## 4 Experimental Investigation  
### Experimental Setup  
Experiments were carried out to verify the improvement of the SMP as shown in Figure 13. The equipment used in the experiments (MSGW30, Zheda Jingyi Tech, Ltd., Hangzhou, China) was designed for magnetostrictive guided wave inspection, which was integrated with modules, such as signal generation, power amplification, pre-amplification and A/D conversion. The equipment has two channels. Under the control of the computer, the equipment can work in the pulse-echo mode (wave generation and detection using Channel 1) and pitch-catch mode (wave generation using Channel 1 and detection using Channel 2). The inspection subject was an aluminum plate with a thickness of 5 mm. For the constant and same bonding conditions of the transducers (T1 and T2), dual adhesive tapes were used for the bonding between the magnetostrictive patches made of Fe-Co alloy and the aluminum plate. The thicknesses of the magnetostrictive patch and dual adhesive tape were 0.12 mm and 0.15 mm, respectively. The FPCs (MF64 and MF128) for the two main frequencies (64 kHz and 128 kHz) were processed with two coils and only one was used in the experiments to avoid the crosstalk. The generated guided wave propagated forward and backward without direction controlling and the wave packs from the far end of the plate in the signals were used for evaluation. The SMP was manufactured by hot-pressing with a mixture of the Mn-Zn ferrite power and rubber, which had a thickness and relative permeability of 0.15 mm and 2800, respectively. The SMP, FPC and magnetostrictive patch were all flexible and the dimensions of the SMP and magnetostrictive patch were 50.8 \(\times\) 70 mm, which was the same as the working region of the FPC. The excitation signals at different frequencies were sinusoidal signals of four cycles filtered by the hanning windows and the sample frequency was 2 MHz. Figure 11: MPTs with the SMPs of different thicknesses: (**a**) The magnetic field intensity in Line 1 in wave generation; and (**b**) the magnetic field intensity in Line 2 in wave detection. Figure 12: MPTs with the SMPs of different lift-off distances: (**a**) The magnetic field intensity in Line 1 in wave generation; and (**b**) the magnetic field intensity in Line 2 in wave detection. The improvement of the SMP for wave generation and detection was first verified. In this experiment, the transducer marked as T1 in Figure 13 was used as an actuator while T2 was a sensor. The MF64 was used for frequencies of 55-90 kHz while the MF128 was used for frequencies of 95-150 kHz. The signals were set as the base data when T1 and T2 both were not covered by the SMPs. Furthermore, the signals received at the cases, such as only T1 covered by the SMP, only T2 covered by the SMP and both covered by the SMPs, were compared with the base data. The magnification was calculated using the peak values of the wave packs from the far end of the plate in the Hilbert envelopes of the signals. The effect of thickness changes was investigated using multiple layers of SMPs. For each frequency, the signals were recorded when the MPT was covered by different layers of the SMPs (four layers at most). The last experiment was focused on the lift-off distance, which was carried out using a single layer of the SMP. The lift-off was achieved by installing multiple layers of epoxy patches between the FPC and SMP (six layers at most). The thickness of one single epoxy patch was 0.5 mm. These experiments were carried out with T1 used as an actuator and sensor in the pitch-catch mode. The signals from the cases in which the transducer was not covered by the SMP were used as the base data for amplification evaluation. It should be mentioned that in order to get rid of the void between the layers of the transducer, a nonmetal heavy weight was set above the transducer during the signal collection.