## 4 Experimental Study
### Experimental Setup  
A 10-cycle sine wave was generated by the Mistras Group Limited (MGL) WaveGen function generator software (MGL, Cambridge, UK) which was connected to the MGL \(\mu\)disp/NB-8 hardware (MGL). This allowed the transducer to input sufficient energy into the structure. The peak-to-peak amplitude of the excitation signal was 160 V. Three frequencies were selected for this experiment, 100 kHz, 250 kHz and 300 kHz. The frequencies investigated primarily excited the two fundamental modes as shown in the calculated dispersion curves for a 3 mm aluminium plate in Figure 3.  
100 kHz was chosen to investigate how a longer wavelength interacted with the stiffener and the disbonded region. The wavelengths for the A\({}_{0}\) and S\({}_{0}\) were calculated to be 15 mm and 55 mm respectively. Though this frequency fell outside the operating resonance window of the chosen  
Figure 2: Dimensions of the stiffened panel (all dimensions in mm).  
transducer, the calibration certificate for the transducer showed that it would function at this frequency but at a reduced amplitude. Therefore, frequencies of 250 kHz and 300 kHz were also selected to investigate the interaction of smaller wavelengths. The wavelengths for the A\({}_{0}\) and S\({}_{0}\) were calculated to be 9 mm and 22 mm respectively for the 250 kHz excitation and 8 mm and 18 mm respectively for the 300 kHz excitation. Frequencies above 300 kHz were not considered due to the constraint of the sampling frequency of the acquisition card in the laser vibrometer used. A 10 V peak-to-peak wave was also generated and used as a reference signal for triggering the acquisition of the vibrometer. A repetitive trigger rate of 20 Hz was used as this gave sufficient time for the induced wave energy to fully dissipate before the next measurement was taken.  
The vibrometer used for this experiment was the Polytec PSV-500-3D-M (Polytec, Waldbronn, Germany). This vibrometer uses three laser heads to measure each scan point. The measurements recorded by each laser head can then be used to calculate both the in-plane and out-of-plane modes using trigonometry. Since measurements are taken from three heads it is less important for the lasers to be perpendicular to the structure under test with a 3D scanning system than it is with a 1D system.  
The sampling frequency was chosen as 2.56 MHz, which gave sufficient resolution for reconstructing the wave. A drawback of the vibrometry system used however is that at this sampling frequency, the lowest sensitivity range is 200 mm/s. This is not ideal as the amplitude of the measured velocities was of the order of 1 mm/s and thus the measured signal had a low signal to noise ratio. This was overcome by taking 200 measurements at each point and averaging the signal.  
4096 samples were taken giving an overall sample time of 1.6 ms and a resolution of 390.625 ns. This sample length was considerably longer than required by this study resulting in the waveforms recorded capturing the Lamb wave interaction with the stiffener followed by the edge reflections with a period of no activity in between. Therefore, the sample time was reduced to capture only the first interaction of the Lamb wave with the stiffener in post-processing.  
A scan area of 145 mm \(\times\) 145 mm was measured which comprised of 5329 measurement points. This area was coated with retro-reflective glass beads which have a diameter of the order of 10 \(\upmu\)m. The glass beads improved the back-scatter of the laser light and hence improved the quality of the signal. The beads were applied to the surface using a spray adhesive and a thin coverage was ensured by removing excess beads.