| Abstract: |
Precise AE signal arrival times of the fundamental Lamb modes can be obtained from the arrival time of the peak wavelet transform (WT) magnitude at a particular frequency of interest. Since these arrival times are not determined from a fixed threshold, they are not affected by dispersion, attenuation and source amplitude. They also correspond to a single group velocity and lead to more accurate source location results than those obtained by traditional AE location calculations based on threshold-based arrival times. In this research, noise-free finite element modeled (FEM) AE signals were combined with experimental wideband electronic noise to form noisy signals. Since the noise-free signal was available, the changes in the WT-based arrival times from noise-free to noise-altered signals could be quantitatively evaluated. Several signal-to-noise (S/N) ratios were examined in a statistical fashion for three important types of AE sources at three propagation distances and at different radiation angles. The WT-determined arrival times were obtained for the two different frequency-mode combinations (A0 at 60 kHz and S0 at 522 kHz) that represent the most energetic portions of the signals in a large 4.7 mm thick aluminum plate. Using the arrival times, statistical calculations of linear source location were also studied to evaluate errors in location accuracy caused by noise. Even at S/N ratios as low as 1 to 1, the location error was 2 % or less for a large majority of the cases. The arrival time errors were also examined using two alternative time-frequency analysis approaches to obtain the arrival times. One algorithm (Choi-Williams) provided significantly improved results for the noisy signals. |
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