Investigation of delta-t mapping source location technique based on the arrival of A0 modes


The main assumptions made for traditional Time of Arrival (TOA) source location techniques are constant wave speed and straight wave path between the acoustic emission (AE) source and the sensors. However, because of material inhomogeneity and structural complexity, wave velocities may vary in different directions and a direct wave path may be very difficult to be achieved. Therefore, to solve these problems, researchers developed delta-T mapping source location technique, which has been shown to have good accuracy for source location in aerospace structure because complex geometric features are considered when training the mapping data and accurate wave speed data are not required. However, this technique relies on identifying the arrival time of S0 (extensional) Lamb mode, which may not be distinguished from background noise due to lowamplitude AE sources such as corrosion or large source-to-sensor distance. As shown in Figure 1, source location accuracy in a corrosion test on a simple plate was greatly improved after the velocity of A0 (flexural) Lamb mode was used in the source location algorithm. Therefore, identifying the arrival time of A0 modes and hence building delta-T maps are more feasible to solve the problem. Experiments have been carried out with an artificial AE source on a thin complex steel plate. A threshold crossing method based on wavelet coefficient was used to estimate the arrival time of A0 modes. The average error (3.9 mm) of the source locations predicted by delta-T mapping based on A0 arrival was larger than that (1.8 mm) of S0 arrival, which can be explained by the difficulties in accurately identifying the arrival of A0 modes.
