## 2 Experimental Setup  
In the experiments conducted for this study, three composite panels were fabricated to investigate the effects of thickness on Lamb wave propagation in composites utilizing embedded PZT transducers. Unidirectional carbon fiber pre-pregs, specifically Hexply(tm) IM7/8852 (Hexcel, Duxford, UK), were used for the lay-up process. The stacking sequences employed were quasi-isotropic, [(0\({}^{\circ}\)/+45\({}^{\circ}\)/\(-45\)\({}^{\circ}\)/+90\({}^{\circ}\))\({}_{\mathrm{h}}\)]\({}_{\mathrm{s}}\), with n representing the number of plies. The panel thicknesses manufactured are 2 mm, 4 mm, and 9 mm. The dimensions of the specimens were 300 mm \(\times\) 225 mm, and the average cured thicknesses for each composite panel were approximately 2 mm, 4 mm, and 9 mm, respectively. The embedding of PZT transducers was accomplished using DuraAct(tm) (PI Ceramic GmbH, Lederhose, Germany) (P-876.K025) PZT transducers. Figure 1a illustrates the drawing of each composite panel, depicting the configuration of the PZT transducers. Additionally, Figure 1b provides a schematic representation of the diagnostic layers with embedded PZT transducers during the embedding process. These composite panels, along with the embedded PZT transducers, were specifically designed to investigate the thickness effects on Lamb wave propagation and enable a comparative analysis with the results obtained from surface-mounted PZT transducers in the author's previous work. The choice of material, lay-up configuration, and PZT transducers facilitates a systematic examination of the influence of thickness on Lamb wave behavior in composite laminates. Figure 2 shows the flow chart of the embedding technique for the composite manufacturing. To prepare the diagnostic layers with printed circuits, a Dimatix materials printer (DMP-2580) was utilized along with a silver-based ink containing nanoparticles at a concentration of 30-35 wt.%. The circuits were printed on Kapton(r) films (DuPont, Stevenage, UK), which had a thickness of 25.4 um and a melting temperature of up to 400 \({}^{\circ}\)C. The piezo voltage for printing was set to 20 V, and a jetting frequency of 5 kHz ensured stable drop formation during the printing process. To achieve optimal printing quality, the substrate temperature was maintained at 55 \({}^{\circ}\)C, and a drop spacing of 35 um was employed. To enhance the conductivity of the circuits and increase signal amplitudes, the circuits were printed three times with a width of 1.4 mm. After printing, the Kapton(r) films with the printed circuits were placed in a laboratory oven at 135 \({}^{\circ}\)C for 1 h to ensure the particles in the ink became conductive. Following the printing process, the nine PZT transducers needed to be connected and pre-bonded to the Kapton(r) films. In the author's previous work [33], there were issues with connecting the circuits and connectors after the curing process, particularly at the cut-out position. The exposed printed circuits were prone to brittleness and damage, which led to the loss of connection for the embedded PZT transducers. This was a labor-intensive problem to resolve. Additionally, the conductive agents used for connecting the circuits and connectors required curing at 80 \({}^{\circ}\)C for 20 min, which was not a convenient method for  
Figure 1: (**a**) Drawing of the composite panel; (**b**) schematic of embedded PZT transducers during lay-up. Figure 2: Flow chart of the manufacturing procedure. making connections. To address these challenges, high-temperature connecting terminals (Techni Measure Ltd., Doncaster, UK) (TML Co. TPF-2 MS) were used. The terminals enabled soldering and ensured more stable connection quality when joining the circuits and connectors. Figure 3 provides an illustration of the high-temperature connecting terminals and their application in connecting the circuits and connectors.