Yuxiang Zhang

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.

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.

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.

Diffuse ultrasound monitoring of stress and damage development on a 15-ton concrete beam.

This paper describes the use of an ultrasonic imaging technique (Locadiff) for the Non-Destructive Testing & Evaluation of a concrete structure. By combining coda wave interferometry and a sensitivity kernel for diffuse waves, Locadiff can monitor the elastic and structural properties of a heterogeneous material with a high sensitivity, and can map changes of these properties over time when a perturbation occurs in the bulk of the material. The applicability of the technique to life-size concrete structures is demonstrated through the monitoring of a 15-ton reinforced concrete beam subject to a four-point bending test causing cracking. The experimental results show that Locadiff achieved to (1) detect and locate the cracking zones in the core of the concrete beam at an early stage by mapping the changes in the concretes micro-structure; (2) monitor the internal stress level in both temporal and spatial domains by mapping the variation in velocity caused by the acousto-elastic effect. The mechanical behavior of the concrete structure is also studied using conventional techniques such as acoustic emission, vibrating wire extensometers, and digital image correlation. The performances of the Locadiff technique in the detection of early stage cracking are assessed and 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.

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.

A Bayesian approach for high resolution imaging of small changes in multiple scattering media.

This paper introduces a Bayesian approach to achieve high-resolution imaging of sub-wavelength changes in the presence of multiple scattering. The approach is based on the minimization of a cost function defined by the decorrelations induced in the measured waveforms by the apparition of a local changes. Minimization is achieved via a Monte Carlo Markov Chain (MCMC) algorithm combined to an analytical model that computes the sensitivity kernel of the medium. In the inversion procedure, the parameters to infer represent the physics of the problem, such as the diffusivity in the medium and/or the geometrical features of the reflector (position and scattering cross-section). The method is successfully compared to the linear inversion approach initially proposed for the so-called Locadiff imaging method through several examples, both numerical and experimental.

Study of concrete's behavior under 4-point bending load using Coda Wave Interferometry (CWI) analysis

Coda Wave Interferometry (CWI) is an ultrasonic NDT method suitable for complex material such as concrete that can precisely measure small propagation velocity variation (10−2%). By measuring variation of propagation velocity in concrete caused by acoustoelasticity phenomena, CWI analysis can be used to monitor concretes internal stress level. For the first time, CWI is used to measure propagation velocity variations due to a stress field in a concrete beam under four-points bending test, which contains simultaneously compressive and tensile stress. Embedded optical-fiber sensors, strain gauges are used in the experiment, in order to confirm and validate the CWI analysis result. Thermocouples are also embedded into concrete beams for monitoring internal temperature fluctuations.

Monitoring the Stress Level of Concrete Structures with CODA Wave Interferometry: Experimental Illustration of an Investigated Zone

The use of Coda Wave Interferometry (CWI) to Non Destructively assess concrete structures is an emerging topic. CWI has recently been proposed to determine the stress level of in situ pre-stressed concrete structure as well as to monitor damage of concrete material. The idea is using ultrasonic waves with a wavelength similar to the aggregate size to be in the diffusive regime, and so to probe very small changes of the material. The velocity change is of the order of 0.1% for classical concrete under a 10MPa strain which can be measured with CWI. This velocity variation can be linked to the stress level modification by Murnaghan’s theory. While CODA theory and experiments are emerging in the civil engineering NDT laboratory, it becomes relevant in parallel to know what is actually the investigated zone for, in fine, relating the velocity changes to a given distribution of stress or damage as a function of depth in in situ condition. In this paper we show an experiment that illustrates the existence of an investigated zone in a concrete beam of 0.8 m x 0.2 m x 0.1 m.

Characterizing extended changes in multiple scattering media using coda wave decorrelation: numerical simulations

In multiple scattering media, the coda wave decorrelation relates linearly to the scattering cross-section of structural change when the change is small compared to the wavelength. In practical applications, we assume that the total decorrelation induced by changes in the medium is the sum of the decorrelation induced by each elementary change. In this article, we investigate the validity of this linear approximation for extended changes larger than the wavelength, and the possible signature of the change orientation. Coda waves are simulated using a 2-D finite-difference model in multiple scattering media. We perform a parametric analysis of the decorrelation induced by extended structural changes of various length and orientation, as well as the mutual influence between two identical changes separated by a varying distance. Our findings are: (1) we underestimate the length of the change when it exceeds one wavelength. (2) the decorrelation value is sensitive to the orientation of extended changes at distances smaller than four mean free paths between the source and the receiver. (3) two simultaneous changes are interacting within a distance of the order of the mean free path, but can be considered independent at a separation distance larger than a few mean free paths.