# Resonant Airborne Acoustic Emission for Nondestructive Testing and Defect Imaging in Composites
## 3 RACE Experimental Evidence  
In the experiments, commercial piezo-actuators manufactured by isi-sys GmbH with a frequency response extended into the 100-200 kHz range were used for activation of defects. To recognise the LDR frequency, the voltage applied is in chirp mode (bandwidth above 100 kHz, input 10-20 V) from the HP 33120A arbitrary waveform generator after the HVA-B100 amplifier. The RACE signal is detected by a half-inch condenser microphone (B&K 4130, sensitivity 10 mV/Pa) combined with 40 dB preamplifier type B&K 2642 and power supply B&K 2810, positioned at a close distance above the specimen surface and attached to a 2D scanner (Isel-automation) (Figure 2). To investigate the RACE field features, the simulated defects of circular-shaped Flat-Bottom Holes (FBH) (different radii and residual back-wall thicknesses) in PMMA plates were used. A close interconnection between RACE and LDR is obvious from Figure 3: the RACE signal closely matches the frequency response of LDR. The latter is measured for a wideband excitation of FBH by using a laser vibrometer (PSV 300 Polytec, Waldbronn, Germany) and is compared with microphone RACE response at different frequencies. Another way to detect and visualize the RACE is concerned with application of the air-coupled vibrometry [18]. The technique is based on a focused laser beam which propagates through the airborne field created by the wave in the solid specimen, is then reflected from a fixed reflector, and is processed by a heterodyne interferometer of the laser vibrometer Polytec 300. Due to the photoelasticity of air, the optical path length of the beam and its frequency are modulated according to the airborne acoustic pressure. The induced frequency shift is recognized by the vibrometer software as a variation in vibration velocity of the sound in air that is visualized and measured. The use of the air-coupled vibrometry in imaging RACE from point-of-impact damage (indicated by the white circle, Figure 4a) in a carbon fibre-reinforced polymer (CFRP) is illustrated in Figure 4b,c, which provides side views of a laser vibrometer C-scan of the air  
Figure 1: (**a**–**c**): FEM simulation of resonant behaviour of FBH: LDR for a circular (**a**) and rectangular (**b**) FBH in polymethyl methacrylate (PMMA); (**c**) RACE for rectangular FBH in an aluminium plate. above the surface of the CFRP. In a separate measurement, laser vibrometry reveals LDR in the small, damaged area at 110 kHz. When excited at a frequency outside of LDR (90 kHz, Figure 4b), the point impact does not affect the wave propagation, and the airborne field is a slanted plane wave propagating at an angle defined by the ratio of the sound velocities in air and in the specimen. Figure 4: The point of impact in the CFRP specimen studied (**a**); airborne field above the CFRP specimen with an impact: (**b**) nonresonant case (90 kHz excitation); (**c**) LDR-induced RACE at 110 kHz excitation. Figure 3: Comparison of RACE and LDR frequency responses for FBH in PMMA plates. Figure 2: Setup for excitation and microphone RACE detection and defect imaging. The airborne field changes dramatically when the wave frequency coincides with LDR frequency (110 kHz) (Figure 4c): the radiation from the defect area turns into a spherical wave at a distance of a few wavelengths from the defect (far field). In the near-field zone, a part of the wave front could be considered as an airborne \"fingerprint\" of the defect, emitted in the vertical direction (inside the dotted area in Figure 4b). The near-field part of radiation is preferable for receiving the RACE signal and imaging of the defect. This conclusion is also apparent from another RACE evidence experiment shown in Figure 5. An impact-induced damage (15 \(\times\) 7 mm\({}^{2}\)) (Figure 5a, reverse side) in a CFRP plate (165 \(\times\) 30 \(\times\) 2.5 mm\({}^{3}\)) manifests LDR at 59,500 Hz.