Efficient determination of local defect resonance frequencies from bicoherence plots using double excitations

Abstract

Abstract In recent years, a lot of developments can be seen in the field of local defect resonance (LDR) based thermosonics which provide thermal image of a defect by exciting the structure at particular defect frequencies. LDR based thermosonics is found to be more promising compared to traditional thermosonics due to its high reproducibility and low power usage. However, fast and accurate detection of these LDR frequencies is a challenging task. In this context, this paper addresses a fast and reliable method of LDR frequency determination using bicoherence plots. Two exciters and one receiver PZTs (lead zirconate titanate) are used for the experiment. One exciter PZT is provided with chirp or Gaussian white noise signal input while a fixed frequency signal is given to the second exciter PZT. Intermodulation of LDR and excitation frequencies can be observed in frequency plots. LDR frequencies can be easily detected from bicoherence plots, while its detection from frequency plots is found to be cumbersome. The efficacy of bicoherence analysis in detection of LDR frequency is shown using an aluminium plate with flat bottom hole and three delaminated glass fibre reinforced composite (GFRP) plates. Second harmonic of LDR frequencies are evaluated from the bicoherence plots which can be used later for defect imaging.