Evaluation of nonlinear interface areas in a multiple scattering medium by Nonlinear Coda Wave Interferometry (NCWI): Experimental studies

Abstract

This paper aims to investigate the application of the Nonlinear Coda Wave Interferometry (NCWI) method in evaluating the nonlinear interface areas in highly heterogeneous materials. An experimental protocol is proposed for quantitatively analyzing the nonlinear interface effects in a multiple scattering medium. The experimental design involves a perforated aluminum plate with a surface ratio of circular voids equal to 17.63%, some of which are threaded to accommodate screws. The nonlinear contacts between the screws and the perforated plate are highlighted by a strong pump wave and investigated using the coda waves. Drawing upon prior numerical simulation findings (Chen et al., 2021), it is anticipated that the CWI observables (namely, the relative variation of coda wave velocity θ and the remnant decorrelation coefficient Kd) will be proportional to the change in crack length within the heterogeneous medium. In this study, the change in crack length through numerical simulation can be simulated by considering the different nonlinear interface areas between the screws and the perforated aluminum plate in the experimental design. Experimental findings demonstrate that the CWI observables are proportional to the nonlinear interface areas, which essentially aligns with previous numerical simulation results. In this paper, suggestions for refining the experimental setup are provided. This guidance is instrumental for further conducting quantitative research on nonlinear interface effects in complex media.