Monitoring Cracks with Nonlinear Coda Wave Interferometry (NCWI): Numerical and Experimental Results

Abstract

In this paper, the Nonlinear Coda Wave Interferometry (NCWI) method is studied for an overall assessment of cracking levels in very heterogeneous materials. NCWI is an ultrasonic method that mixes the performance of coda wave interferometry, which takes advantage of the heterogeneity of the medium to detect small changes, and nonlinear acoustics that displays high sensitivity to cracks. First, a series of experimental results are summarized. They include laboratory measurements on crack concrete samples as well as on perforated aluminium plates prepared so as to mimic cracks growth, and on-site measurements on a reduced 1:3 scale concrete nuclear containment plant (EDF VeRCoRs mock-up) under loading conditions that replicates decennial inspection. All experimental results demonstrate that the NCWI observables are sensitive to crack length (even when closed by post-tensioning) and can thus be used to monitor crack status in very heterogeneous material. Even though the experiments show a clear relation between crack length and crack opening, until very recently it was not possible to quantitatively recover information about the cracks as no model exists that can predict the NCWI experimental results. To tackle this problem the Spectral Element Method (SEM) is used to perform parametric numerical sensitivity studies of the NCWI observable to crack status. The aim here is to find simple and robust laws that links the NCWI observables to quantitative information about cracks (here the total crack length or the viscoelastic properties of an equivalent damaged zone). In this paper specific attention is given to the confrontation between numerical and experimental data obtained in perforated aluminium plates. A linear law between a CWI observable and the crack length is validated. Finally, perspective of an imagery NCWI based method is discussed.