## 2 Materials and Methods
### Lamb Waves
#### 2.2.2 Experimental Investigation  
To show the signals of the Lamb waves and their waveform changes, the signals of Lamb waves propagating in the WTB obtained during an experiment are registered. The artificial defect having a diameter of 81 mm was made by drilling out the region of the main spar, simulating the real disbond type defect between the main spar and covering shell. The typical measurement systems used in the SHM and NDT are presented in Figure 3b,c respectively. The more accurate hybrid experimental measurement setup for the WTB testing using SHM and NDT techniques is presented in Figure 6. The low-frequency ultrasonic measurement system 'Ultralab' developed and manufactured at Ultrasound Institute, Kaunas University of Technology, consists of the high voltage generator, the low noise amplifier and an analogue to digital converter. The maximum output voltage of the generator is 750 V. Gain of the amplifier can be changed from 10 dB to 50 dB. The low noise 13 dB preamplifier is connected directly to the receiver in order to improve the signal-to-noise ratio. The ultrasonic transmitter was excited by a single rectangular pulse with a duration of 11.6 ms (it corresponds to half of the period of frequency 43 kHz), and the voltage of 120 V. Scanning was performed with 0.1 mm scanning step along x direction. The permanently fixed macrofibre composite (MFC) transmitting transducer was used as a transmitter in order to generate the A\({}_{0}\) mode of 43 kHz. The contact type receiving transducer was scanned away from the transmitter from x1 = 30 mm up to x2 = 160 mm in order to obtain the B-scan image. The B-scan image and waveforms of the experimental signals (mainly the A\({}_{0}\) mode) of Lamb waves (before the defect, in the region of the defect, and behind the defect) are presented in Figure 7.  
Waveforms of the presented Lamb wave signals, propagating along the complicated structure, are distorted due to frequency flicker of the interfering multiple modes, from an overlapping of directly propagating, scattered, and converted modes at edges of the defect, etc. The phenomenon of dispersion and relatively elongated waveforms of the signals in the time domain significantly complicate the applicability of conventional SPMs (windowing, threshold detection of amplitude, etc.) in the analysis of time or frequency parameters of the mentioned signals. So to identify and characterise faults using these  
Figure 5: Dispersion curves of the phase (**a**) and group (**b**) velocities up to 250 kHz calculated by the SAFE method.  
types of waves, specific signal analysis algorithms are required [22]. That is why specific signal processing techniques are necessary to be developed, investigated, and used.  
Figure 6: The hybrid experimental setup of WTB inspection using embedded MFC transmitter and mechanically scanned receiver (contact type transducer).  
Figure 7: The B-scan image and waveforms of the experimental signals of Lamb waves (the A\({}_{0}\) mode). Distance from transmitter (**a**) 40 mm (position in the B-scan before the defect: x = 10 mm), (**b**) 95 mm (position in the B-scan, in the region of the defect: x = 65 mm), (**c**) 170 mm (position in the B-scan, (**d**) behind the defect: x = 140 mm).