Detection of local defect resonance intermodulation peaks using bicoherence analysis

Abstract

The nonlinear ultrasonic technique is a popular and efficient way for the detection of small defects in any structural component. Local defect resonance (LDR) is one of the nonlinear ultrasonic techniques introduced recently for detection of damages in composites structures. LDR frequency is used to characterize small defects like delamination, cracks by generating high vibration amplitudes at the defect location. In this paper, an analytical study of the nonlinear intermodulation frequencies generated due to the interaction of two different exciting frequencies and their bicoherence analysis is presented in the case of aluminium plate having a flat bottom hole (FBH) and glass fiber reinforced polymer (GFRP) plates having multiple delaminations. The flexural wave propagation in asymmetric composite plate is shown by considering quadratic nonlinearity. The excitation frequency of the first transmitter is varying from LDR frequency to it's subharmonic and superharmonics while the other one is excited with a single periodic frequency (SPF). The analytical results are further verified by experimental investigations and good agreement is found. Local defect resonance intermodulation (LDRI) and single periodic frequency intermodulation (SPFI) are obtained in Fast Fourier transform (FFT) plots. In Bicoherence analysis, LDRI peaks are observed to possess higher bicoherence value and hence, their detection chance is higher during wideband excitation. Moreover, inclusion of subharmonic LDR frequencies will create lesser peaks and detection of all LDR frequencies is possible.