Detection of local defect resonance frequencies using bicoherence analysis

Abstract The nonlinear acoustic wave spectroscopy and defectometry (NAWSD) technique is found to be an efficient tool for monitoring intricate and complex defects in composite structures due to its better resolution and accuracy compared to the traditional methods. Local defect resonance (LDR) is one of such methods based on amplifying vibrations at the defect regions only, resulting in defect-selective and high resolution image. In the present work, numerical as well as experimental approach has been presented in order to detect the LDR frequency of aluminium plate having circular flat bottom hole (FBH) and delaminated glass fibre reinforced polymer composite (GFRP) plates. First, a wave propagation study using explicit dynamic analysis is performed followed by, bicoherence analysis to detect the LDR frequency numerically. The LDR frequency modes are also verified by steady state dynamic analysis. A set of experiments are then carried out using Gaussian white noise as well as chirp signal to determine the LDR frequencies from the Fast Fourier Transform (FFT) and bicoherence plots. In every cases, the bicoherence peaks corresponds to the second harmonic of LDR frequency. The bicoherence plots are found to be robust and effective tool for detection of LDR frequencies compared to FFT plots. Bicoherence analysis based LDR frequency determination technique can be further utilized in delamination and disbond imaging of composite structures.