## 2 Description of the Experiment
### Experimental Equipment and Principle  
The active Lamb wave is excited by the driving element to the monitored structure. The active Lamb wave is received by the receiving sensor after propagating a certain distance in the structure. Then the Lamb wave signal containing damage information is processed by using a specific signal processing method, and the characteristic parameters related to the monitored structure damage are extracted, and the structural damage is identified by combining the corresponding damage identification methods. The ultrasonic excitation system of this experiment is the RETIC RAM SNAP 5000 high-energy ultrasonic measurement system. The RETIC RAM SNAP 5000 high-energy ultrasonic measurement system is used for the excitation and measurement of ultrasonic Lamb waves. This measurement system was specially developed by the RITEC Corporation of the United States for the measurement of nonlinear ultrasound. It is a commercial nonlinear ultrasonic measurement system with advanced technology at present. It mainly includes a broadband RF pulse amplifier, signal tracking receiver, phase shift sensitive detector, gate integrator and multiple frequency synthesizers. The ultrasonic measurement system generates pulse signals according to the indication information, which are transmitted to the ultrasonic piezoelectric transducer through the attenuator and low-pass filter through the connecting line. The transducer converts ultrasound signals into vibration signals through the reverse piezoelectric effect, and injects them into the plate through the piezoelectric element, so that they propagate into the plate. The excited Lamb wave propagates in the sheet and interacts strongly with the sheet defect. The signal with a large amount of information required for detection enters the receiving transducer, and is displayed and stored by the oscilloscope after passing through the high-pass filter and amplifier. The received signal is processed to obtain the desired information. The specific experimental equipment diagram and recommended schematic diagram are shown in Figure 4.  
The RETIC RAM SNAP 5000 high-energy ultrasonic measurement system generates 5-cycle Hanning window modulated sine pulse signals, drives ultrasonic piezoelectric transducers to generate ultrasonic vibration signals after passing through attenuators, and applies these vibration signals to sample objects for experiments. A signal with 5 cycles ensures that the waveform in the time domain is easy to distinguish while maintaining a narrow bandwidth. The Hanning window modulation used in our experiment can increase the main lobe, reduce the side lobes, and reduce spectral energy leakage. The Lamb wave is generated in the flat sample and propagated to the receiving piezoelectric transducer. Then the signal is amplified by the power high voltage amplifier, and then the signal is converted from an electrical signal to a mechanical signal through the PZT driver and propagated in the plate structure. Then it is received by the PZT sensor and converted into electrical \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline E\({}_{11}\)/MPa & E\({}_{22}\)/MPa & E\({}_{33}\)/MPa & \(\mathbf{\
u_{12}}\) & \(\mathbf{\
u_{13}}\) & \(\mathbf{\
u_{23}}\) & G\({}_{12}\)/MPa & G\({}_{12}\)/MPa & G\({}_{13}\)/MPa \\ \hline
172,000 & 7000 & 7000 & 0.35 & 0.35 & 0.35 & 3900 & 3900 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Materials’ parameters of carbon fiber/epoxy composites. signal again. Finally, the received signal is input into the RETIC RAM SNAP 5000 system, oscilloscope and computer for storage, display, and subsequent processing and analysis. For this experimental design, Lamb wave is selected as the detection method. The wave velocity of the same guided wave mode will change with the change in frequency. At the same time, the increase in frequency will also lead to more complex guided wave modes.