# Resonant Airborne Acoustic Emission for Nondestructive Testing and Defect Imaging in Composites
## 8 Conclusions  
Resonant airborne emission is the effect of acoustic waves generated from the defect area in ambient air by local standing wave vibrations developed in this area at LDR frequency. A monochromatic, resonant activation of defects substantially enhances the efficiency of AE, which is readily detected by a commercial microphone or an air-coupled transducer. An approach which does not require preliminary knowledge of LDR frequency is one based on wideband acoustic activation by using a noise-like input signal. The FFT of the signal obtained in the noisy activation mode resolves the inverse problem of finding LDR frequencies.  
The AE field from the defect area is a footprint of the radiation source, and thus is applicable to AE detection and imaging of defects in composites via microphone C-scanning of the specimens. The quality of the resonant AE images is comparable with the images obtained via scanning laser vibrometry. Activation of LDR in combination with high nonlinearity of cracked defects in composites greatly increases the nonlinear AE of defects in the form of the higher harmonics and mixed frequencies. The noisy mode provides full-scale imaging of the shape of the defect due to excitation of the higher-order LDR.  
Unlike conventional AE scanning of flat composite plates, the use of robots brings forth an opportunity for detecting and imaging of the defects in complicated-shape objects. The robotic AE scanning mode supplemented with noisy excitation enables imaging of multiple defects without a prior search for LDR frequencies. A newly developed full-field AE imaging method based on commercial acoustic cameras changes a slow and tedious scanning operation into a rapid visualization of defects. Multiple case studies of the full-field AE approach validate the reliable imaging of various risk defects in composites. The  
Figure 22: (**a**) crack in CFRP stringer-reinforced sample, SoundCam image (**b**), laser vibrometer LDR image (**c**); (**d**) BVID in non-crimp fabric CFRP; fractional LDR 32,200 Hz vibrometry image (**f**) and full-field AE image using acoustic camera (**e**).  
resonant AE approach simplifies integration of the proposed NDT imaging system, which includes inexpensive, fully acoustic, instrumental components.  
[PERSON] implemented the major bulk of the RACE analysis and measurements in the scanning mode. [PERSON] was responsible for the robotic and the full-field modes. [PERSON] initiated and coordinated experimental work and perception of the results. All authors have read and agreed to the published version of the manuscript.  
This research was funded by DFG, grant number KR 31 2131/12-1.  
Not applicable.  
The data presented in this study are available in references [8; 9; 16; 18; 19; 20; 21; 22; 23; 24].  
The authors acknowledge funding of this study in the framework of DFG project KR 31 2131/12-1.  
The authors hereby declare that the submitted work was carried out in the lack of any personal, professional or financial relationships that could potentially be considered as a conflict of interests.