Odile Abraham

Characterization of surface cracks with Rayleigh waves: a numerical model

Abstract In the non-destructive testing of concrete structures, the use of Rayleigh waves shows some advantages to characterise surface cracks: easiness of excitation and recording, access to only one surface of the structure required, great spectral sensitivity to the propagation medium…. But the behaviour of Rayleigh waves on surface defects in concrete is difficult to perceive in the field, even if the dependence of the diffraction pattern on the cracks geometrical features is significant. A numerical model is adapted from earth physics in order to better understand the influence of the crack geometry on Rayleigh-wave propagation. This model, based on an indirect boundary element method, calculates the three-dimensional seismic response of two-dimensional structures. Synthetic seismograms are obtained for the propagation of a Rayleigh wave across various crack geometries. The variations of spectral ratios between the transmitted and incident waves are studied as a function of the crack depth. They are used to design an efficient procedure for the determination of crack depths.

Study of stress-induced velocity variation in concrete under direct tensile force and monitoring of the damage level by using thermally-compensated Coda Wave Interferometry

In this paper, we describe an experimental study of concrete behavior under a uniaxial tensile load by use of the thermally-compensated Coda Wave Interferometry (CWI) analysis. Under laboratory conditions, uniaxial tensile load cycles are imposed on a cylindrical concrete specimen, with continuous ultrasonic measurements being recorded within the scope of bias control protocols. A thermally-compensated CWI analysis of multiple scattering waves is performed in order to evaluate the stress-induced velocity variation. Concrete behavior under a tensile load can then be studied, along with CWI results from both its elastic performance (acoustoelasticity) and plastic performance (microcracking corresponding to the Kaiser effect). This work program includes a creep test with a sustained, high tensile load; the acoustoelastic coefficients are estimated before and after conducting the creep test and then used to demonstrate the effect of creep load.

Analysis of coherent surface wave dispersion and attenuation for non-destructive testing of concrete

Rayleigh waves measurements are used to characterise cover concrete and mortar in the frequency range 60-180 kHz. At these frequencies, the wavelength is comparable to the size of the aggregates, and waves propagate in a multiple scattering regime. Acquired signals are then difficult to interpret due to an important incoherent part. The method proposed here is the study of the coherent waves, obtained by averaging signals over several configurations of disorder. Coherent waves give information on an equivalent homogeneous medium. To acquire a large amount of measurements with accuracy, an optimised piezoelectric source is used with a laser interferometer for reception. Adapted signal processing technique are presented to evaluate the coherent phase and group velocities and also the coherent attenuation parameter. The sensitivity of these three parameters with the properties of concrete is discussed, as well as the necessity to use coherent waves to obtain accurate results.

Validation of a thermal bias control technique for Coda Wave Interferometry (CWI)

The Coda Wave Interferometry (CWI) analysis serves to monitor the variation of propagation velocity in a heterogeneous medium with high precision (10(-3)% in relative terms). In combination with acoustoelastic theory, this type of analysis offers an NDT method for stress evaluation and/or damage detection. Since the CWI method is intended to evaluate extreme levels of accuracy, the presence of bias under certain circumstances can undermine evaluation results and/or test repeatability. In this paper, we offer a bias control technique involving the use of a second (reference) specimen for CWI analysis that is designed to compensate: (1) the thermally-induced velocity variation due to environmental temperature fluctuations; and (2) bias originating from experimental procedures. The presentation of this technique contains both a theoretical analysis and experimental protocol for the purpose of implementation. Furthermore, comparisons of experimental results have been included in order to demonstrate the utility of this bias control technique under laboratory conditions.

2D elastic full-waveform imaging of the near-surface: application to synthetic and physical modelling data sets

Standard seismic methods are generally not well adapted to provide sharp quantitative images of the first few metres of underground. A two-dimensional full-waveform inversion of land seismic data, based on frequency-domain viscoelastic modelling, offers a promising approach to take advantage of the full complexity of seismograms and to simultaneously build 2D images of P and VS parameters. In order to understand the behaviour of this method in a near-surface context and anticipate the corresponding field applications, we perform this investigation by applying waveform inversion on a simple layered medium. We first use synthetic data obtained from numerical modelling and then we employ laboratory data obtained by small-scale physical modelling. We demonstrate that such a near-surface 2D model can be quantitatively determined even in a realistic situation where the data are dominated by high-amplitude surface waves. A comparison of results derived for the same medium from ideal synthetic data and noisy experimental data allows detecting anomalies in the reconstruction of velocity models due to the experimental nature of the data used.

NON-DESTRUCTIVE TESTING OF FIRED TUNNEL WALLS: THE MONT-BLANC TUNNEL CASE STUDY

The investigation of fired tunnel walls typically relies on visual inspection and a comprehensive study of core samples. Visual inspection is limited to surface diagnosis, while core samples only provide a detailed image of the damaged zone at a single point. In order to gain an extensive view of the entire depth of the damaged zone as well as specific material properties, both the seismic refraction method and ground-penetrating radar investigation may be carried out. In the case of the Mont-Blanc Tunnel, these non-destructive methods have been tested in three zones: heavily damaged, moderately damaged, and sound. On the radargrams generated, several layers can be roughly identified; seismic refraction then confirms and characterises the existence of layers with contrasting mechanical properties.

Nonlinear mixing of ultrasonic coda waves with lower frequency-swept pump waves for a global detection of defects in multiple scattering media

An ultrasonic method providing for an efficient global detection of defects in complex media (multiple scattering or reverberating media) is reported herein; this method is based on the nonlinear acoustic mixing of coda waves (stemming from multiple scattering) with lower frequency-swept pump waves. Such a nonlinear mixing step is made possible by the presence of nonlinear scatterers, such as cracks and delamination, yet remains absent when the waves are scattered only by linear scatterers, as is the case in a complex but defect-free medium. A global inspection is achieved thanks to the use of wide-band coda and pump signals, which ensure the excitation of many resonances along with a homogeneous acoustic energy distribution in the medium. We introduce the existing sensitivity tools developed for Coda Wave Interferometry in extracting the pump amplitude-dependent parameters of the coda waves associated with effective nonlinear parameters of the medium. By comparing results at two damage levels, these effective nonlinear parameters are shown to be correlated with crack presence in glass samples. The mechanisms potentially responsible for the observed amplitude dependence on the tested elastic parameters and waveform modification are discussed.

FOLLOWING STRESS LEVEL MODIFICATION OF REAL SIZE CONCRETE STRUCTURES WITH CODA WAVE INTERFEROMETRY (CWI)

The determination of the stress level in in‐situ concrete structures is of paramount importance for in the engineering field. CODA wave interferometry (CWI) has been recently proposed to monitor stress levels in pre‐stressed concrete structures in a non‐destructive manner. The idea is to follow very small changes of the ultrasonic wave velocity linked to stress level modifications, through Murnaghan’s theory. The change in velocity, which is of the order of 0.1% for 10 MPa for classical concrete, can be measured by taking advantage of the heterogeneity of concrete. The accurate measurement of travel time delay is made possible by a source generating a wave train with wavelength sizes comparable to that of the aggregates in the concrete, resulting in multiple scattering. This method is sufficiently sensitive to record small changes in the medium’s mechanical properties. An experiment on a real size concrete structure subjected to time varying loading is described.

Mechanical characterization of heterogeneous soils with surface waves: experimental validation on reduced-scale physical models

The characterization of heterogeneous soils using common geotechnical techniques often proves impossible when the size of the heterogeneity is larger than a few tens of centimetres. Geophysical investigation techniques based on seismic wave propagation can help engineers to characterize the mechanical properties of such materials. In this paper, both refracted and surface waves are used to estimate the mechanical properties of an equivalent homogeneous medium. A summary of the main results obtained numerically using finite-element computations and homogenization theory is presented. It is shown that, for first-mode surface-wave wavelengths larger than 7.5 times the nominal size of the heterogeneity and within certain heterogeneity concentration ranges (up to 50% for matrix dominant soils), surface waves homogenize the soil in accordance with classical homogenization theory. To validate these numerical results, a reduced-scale model was built and seismograms, generated with a falling weight, were recorded. The phase-velocity dispersion curve of the generated surface waves is inverted in order to obtain the shear-wave velocity of the heterogeneous layer. The compressional-wave velocity is calculated by means of seismic refraction analysis. Velocities obtained on the reduced-scale model correspond to those predicted by homogenization theory from individual measurements of matrix and inclusion velocities.

Nonlinear coda wave interferometry for the global evaluation of damage levels in complex solids

HIGHLIGHTSNonlinear modulation method is combined with CWI for global damage assessment.Propagation mediums elastic nonlinear level is evaluated effectively and globally.Damage degree of Pyrex samples are related to their effective nonlinear level.Influences from temperature change to the method are discussed. ABSTRACT A nonlinear acoustic method to assess the damage level of a complex medium is discussed herein. Thanks to the highly nonlinear elastic signatures of cracks or, more generally, internal solid contacts, this method is able to distinguish between contributions from linear wave scattering by a heterogeneity and contributions from nonlinear scattering by a crack or unbounded interface. The coda wave interferometry (CWI) technique is applied to reverberated and scattered waves in glass plate samples featuring various levels of damage. The ultrasonic coda signals are recorded in both the absence and presence of an independent and lower‐frequency elastic “pump” wave, before being analyzed by CWI. The monitored CWI parameters quantifying changes in these coda signals, which therefore quantify the nonlinear wave‐mixing effects between the coda and pump waves, are found to be dependent on the damage level in the sample. A parametric study is also performed to analyze the influence of sensor positions and average temperature on the methods output. The reported results could be applied to the non‐destructive testing and evaluation of complex‐shape materials and multiple scattering samples, for which conventional ultrasonic methods show strong limitations.