Comparison of wavelet transform and Choi-Williams distribution to determine group velocities for different acoustic emission sensors

• Published in: Journal of acoustic emission 2008-01, Vol.26, p.40
• Main Author: Hamstad, M.A
• Format: Article
• Language: English
• Subjects: Acoustic emission testing; Analysis; Elastic waves; Mechanical properties; Methods; Plates, Iron and steel
• ISSN: 0730-0050

Acoustic emission (AE) sources in a plate generate dispersive Lamb waves. In this research, pencil lead breaks (PLBs) were made on an edge of a 3.1 mm thick aluminum alloy plate to generate such AE waves. The PLBs were made near the plate mid-plane and near the bottom of the edge. The waves were sensed by pairs of AE sensors (both resonant or both non-resonant wideband). Each sensor of each pair was located at a different distance from the plate edge PLB position. The signals from the different sensors and source positions were analyzed and contrasted. The recorded signals were processed by a wavelet transform (WT), and the group velocity curves for the appropriate Lamb modes were superimposed to clearly identify the modes in the signal. The threshold-independent mode arrival times at specific intense signal frequencies were determined by the time of the peak WT magnitude at the intense frequency. Since the two sensors were located at different known distances from the PLB position, an experimental group velocity was calculated based on the difference in arrival times and the difference in distance. This velocity was then compared to the theoretical group velocity for the relevant mode and frequency. In addition, an alternate frequency/time analysis method was used. This was the Choi-Williams distribution (CWD). The same procedures used for the WT results were followed for the CWD distribution. In addition the experimental results were compared to finite element calculated results for plate-edge monopoles both near the mid-plane and near the top plate edge. Group velocities within 6.5 % of the theoretical values were obtained with all the sensor types when certain intense resonant frequencies were ignored for two of the three resonant sensor types. Keywords: AE arrival times, AE sensors, Choi-Williams distribution, group velocities, Lamb modes, thin plate, wavelet transform