134 A. Gannon et al. Fig. 12.9 Generated map of suspected damage location using RMLE filter (left) and generated image of damage from wavenumber data (right) the likelihood of damage at each location on the plate was generated from the results (Fig. 12.9, left). Note that this map was generated from the front view of the plate and the sensor layout and damage location are a mirrored from their locations in Fig. 12.8 (left). The smaller black box shows the scan area for Stage 2, chosen to be a rectangle that includes all pixels whose likelihood estimates are in the top 90 %. A subset of transducers that are closest to the damage while surrounding it were selected to provide the excitation for the LDV according to the procedure in Sect. 12.3.5.2. Sensors 1, 2, 4, and 5 were automatically selected and are shown highlighted in red in Fig. 12.9 (left). Stage 2 of the DSSHM methodology then started. The scanning laser Doppler vibrometer scanned the area where Stage 1 determined the most likely location of damage to be. Figure 12.9 (right) shows the image generated from the wavenumber data. The image provides a better representation of the actual size and shape of the damage than Fig. 12.9 (left) from Stage 1. From the wavenumber estimate we can also infer the depth of the damage. 12.4.2 Field Experiments Testing was conducted on a retired Boeing 737–200 at the Sandia National Labs Aging Aircraft NDI Validation Center in Albuquerque, New Mexico. 12.4.2.1 Test Setup Thirteen piezoelectric transducers were attached to the Boeing 737, once again using superglue (Fig. 12.10, left). To simulate exfoliation corrosion, a 6.35 4.45 cm area of the aircraft fuselage had 20 % of its skin depth removed, Fig. 12.10 (right). The damage was created with a Dremel rotary tool and the depth verified using Eddy Current. 12.4.2.2 Results and Discussion Similar to the procedure described in the laboratory testing (Sect. 12.4.1), baselines were recorded, damage was introduced, and the DSSHM methodology was performed. Due to time restraints, a total of three baselines were recorded over a time period with an average temperature of 28 ıC. Figure 12.11 shows the generated map of suspected damage along with the determined scan area of the laser Doppler vibrometer.
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