ABSTRACT
This paper reports a numerical study of the sensitivity and applicability of the Nonlinear Coda Wave Interferometry (NCWI) method in a heterogeneous material with a localized microcracked zone. We model the influence of a strong pump wave on the localized microcracked zone as a small average increase in the length of each crack. Further probing of this microcracked zone with a multiply scattered ultrasonic wave induces small changes to the coda-type signal, which are quantified with coda wave interferometry. A parametric sensitivity study of the CWI observables with respect to the changes in crack length is established via numerical simulations of the problem using a 2D spectral element method (SEM2D). The stretching of the signal, proportional to the relative variation in effective velocity, is found to be linearly proportional to the global change in crack length, while the other CWI parameter, the remnant decorrelation coefficient, is found to be quadratically proportional to the crack length change. The NCWI method is shown to be relevant for the detection of different damaged material states in complex solids. The reported numerical results are especially significant in the context of quantitative nondestructive evaluation of micro-damage level of a heterogeneous materials using nonlinear ultrasound signals.
ABSTRACT
The numerical studies conducted in this paper are based on our previous research (Chen et al., 2017); through use of the spectral element method, parametric sensitivity studies of Nonlinear Coda Wave Interferometry (NCWI) are established here and divided into two parts. In the first part, CWI observables are found to be proportional to the product of the changes in elastic modulus within the Effective Damaged Zone (EDZ) and the EDZ surface area. The modifications to intrinsic properties are quantified via an overall wave velocity variation, as probed by a reverberated coda wave. However, for high-level changes, CWI may fail due to meaningless decorrelation values. In this context, parametric studies are conducted to yield a maximum range for EDZ contrast and area. To further validate these observations using a more realistic numerical model, instead of introducing a homogeneous EDZ model, the second part of this paper adds random cracks with random orientations into the EDZ of a material sample. The influence of a strong pump wave on localized nonlinear damage is reestablished; results show that the cracks added into the EDZ reduce the property changes required to match the previous experimental dataset.
