Bart Van Damme

Time reversed acoustics techniques for elastic imaging in reverberant and nonreverberant media: An experimental study of the chaotic cavity transducer concept

In view of emerging imaging technologies based on the combination of Time Reversed Acoustics (TRA) with Nonlinear Elastic Wave Spectroscopy (NEWS) for the detection and localization of micro-damage in solids, we have investigated the benefits of chirped source signal excitation, inverse filtering techniques, and the implementation of chaotic cavity transducers to improve the quality of energy focusing, especially for weakly reverberant media. 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. Experimental data for reverberant and nonreverberant composite plates show that the use of chirps, inverse filtering and chaotic cavity transducers significantly enhances the focusing process, and enables focusing in a nonreverberant medium using only one transducer. As a potential exploitation, the application of the chaotic c...

The vibration dipole: A time reversed acoustics scheme for the experimental localisation of surface breaking cracks

A combination of time reversed acoustics and nonlinear elastic wave spectroscopy techniques is introduced to localize surface breaking defects in a non-destructive manner. Reciprocal time reversal is applied at two neighbouring positions in order to create a vibration dipole with high amplitudes. At surface breaking cracks, nonlinear elastic effects are triggered by the shear forces due to induced friction of the crack interfaces. By mapping the nonlinearity generated by the vibration dipole over the sample surface, the position of a surface breaking crack can be visualized. The technique is tested on an industrial steel sample containing a closed crack.

Analytical analysis of slow and fast pressure waves in a two-dimensional cellular solid with fluid-filled cells.

Wave propagation in cellular and porous media is widely studied due to its abundance in nature and industrial applications. Biots theory for open-cell media predicts the existence of two simultaneous pressure waves, distinguished by its velocity. A fast wave travels through the solid matrix, whereas a much slower wave is carried by fluid channels. In closed-cell materials, the slow wave disappears due to a lack of a continuous fluid path. However, recent finite element (FE) simulations done by the authors of this paper also predict the presence of slow pressure waves in saturated closed-cell materials. The nature of the slow wave is not clear. In this paper, an equivalent unit cell of a medium with square cells is proposed to permit an analytical description of the dynamics of such a material. A simplified FE model suggests that the fluid-structure interaction can be fully captured using a wavenumber-dependent spring support of the vibrating cell walls. Using this approach, the pressure wave behavior can be calculated with high accuracy, but with less numerical effort. Finally, Rayleighs energy method is used to investigate the coexistence of two waves with different velocities.

Application of the chaotic cavity transducer concept for imaging in non‐reverberant media.

The concept of chaotic cavity transducer utilizes a combination of a piezoelectric ceramic disk glued on a cavity of chaotic shape on the hardware side, with the time reversal (or inverse filter) technique on the software side. Using a two step procedure of direct excitation‐reception and time reversed excitation‐reception, it is possible to focus energy anywhere inside a non‐reverberating sample, thanks to the ergodicity of the chaotic cavity. Moreover, the same basic concept can be used to create a virtual phased array using a single channel device. Both experimental data and simulations will be provided to illustrate the concepts. The goal is to use the chaotic cavity transducer concept to enhance the localization of micro‐damage coupled to nonlinear elastic wave spectroscopy methods.

Modelling the transmission of ultrasound pulses through grain-to-grain contacts

Wave propagation through packed spherical particles is characterized by two distinct mechanisms, depending on the frequency of the wave content. Coherent wave pulses occur when the Hertzian contact model can be used, i.e., for frequencies low enough so that the granules behave as rigid bodies. Above a certain frequency, a chaotic time signal is the result of diffusive energy transmission through the grain contacts. This so-called coda wave is important for applications tracking the microstructure of materials, e.g., in nondestructive testing. This work looks for parallels between the two approaches by investigating the transmission of short ultrasound pulses in the unit cell of a granular material: two spheres in contact. Laser measurements on two identical large steel spheres show that the energy transmission happens in discrete steps, due to guided surface waves. The measured and modeled energy distribution evolution are similar to the one predicted by the diffusive theory. However, we show that the Her...

Low frequency bandgaps in lightweight metamaterial panels using rotation inertia multiplication

Of all possible features of structural metamaterials, the formation of bandgaps is the most studied one due to its direct application for sound and vibration isolation. While achieving low frequency values for the position of the first bandgap is, in general terms, not an unsurmountable challenge, the combination of material properties such as high stiffness, low density, and reduced size of the unit cell, with low (in absolute terms) frequency bandgaps, may well require some careful consideration. In previous work, we designed panels with a 3D network of resonators, clearly improving the vibration isolation compared to a homogeneous panel with the same weight. Recently, we have devised a novel implementation of inertia amplification, based on coupling the energy of longitudinal waves into the rotational oscillation of inertia elements within the unit cell. In this contribution, we present examples of phononic crystals based on this approach, and we discuss the interaction of acoustic waves with the discu...

Microdamage detection and monitoring using nonlinear elastic wave spectroscopy over a wide frequency range

Innovative diagnostic techniques based on Nonlinear Elastic Wave Spectroscopy (NEWS) have been implemented for global inspection of microdamage in intact and damaged aeronautical components using low and high frequency sound waves. For low frequency analysis, we focused on the amplitude dependent analysis of the resonance behavior, both in time and frequency domain. Alternatively, we have analyzed the time‐windowed interaction of a hammer impact with a high frequency continuous wave. The results of both techniques agree extremely well, and prove their potential for quick and global microdamage detection. For local inspection, we have implemented a combination of the traditional elastic wave time reversal technique with a phase coded pulse sequence selective excitation. Alternatively, sparse array tomographic reconstruction with NEWS pre‐treatment could be used. The nonlinearity based imaging techniques are illustrated on aeronautical components, and discussed in terms of the feasibility and usefulness of ...