# Damage Imaging in Lamb Wave-Based Inspection of Adhesive Joints
## Materials and Methods
### Experimental Setup  
Experimental measurements consisted of the excitation of Lamb waves in the lower adherend and the collection of wave propagation signals in the selected area of the specimen. The experimental setup is presented in Figure 4. The excitation signal was generated by means of the arbitrary function generator AFG 3022 (Tektronix, Inc., Beaverton, OR, USA) and the high-voltage amplifier PPA 2000  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**No. of Sample**} & \multirow{2}{*}{\begin{tabular}{c} **Design Surface of** \\ **Defect Area (cm\({}^{2}\))** \\ \end{tabular} } & \begin{tabular}{c} **Determined Surface of** \\ **Defect Area Including** \\ **Kissing Defects (cm\({}^{2}\))** \\ \end{tabular} & \
\begin{tabular}{c} **Design Relative** \\ **Defect Surface (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Detemined** \\ **Relative** \\ **Defect Surface (\%)** \\ \end{tabular} \\ \hline \#1 & - & - & - & - \\ \#2 & 24.00 & 24.00 & 33.33\% & 33.33\% \\ \#3 & 48.00 & 49.11 & 66.67\% & 68.21\% \\ \#4 & 15.00 & 17.64 & 20.83\% & 24.50\% \\ \#5 & 24.00 & 26.03 & 33.33\% & 36.16\% \\ \#6 & 24.00 & 26.33 & 33.33\% & 36.57\% \\ \#7 & 24.00 & 29.55 & 33.33\% & 41.05\% \\ \#8 & 8.00 & 9.09 & 11.11\% & 12.63\% \\ \#9 & 8.00 & 13.34 & 11.11\% & 18.53\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Designed and determined area of defects.  
Figure 3: Photographs of the investigated specimens after separation (regions outlined with yellow line denote kissing defects).  
(EC Electronics, Krakow, Poland). Lamb waves were excited by the plate piezoelectric actuator NAC2024 (Noliac, Kvistgaard, Denmark), with dimensions of 3 \(\times\) 3 \(\times\) 3 mm\({}^{3}\) and a resonant frequency of over 500 kHz. The actuator was bonded to the bottom side of the specimen (Figure 4b) using petro wax 080A109 (PCB Piezotronics, Inc., Depew, NY, USA). The excitation signal was a five-peak wave packet obtained from a sinusoidal burst with a carrier frequency of 200 kHz modulated by the Hanning window. There were also additional excitation signals for specimens #3 and #6 that differed in the frequency value from the basic variant (100 kHz and 50 kHz, instead of 200 kHz). Acquisition of the wave propagation signals was realized by the scanning head of the laser vibrometer PSV-3D-400-M (Polytec GmbH, Berlin, Germany) with a VD-09 velocity decoder (Figure 4a). Time signals were acquired with a sampling frequency of 2.56 MHz and were averaged 40 times to improve the signal-to-noise ratio. The excitation signal was triggered as external and monitored using a reference channel of the vibrometer. To improve the backscatter of light, the scanned surface was covered with retro-reflective sheeting. Velocity values (out-of-plane components) were recorded in the time domain in 7381 points that were distributed at the top side of the specimen (on the surface of the upper plate with no PTFE tape attached, see Figure 3). The area was scanned point by point in the mesh of 61 rows and 121 columns (Figure 5), giving a scan resolution of 1.933 mm and about nine scan points per the shortest wavelength. Signals sensed at all of the points allowed for us to prepare a map of the propagating waves, i.e., a two-dimensional plane view at a specific time instance.