Thomas Goursolle

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.

Investigation of damage mechanisms of composite materials: Multivariable analysis based on temporal and wavelet features extracted from acoustic emission signals

Identification methods of nonlinear parameters are developed to characterize solid materials and soft tissues. In the frame of classical parametric interaction methods, the parameter evaluation is based on the measurement of quantities related to the acoustic wave propagation medium. They appear at frequencies combinations and/or multiple harmonic of primary wave frequencies, and are generally weak in comparison to the amplitudes of the sources. Hence, the use of experimental devices excited at high amplitude levels is required. One problem in the identification of nonlinear parameters is that the ultrasonic device itself is not exempt of nonlinearity. Generally speaking the problem is to ensure, for each specific configuration, that the involved instrumentation (waveform generator, voltage amplifier, transducer), the coupling medium, mutual interactions between many sources… do not perturb the evaluation of the investigated nonlinear effects.

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,...

7D-2 3D PSTD Simulations of NEWS-TR and TR-NEWS Methods: Application to Nonclassical Nonlinearity Ultrasonic Imaging

Nonlinear elastic wave spectroscopy with pretreatment and post-treatment of time reversal, called NEWS-TR and TR-NEWS, respectively, have been investigated numerically in this work. Ultrasonic Imaging of a localized 3D damaged region, modeled by a nonclassical nonlinear behavior has been demonstrated. With NEWS-TR localization is performed with a rebroadcast of the nonlinear signature after TR signal processing. Chirp-coded excitation has also been introduced as an alternative for excitation. With TR-NEWS, 3D reciprocity has been numerically demonstrated and nonlinear ultrasonic imaging is performed with classical NEWS analysis after local focusing of ultrasound generated by TR tool.

3D-4 Nonlinear Time Reversal Acoustics for Defect Localization: Numerical Study of Retrofocusing Properties

Nonlinearity based time reversal (NL-TR) for retrofocusing of ultrasound appears to be a new tool in nonlinear imaging because of its ability to efficiently concentrate ultrasound in heterogeneous media for defect localization. A 2D pseudo-spectral time domain algorithm has been developed with an Adams-Bashforth method for solving elastic wave equation in nonlinear hysteretic heterogeneous solids. A 200 kHz central frequency pulse with 50 kPa source pressure was used as initial excitation in a 28 times 50 cm2 plate with a 56.25 mm2 defect area having nonlinear hysteretic behavior. Nonlinear elastic wave spectroscopy pre-treatment of time reversal, called NEWS-TR has been investigated in this work. After ultrasonic propagation in the plate, signal is recorded, filtered and time reversed and sent back in order to evaluate the ability to efficiently retrofocuse on the defect. A new imaging method of nonlinear zone with harmonic generation measurements is studied

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....

Evaluation of local nonlinearity using time reversal acoustic focusing

Time reversal acoustic focusing can be used for local assessment of nonlinear properties of media [Sarvazyan A., Sutin A. , JASA, 115 (5): 2595 (2004)]. Time Reversal focusing provides high acoustic pressure leading to a nonlinear response localized in space and time. Interaction area of two overlapping focused beams acts as a virtual localized source generating ultrasonic waves with combination frequencies. Measurements of the intensity of this signal allow estimation of the nonlinear parameter in the focal region. The first experiment was conducted in water with spatially varying acoustical nonlinearity produced by a microbubble column or contrast agent Optison. We used two synchronized TRA aluminium reverberators to focus ultrasonic waves with frequencies 660 kHz and 776 kHz. Various parameters such as amplitude and frequency of the primary field, the position and number of emitters, choice of initial excitation (tone burst, binary, shirp‐coded) were optimized for the nonlinear signature extraction. Th...