## 4 Experimental Results and Discussion  
### Pencil-Lead Breaks Experiment  
Pencil-lead breaks (PLB) in plates were used as the typical AE test source in literature [19; 21]. In order to verify the effectiveness of the proposed approach, here a PLB signal is captured by an AE detection system. The schematic diagram of the experimental system is displayed in Figure 10. It consists of a CFRP plate, PLB (on the top surface of the plate), AE sensor, pre-amplifier, AE system and industrial computer. The AE sensor is mounted on the top surface of CFRP plate, and the AE signal is captured in the experiment. During the experiment, the original AE signal is amplified by 40 dB, and the sample rate is 10 MHz.  
Figure 10: (**a**) The experimental system of PLB and (**b**) the configuration of CFRP plate with T stiffeners.  
The detailed information of the experimental system is shown in Table 1.  
From the acquired AE signal, the waveform and spectrum of PLB can be obtained. The waveform in Figure 11 shows the duration of PLB signal is about 0.4 ms, and it can be seen that both the extensional mode and flexural mode are present. There are many peak frequencies, due to the big difference of the amplitude, the high frequency components are not obvious in Figure 12a, but from the power spectral density (PSD) in Figure 12b, it can be seen that the three main spectrum bands are in the near zone of 100, 200 and 500 kHz. Then the main difficulty of analyzing this signal would be separating the three main components from the high level of noise, especially the nearby peak frequencies.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **Type** & **Parameter** \\ \hline CFRP plate with & TR50S12L & 650 mm \(\times\) 650 mm \(\times\) 3 mm, 20 layers, [0, 90]\({}_{\text{105}}\); T shape \\ stiffness & TR50S12L & stiffeners (cross section): 40 mm \(\times\) 20 mm \(\times\) 3 mm \\ \hline Pencil-lead & Mitsu Nano Dia & Diameter: 0.5 mm \\ \hline AE sensor & PAC \({}^{1}\) WS \(\alpha\) & Wide-band (WD) Sensor; \\  & & Operating Frequency Range: 100–1000 kHz; \\  & & Temperature Range: \(-\)65–175 \({}^{\circ}\)C. \\ \hline Pre-amplifier & PAC 2/4/6 & Gain Selectable: 20/40/60 dB \\  & & Bandwidth: 10 kHz–2.5 MHz (20 dB), 10 kHz–2.0 MHz (40 dB), 10 kHz–900 kHz (60 dB); Temperature Range: \(-\)40–65 \({}^{\circ}\)C \\ \hline AE system & PAC PCI-2 & 2 simultaneous channels, 18 bit resolution, 40 MS/s \\ \hline Industrial computer & IEI RACK-360 GBATX & E5800 3.2 GHz, 4G memory, 7 PCI slots \\ \hline \end{tabular} \({}^{1}\) Physical Acoustics Corporation.  
\end{table}
Table 1: The detailed information of the experimental system.  
Figure 11: The waveform of the PLB signal.  
When the window value is 50 kHz and the segmentation numbers are three, the detected local maxima using the original EWT and LWM methods are shown in Figure 13, where they are denoted by the red circles and red stars, respectively.  
Figure 12: (**a**) The spectrum and (**b**) the PSD of PLB signal. (Include three different main spectrum bands, which are near the 100, 200 and 500 kHz marks).  
Figure 13: Local maxima of (**a**) the original EWT method and (**b**) the proposed LWM method.  
In Figure 13a, the three maxima are very close to each other, they should not be separated to three different components, but by using the traditional method, they could be roughly separated. Therefore, the results through the proposed LWM method are better than the original EWT, where the three local maxima are located in the three main frequency bands. The separated results based on the proposed LWM-EWT method are depicted in Figure 14. It contains the waveform and spectrum of three dominant components decomposed from the PLB signal.  
Figure 14: (**a**) The waveforms and (**b**) the spectrum of separated signals by the LWM-EWT method.