Distinction between crack echoes and rebar echoes based on Morlet Wavelet Transform of impact echo signals

Abstract

In an Impact-Echo (IE) test, the Fast Fourier transform (FFT) of the impacted surface responses reveals frequency peaks of major periodical events and can be used to determine depth information, such as the thickness of a target specimen and the depth of a delaminated crack. Since rebar interfaces also result in comparable spectral frequencies but with different depth characteristics, IE testers might have difficulty to determine whether a certain peak frequency is associated with a crack or a rebar. This paper aims to illustrate the use of the time-frequency analysis with Morlet Wavelet transform (MWT) to discriminate the crack response from the rebar response. In additional to the traditional FFT spectrum, the time-frequency technique provides more informative data to assist the analysis task.The proposed method for recognizing crack and rebar can be carried out first by analyzing the peak frequencies in the FFT spectrum. Next, the amplitude histogram at the selected frequency is extracted from the time-frequency spectrogram. Through evaluating the decay pattern of the amplitude curve, an index representing the “duration of amplitude-decay” is defined. Based on the comparison between results of the two studied interfaces, namely concrete-void and concrete-steel, it is preliminarily summarized that the reflected signals are generally less significant at the concrete-steel interface. A rebar leads to weaker reflective energy and thus shortens duration of amplitude-decay. Therefore, it is concluded that the proposed methodology can be applicable to distinguish the difference between the crack echoes and the rebar reflections based on the spectrogram obtained with the time-frequency analysis. Accordingly, this innovated technique provides analysts with an alternative scheme in the recognition/verification of cracks and rebars via the IE tests.