# Resonant Airborne Acoustic Emission for Nondestructive Testing and Defect Imaging in Composites
## 5 Nonlinear AE-Mode  
It is well known that planar defects in composites are accompanied by the damage that results from a combination of matrix-fibre debonding, cracking and delaminations. Such a mixture of the fractured constituents makes the defect area highly nonlinear due to contact acoustic nonlinearity (CAN) [21]. Due to CAN, local, nonlinear vibration spectra  
Figure 8: Multiple-defect imaging in noisy mode of RACE: 4 circular FBH of different depths (**a**) and 4 square inserts at various depths (**b**) in CFRP plate. Figure 7: RACE imaging of BVID defect in CFRP plate: (**a**) photo of the defect; (**b**) AE scanning image. acquire a number of new frequency components which are used as sensitive signatures of emerging damage in the nonlinear approach to NDT. The highly nonlinear vibrations of the defects apparently contribute to the RACE spectrum and bring about the nonlinear AE [22; 23]. The experimental setup capable of detecting nonlinear AE and applying it to the imaging of defects (Figure 9) uses a high-frequency (~450 kHz (3 dB-bandwidth of ~20 kHz)) focused, air-coupled transducer as a receiver. A strong rejection of low-frequency signals used for the defect piezo-excitation is provided by its band-pass frequency response and high-pass filtering circuit. The nonlinear frequency components (higher harmonics (HH)) are then filtered through and used as an input to air-coupled scanning equipment (AirTech 4000) for mapping of the nonlinear components' distributions (C-scan). The feasibility of the nonlinear AE is illustrated in Figure 10 for a damaged, glass fibre-reinforced polymer (GFRP) specimen (2 \(\times\) 30 \(\times\) 200 mm\({}^{3}\)) with an embedded ceramic actuator (\"smart structure\"). Repetitive voltage application caused an oval delamination between the actuator and the substrate. The nonlinear bandpass spectra of the receiver that contain (2-4) higher-order harmonics were used for nonlinear AE imaging of the delamination and are compared with nonlinear laser vibrometry in Figure 10. Both techniques reliably visualize the delaminated part of the defect with virtually comparable sensitivity and resolution. A certain difference in images is apparently concerned with the different nonlinear outputs used, namely the third harmonic image (~58 kHz) in the optical case Figure 10c and (23 + 24) HH (around 450 kHz) AE image Figure 10b. A high spatial resolution of the nonlinear AE imaging is also demonstrated in Figure 11. The mm-size contour details of the complex shape delamination (5 \(\times\) 10 mm\({}^{2}\), Figure 11a) in the multiplied GFRP are traced closely in the airborne HH image (Figure 11b). Figure 10: Imaging of the oval delamination (**a**) in smart structure: nonlinear AE (**b**) and laser vibrometry image (**c**). Figure 9: Setup for detecting nonlinear AE and applying it to imaging of defects. The result in Figure 11 is obtained in nonresonant mode, i.e., the excitation frequency (20 kHz) was quite distant from the defects' LDR frequencies. For this reason, piezo-stack transducers and a high-power supply (Branson, USA) have been applied to generate the nonlinear AE. By combining the resonance conditions provided by LDR with highly efficient CAN, a substantial improvement in the efficiency of nonlinear AE as a signature of cracked defects can be expected. The LDR-induced increase in local vibration amplitude enables lowering of the excitation level and observation of a highly efficient HH AE at lower acoustic inputs, using conventional ultrasonic equipment. The two examples of resonant HH AE (Figure 12) images are obtained with excitation by low-cost piezo-elements of 2-5 kHz fundamental frequencies, distributed by Conrad Elektronik GmbH. The image of a circular FBH (8 mm diameter, 50 kHz LDR frequency, Figure 12b) is observed at the 9 th HH (450 kHz), while for the point impact in CFRP (~5 \(\times\) 5 mm\({}^{2}\), Figure 12c), the output frequency of 440 kHz corresponds to the 4 th HH of its LDR frequency, 110 kHz.