A Modified Synthetic Aperture Focusing Technique (SAFT) by the Hilbert-Huang Transform (HHT)

Combing the stress wave signals with Synthetic Aperture Focusing Technique (SAFT) to inspect and image the interior defect of concrete structural has shown its feasibility by some researchers. However, some bright zones displayed in the image plot are not corresponding to the Rayleigh waves and reflection signals from the bottom surface of the structural component or the embedded defect. This type of image is treated as a fictitious defect and will affect the judgment of nondestructive evaluation. Since the theorem of SAFT is based on the superposition of all the amplitudes of receiving signal at the flight time corresponding to each image cell in the domain of inspection, the irregular waveform in the time domain can generate the abovementioned fictitious defects. In this paper, the synthetic aperture focusing technique is modified by using the frequency, time and energy information from the Hilbert-Huang Transform (HHT) method to improve its image quality. It is found that the reflected signal from defect will be located in a certain band of frequency and the energy level at the time corresponding to a defect is relatively high. Numerical simulation results reveal this feature of HHT method can filter out the fictitious image displayed in the previous method and provides a clearer and sharper image of real defect.