Acoustical properties of an immersed corner-cube retroreflector alone and behind screen for ultrasonic telemetry applications.

Ultrasonic telemetry measurements consist in remotely detecting and locating an object. To maximize the signal-to-noise ratio, a target may be used, positioned at a reference point. In this framework, the ultrasonic reflective characteristics of a corner-cube retroreflector (CCR) are investigated. The most interesting property of a CCR is its ability to fully reverse an incoming wave in the same direction under certain conditions. Theoretical developments are performed in order to understand its acoustic behaviour, and experimentations are made in various configurations: CCR alone in water, and behind an immersed plate that acts as a screen, with normal and non-normal incidence. The results highlight its strong performance. Moreover, the study of two other couples of CCR material and surrounding fluid underlines the relevance of considering the acoustic properties of each medium, as they have a strong influence on the acoustic response of the CCR.

Multi-modal leaky Lamb waves in two parallel and immersed plates: Theoretical considerations, simulations, and measurements.

Leaky Lamb waves have the potential to be used to perform non-destructive testing on a set of several parallel and immersed plates. Short-time Fourier transform and two-dimensional Fourier transform have both been successfully used to measure the propagation properties: phase and group velocity, and leaky attenuation. Experimental measurements were validated by comparison between theory, experimentation and finite-element simulations (using comsol multiphysics® software) in the case of one immersed plate in water. These signal processing techniques proved to be efficient in the case of multi-modal propagation. They were applied to two immersed plates to identify the leaky Lamb mode generated in the second plate. Dispersion curves of the system composed by two immersed and parallel plates are computed. When plates have the same thickness, leaky Lamb modes propagate from the first to the second plate without any mode change, with the apparent attenuation being weaker in the second plate. Considering that the second plate is continuously supplied in energy by the first one, an energy-based model is proposed herein to estimate the apparent attenuation in the second plate. Despite our extremely simplifying assumption, this model proved to be in good agreement with both finite-element modelling and experimentation.

Simulations of ultrasonic wave propagation in concrete based on a two-dimensional numerical model validated analytically and experimentally.

Several non-destructive evaluation techniques to characterize concrete structures are based on ultrasonic wave propagation. The interpretation of the results is often limited by the scattering phenomena between the ultrasonic wave and the high concentration aggregates contained in the cement matrix. Numerical simulations allow for further insights. This study aims to build a two-dimensional numerical model in order to reproduce and interpret ultrasonic wave propagations in concrete. The model is built in a spectral-element software package called SPECFEM2D. The validation of the numerical tool is based on the use of resin samples containing different amount of aluminum rods from low (5%) to high concentration (40%), the last one being representative of aggregate concentration in concrete. These samples are characterized using an ultrasonic testing bench (ultrasonic water tank) from 150 kHz to 370 kHz. The measured results are analyzed in terms of phase velocity and attenuation which are the main parameters of coherent waves. As homogenization models such as the Waterman-Truell or Conoir-Norris models are usually used to model coherent waves in two-phase systems, we also compare the experimental and numerical results against them. We confirm that the use of these homogenization models is limited to low concentration of scattering phase, which is not adapted to applications to concrete. Finally, we use our numerical tool to carry out a parametric study on scatterer concentration, shape, orientation and size distribution of aggregates in concrete. We show that aggregate orientation has an influence on coherent wave parameters, but aggregate shape has not.

Weld inspection by focused adjoint method.

This paper reports a methodology for the non-destructive ultrasonic evaluation of welds, based on probing, residue back-focusing and topological energy calculation using an enhanced (focused) adjoint method. The proposed method combines the advantages of time reversal to compensate for the cumulative distorsions experienced by a wave propagating in a heterogeneous medium, and topological imaging to highlight the defect location. The synergistic effect of this combined approach makes it possible to detect anomalies in the most efficient way. The method paves the way towards a matched-insonification imaging of anomalies in anisotropic media.

Topological imaging in bounded elastic media.

Detecting, imaging and sizing defects in a bounded elastic medium is still a difficult task, especially when access is complex. Adjoint methods simplify the task as they take advantage of prior information such as the geometry and material properties. However, they still reveal a number of important limitations. Artifacts observed on the conventional topological energy image result from wave interactions with the boundaries of the inspected medium. The paper describes a method for addressing these artifacts, which involves forward and adjoint fields specified in terms of the boundary conditions. Modified topological energies are then defined according to the type of analyzed flaw (open slit or inclusion). Comparison of the numerical results with the experimental data confirms the relevance of the approach.

Locating and characterizing a crack in concrete with diffuse ultrasound: A four-point bending test.

This paper describes an original imaging technique, named Locadiff, that benefits from the diffuse effect of ultrasound waves in concrete to detect and locate mechanical changes associated with the opening of pre-existing cracks, and/or to the development of diffuse damage at the tip of the crack. After giving a brief overview of the theoretical model to describe the decorrelation of diffuse waveforms induced by a local change, the article introduces the inversion procedure that produces the three dimensional maps of density of changes. These maps are interpreted in terms of mechanical changes, fracture opening, and damage development. In addition, each fracture is characterized by its effective scattering cross section.

An experimental evaluation of two effective medium theories for ultrasonic wave propagation in concrete.

This study compares ultrasonic wave propagation modeling and experimental data in concrete. As a consequence of its composition and manufacturing process, this material has a high elastic scattering (sand and aggregates) and air (microcracks and porosities) content. The behavior of the "Waterman-Truell" and "Generalized Self Consistent Method" dynamic homogenization models are analyzed in the context of an application for strong heterogeneous solid materials, in which the scatterers are of various concentrations and types. The experimental validations of results predicted by the models are carried out by making use of the phase velocity and the attenuation of longitudinal waves, as measured by an immersed transmission setup. The test specimen material has a cement-like matrix containing spherical inclusions of air or glass, with radius close to the ultrasonic wavelength. The models are adapted to the case of materials presenting several types of scattering particle, and allow the propagation of longitudinal waves to be described at the scale of materials such as concrete. The validity limits for frequency and for particle volume ratio can be approached through a comparison with experimental data. The potential of these homogenization models for the prediction of phase velocity and attenuation in strongly heterogeneous solids is demonstrated.

Ultrasonic wave propagation in heterogeneous solid media: theoretical analysis and experimental validation.

This research deals with the ultrasonic characterization of thermal damage in concrete. This damage leads to the appearance of microcracks which then evolve in terms of volume rate and size in the material. The scattering of ultrasonic waves from the inclusions is present in this type of medium. The propagation of the longitudinal wave in the heterogeneous media is studied via a homogenization model that integrates the multiple scattering of waves. The model allows us to determine the phase velocity and the attenuation according to the elements which make the medium. Simulations adapted to the concrete are developed in order to test the responses of the model. These behaviors are validated by an experimental study: the measurements of phase velocity and attenuation are performed in immersion, with a comparison method, on a frequency domain which ranges from 160 kHz to 1.3 MHz. The analysis of different theoretical and experimental results obtained on cement-based media leads to the model validation, on the phase velocity behavior, in the case of a damage simulated by expanded polystyrene spheres in granular media. The application to the case of a thermally damaged concrete shows a good qualitative agreement for the changes in velocity and attenuation.