Contact acoustic nonlinearity effect on the vibro-acoustic modulation of delaminated composite structures

In recent years, Vibro-Acoustic Modulation (VAM) techniques for structural health monitoring have received increasing attention. For such techniques, the sidebands and higher-order harmonics generated by double/single sinusoidal excitations are utilized to identify a series of damages. Currently, most VAM investigations are experimental, mainly involving signal processing, while few studies have paid attention to the mechanics of VAM generation. This paper presents a comprehensive investigation which studies the effects of Contact Acoustic Nonlinearity (CAN) on VAM for delaminated composite structures. The paper includes theoretical analysis, simulations, and experiments. Considering both a nonlinear contact constitutive model and the clapping/rubbing discontinuity, an approximate solution for nonlinear motional equation was established by using Fourier series expansion. A modified Greenwood-Williamson (GW) model for physical contact was implemented into the commercial finite element software ABAQUS by a UINTER subroutine, which described the contact behaviors between rough surfaces. The calculated signal responses from the delaminated composite plates were compared to experimental results. A good agreement was qualitatively and quantitatively achieved with acceptable error. Particularly, some specific features of higher-order sidebands existing in the experiment were identified. Results showed that the combined effect of the nonlinear contact constitutive model and the clapping/rubbing mechanism caused odd-even order differences. The asymmetry between the sidebands indicates the existence of amplitude and frequency modulations, which can be used to extract nonlinear damage indexes. These indexes are capable of characterizing the degree and range of damage.