# Resonant Airborne Acoustic Emission for Nondestructive Testing and Defect Imaging in Composites
## 7 Full-Field AE Mode Imaging  
Despite a reasonable image quality obtained by AE scanning, as displayed above, the technique is relatively time consuming. For instance, for the scanning speed of 10 mm/s, it took from 10 to 20 min to scan the area of the defects in the above experiments. To avoid this drawback, a new full-field AE imaging method is developed that applies acoustic cameras for instant detection and imaging of the defects.  
The viability of the method is elucidated below by experimenting with an acoustic camera SoundCam by CAE systems, Gutersloh, Germany. It includes 64 MEMS-microphones, the 24-bit acquisition system of sampling rate 48 kHz, and an operating frequency range 10 Hz-24 kHz. The experimental setup for full-field AE imaging with this camera is shown in Figure 18. The operation parameters include adjustment of the distance, dynamic range and the frequency of the receiver.  
Figure 17: (**a**) robotic AE image of a ring delamination in CFRP; (**b**) wideband laser vibrometry image.  
Figure 18: SoundCam setup for full field AE imaging.  
To test its performance, the airborne acoustic radiation of the piezo-actuator was imaged first (Figure 19). As seen, the radiation circular part of the transducer (diameter \(\sim\)5 mm) is clearly spotlighted in the image. The SoundCam interface in Figure 19 also indicates a fairly high (>10 dB) dynamic range of the image and displays the frequency radiated (\(\approx\)13,100 Hz).  
The noisy mode of the transducer was then used for imaging of AE excitation from a pair of FBH of slightly different LDR frequencies (12,200 Hz and 13,300 Hz), (Figure 20). To distinguish between these frequencies of the radiation, the bandwidth of the SoundCam was packed closely to the values of the LDR frequencies (12,200 and 13,300 Hz, Figure 20d) to demonstrate successive frequency-selective AE imaging of the defects.  
The full-field mode operates well for the imaging of real defects in composites, as demonstrated in Figure 21a,b, for the delamination in the actuator imbedded in the GFRP plate. The defect is easily seen in the laser vibrometry image at LDR frequency 18,900 Hz (Figure 21b). The full-field AE image obtained at this frequency (Figure 21a) indicates the resonance part of the delamination, but provides essentially instant testing.  
Figure 22 shows a few other examples of acoustic camera imaging for impact-induced damage in CFRP. The damage in the stringer-reinforced CFRP specimen results in a visible crack on the reverse side (Figure 22a) that is evident in the laser vibrometry image at LDR frequency 17,940 Hz (Figure 22c). Instead, the AE image detected by the acoustic camera reveals the sound radiation from the vibrating edges of the defect only (Figure 22b). The AE image of the crack is thus similar to the full field ultrasonic thermography image [25], where the vibration of the crack tips results in a strong energy dissipation and maximal temperature rise.  
Figure 19: The SoundCam interface and full-field image of airborne radiation from piezo-actuator (frequency 13,100 Hz).  
Figure 21: Soundcam imaging of a delamination in piezo-actuator imbedded in GFRP plate (**a**), 18,900 Hz LDR laser vibrometry image of the delamination (**b**).  
Figure 20: Laser vibrometry (**a**,**b**) and frequency-selective full-field AE imaging (**c**,**d**) of a pair of circular FBH of close LDR frequencies.  
The AE imaging of a hard-to-detect BVID (Figure 22d) induced by a 20 Joule-impact in non-crimp fabric CFRP sample (150 \(\times\) 100 \(\times\) 3 mm\({}^{3}\)) is illustrated Figure 22. The laser vibrometry proves multiple fractional LDR of the damage that lie in a reasonably high frequency range (above the frequency range of the acoustic camera), e.g., at LDR frequency 32,200 Hz (Figure 22f). Despite this value being above the maximal frequency of the SoundCam, the camera still provides a fair AE image of the hidden defect, and evidently relays the presence and the position of the damage in the noisy mode (Figure 22e).