12 A High-Speed Dual-Stage Ultrasonic Guided Wave System for Localization and Characterization of Defects 135 Fig. 12.10 Sensor layout with simulated damage identified (left) and simulated corrosion (right) Fig. 12.11 Boeing 737 damage localization results (left) and scan results (right) Transducers 2, 4, 5, and 7 were selected to provide the needed excitation for the laser Doppler vibrometer. The generated image based on the scan area determined in Stage 1, is shown in Fig. 12.11 (left). Once again, Stage 2 (result shown in Fig. 12.11, right) was able to produce an accurate characterization of the damage, which is unable to be achieved with just Stage 1 alone. 12.5 Conclusion In this work we have described a two-stage process for the detection, localization, and characterization of structural defects. An array of embedded transducers is used to measure the response of the structure to ultrasonic guided waves. In Stage 1, a test measurement is compared to a set of baseline measurements of the undamaged structure taken over a range of environmental conditions. Using the results from Stage 1, we then isolate an area of the structure where the damage is most likely to be located. This area is the target for the laser scan of Stage 2 where the same nearby embedded transducers to set up a steady state vibration. A scanning laser Doppler vibrometer measures this response and the data is processed to yield an estimate of wavenumber. The high-resolution image created from this wavenumber data gives information on the shape and depth of the damage and can be used to make informed decisions on the structure’s repair and maintenance. In this way Stage 1 uses the information it has gathered and its existing hardware to help Stage 2 produce more detailed information. Our system was validated with tests in the laboratory and in the field. Due to time constraints, only a few field tests could be performed. It would be valuable for future work to include tests on larger areas of the aircraft and run for longer durations. As mentioned above, a more rigorous statistical analysis could be developed for selecting the size of the scan area. We
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