‐processing method has been coupled to a 100‐250 kHz bandwidth chirp‐coded ultrasonic excitation, realized at various amplitude (A, A/2, 31/2A/2, etc.), and performed with DAKEL IDK09 200 kHz PZT sensors. Signal processing based on correlation measurements,...

Symmetry analysis for nonlinear time reversal methods applied to nonlinear acoustic imaging

Using symmetry invariance, nonlinear Time Reversal (TR) and reciprocity properties, the classical NEWS methods are supplemented and improved by new excitations having the intrinsic property of enlarging frequency analysis bandwidth and time domain scales, with now both medical acoustics and electromagnetic applications. The analysis of invariant quantities is a well-known tool which is often used in nonlinear acoustics in order to simplify complex equations. Based on a fundamental physical principle known as symmetry analysis, this approach consists in finding judicious variables, intrinsically scale dependant, and able to describe all stages of behaviour on the same theoretical foundation. Based on previously published results within the nonlinear acoustic areas, some practical implementation will be proposed as a new way to define TR-NEWS based methods applied to NDT and medical bubble based non-destructive imaging. This paper tends to show how symmetry analysis can help us to define new methodologies a...

Optimized excitation for nonlinear wave propagation in complex media: From biomedical acoustic imaging to nondestructive testing of cultural heritage

The interaction between an acoustic wave and a complex media has an increase interest for both nondestructive testing (NDT) applications and for biomedical ultrasound. Today, new optimized excitations are generated thanks to the analysis of symmetry properties of the system such as reciprocity, nonlinear time reversal (TR) and other pulse-inverted (PI) techniques. Generalized TR based NEWS (Nonlinear Elastic Wave Spectroscopy) methods and their associate symmetry skeleton will be taken as an example. Among these family of “pulse coded excitation,” solitonic coding constitutes a new scheme in the sense that solitary waves are the best candidates for pulse propagation in nonlinear and layered media, such as tooth or skin. As another application of mixing properties in a wide frequency range, new broadband techniques are needed in the domain of the preservation of cultural heritage. The analysis of the composition of the stones is one of the key parameters in the study of aging historic buildings. The use of...

Nonlinear Time Reversal Tomography of Structural Defects

Nonlinear Elastic Wave Spectroscopy (NEWS) comprises a new class of NDT techniques providing sensitive detecting and imaging of damages like micro-cracks, etc. The main advantage of nonlinear methods is their high sensitivity and ability to detect small defects relatively far from ultrasonic actuators and sensors. Some limitations of their use are due to difficulties with defect localization when global excitation of complicated structures is considered. To overcome these constraints, a modified TR-NEWS procedure is proposed in this paper. Procedure uses transducer arrays providing improvement of damaged zone location. One array transmits and another array records signals, which are then processed as to obtain nonlinear parameters corresponding to wave-paths between transducers. DORT method is used to derive Greens function of defects and TR-NEWS procedure is completed by the Excitation Symmetry Analysis Method (ESAM) as to extract nonlinear terms from acoustic responses along different wave-paths, appro...

Modulational instability of microbubbles surface modes

Starting from the continuous Marmottant model[1], for the vibration of the bubble radius R(t), and a discrete nonlinear model of the discretized radius with periodic boundary conditions and nonlinear coupling between surface modes, MI in bubbles is investigated numerically. After a first step of Symmetry Analysis[2] applied to the nonlinear equations associated to the models, invariant properties allow an identification of dimensionless similarity variables that link both continuous and discrete models. Thanks to this similarity analysis, the bubble can be studied with a macroscopic mechanical 1D ring chain (diameter =73 cm) using N=48 nonlinearly coupled pendulum of mass m=6g and length =3.2cm with periodic conditions. The second step consists in studying numerically the MI criterion versus the amplitude of external field applied to the bubble. The analysis and the experiments reveals the existence of Intrinsic Localized Modes (ILMs), similar to those found in other more generic systems of nonlinearly vi...

Nonlinear time reversal signal processing techniques applied to acousto-mechanical imaging of complex materials

Recent ten years have seen considerable development of experimental techniques for improving nonlinear NDT methods and harmonic imaging derived from Nonlinear Elastic Wave Spectroscopy (NEWS). Diagnostic ultrasonic imaging based on higher harmonics yields, among others, a better resolution in view of the decreased wavelength in comparison with the fundamental. Furthermore, using symmetry invariance, nonlinear Time Reversal (TR) and reciprocity properties, the classical NEWS methods are supplemented and improved by new excitations having the intrinsic property of enlarging frequency analysis bandwidth and time domain scales. The purpose of this paper is to present the extension of TR-NEWS for skin aging characterization using the Nonlinear Time Reversal signal processing tool known to localize, in a complex medium, sources of nonlinearity potentially responsible of complex material aging. Linear and nonlinear behavior of skin elasticity is measured locally thanks to an acousto-mechanical loading of the ski...

Macroscopic acousto-mechanical analogy of a microbubble

Microbubbles, either in the form of free gas bubbles surrounded by a fluid or encapsulated bubbles used currently as contrast agents for medical echography, exhibit complex dynamics under specific acoustic excitations. Nonetheless, considering their micron size and the complexity of their interaction phenomenon with ultrasound waves, expensive and complex experiments and/or simulations are required for their analysis. The behavior of a microbubble along its equator can be linked to a system of coupled oscillators. In this study, the oscillatory behavior of a microbubble has been investigated through an acousto-mechanical analogy based on a ring-shaped chain of coupled pendula. Observation of parametric vibration modes of the pendula ring excited at frequencies between 1 and 5 Hz is presented. Simulations have been carried out and show mode mixing phenomena. The relevance of the analogy between a microbubble and the macroscopic acousto-mechanical setup is discussed and suggested as an alternative way to investigate the complexity of microbubble dynamics.

Lattice of nonlinear coupled oscillators: An acousto-mechanical analogy of gas microbubble

Expensive and complex experiments and/or simulations are required to study microbubbles dynamics considering its size and the complexity of the associated physical phenomenon of bubble-ultrasound interactions. To overcome this, the use of a macroscopic acousto-mechanical analogy is proposed. In this study, microbubbles surface modes are investigated through an experimental analogy based on coupled pendula. The data are further compared to numerical simulations. Vibration modes up to n = 22 have been observed for excitation frequency ranging from 1Hz to 5Hz. Simulations shows mode mixing phenomena and an influence of the amplitude on the dispersion relation. The relevance of the analogy is discussed.

3D numerical study for defect detection with nonlinear elastic wave spectroscopy and time reversal techniques

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.be/aeronews), 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....