## 4 Experiment Investigation
### The Traditional TRM  
In the traditional TRM, transducers change their roles from actuator to sensor and vice versa to reverse the path direction. All the signals in this experiment are measured 128 times at each receiver position to improve the signal-to-noise ratio (SNR). A five-cycle 40-kHz tone burst signal is launched from PZT A. The response signal captured by PZT B is shown in Figure 5a. As mentioned in Section 2, the re-emitted components are extracted from that response signal. To minimize false warning of damage, only the A0 mode components travelling along the direct path between PZT A and PZT B are extracted. In this case, the time interval [\(t_{01}\), \(t_{02}\)], in which the components are extracted and time inversed, is a key parameter. Let _weight1_ to represent the amplitude drop of envelope of the A0 direct wave. The time interval is the time range where the magnitude of direct wave are larger than _weight1_ times the peak value. As Figure 5a shows, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material Properties** & \(\rho\) **(kg/m\({}^{3}\))** & \(E_{1}\) **(GPa)** & \(E_{2}\) **(GPa)** & \(G_{12}\) **(GPa)** & \(v_{12}\) & \(v_{23}\) \\ \hline
**Value** & 1560 & 135 & 10.9 & 4.7 & 0.285 & 0.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material properties of a single py of the composite plate. Figure 4: Schematic diagram of the composite plate with damage and transducers. when _weight1_ = 0.1, components in the time interval (the red solid line) contains both the direct wave and reflected wave. In contrast, when _weight1_ = 0.3 and 0.5 (the yellow dashed line and the green dash-dotted line), the extracted components only contain the direct wave, however, the direct wave components are incomplete. Take _weight1_ = 0.3 as an example; the extracted component is displayed in Figure 5b as the purple dashed-dot line. Then the extracted components are time reversed and re-emitted by PZT B. Following this, the reconstructed signal captured by PZT A is shown in Figure 5c. Take _weight2_ to represent amplitude drop of envelope of the reconstructed signal. The vertical lines in Figure 5c indicate the ranges at different _weight2_ values, in which the reconstructed components are compared with the original input signal. For illustration, Figure 5d displays the extracted reconstructed signal when _weight2_ = 0.3 by the blue solid line. It can be seen that the extracted signal consists of two wave packets, which is quite different from the original input signal (the red dashed-dot line). Subsequently, the effects of the two time intervals on the accuracy of damage detection are studied. An added mass (20 mm \(\times\) 20 mm \(\times\) 4 mm), is bonded to the specimen as a simulated delamination damage [3; 36]. With a fixed _weight2_ value (i.e., _weight2_ = 0.4), the evolution of DI values with the variation of _weight1_ values under healthy and damaged conditions are shown in Figure 6a. It can be seen that, when _weight1_ = 0.1 and 0.2, the DI values of healthy condition are even larger than that of the damaged condition. Consequently, it indicates a false warning of damage. What is more, at other _weight1_ values, the DI values in healthy and damaged conditions are not significantly different. This will restrict its ability to determine the presence of damage. Figure 5b shows the results, where _weight1_ is fixed while _weight2_ changes. It can be seen that _weight2_ also has a great influence on the DI value. Figure 5: (**a**) The time interval for extracting re-emitted components varies with _weight1_; (**b**) the extract re-emitted components when _weight1_ = 0.3; (**c**) the time interval for extracting reconstructed components varies with _weight2_; and (**d**) the comparison between the reconstructed components and the original input signal when _weight2_ = 0.3. In practical applications, the determination of _weight1_ and _weight2_ is related to the physical information and boundary conditions. Thus, it may be time consuming.