## 4 Results and Discussion  
The relative nonlinearity of the fabricated EMAT sensor was measured for each specimen in the longitudinal direction of the steel wire. The relative nonlinearity tendency was studied. As mentioned above, the nonlinear guided wave technique is a technique to diagnose changes in material properties and microdefects by using harmonic components unlike the conventional technique. EMAT shows higher performance than PZT to measure nonlinearity of material because it does not need contact medium and uses electromagnetic in flaw detection. In case of using PZT, the error due to the individual difference of the experimenter cannot be ignored. Therefore, the method using EMAT is very appropriate. Figure 14 shows the signals for linear ultrasonic wave in three specimens. The time domain signal for each specimen can distinguish between the distance and the speed relative to the time, however, microdefects cannot be confirmed. In addition, the analysis of the modes showed that the L\({}_{1}\) mode was well implemented for SWOSC-V and SWOCS-VHV, but SWOSC-VHS was able to confirm that the various modes were mixed. Therefore, the experiment was conducted using a nonlinear guided ultrasonic technique using second harmonic. Figure 14: Time domain signal. **(a)** SWOSC-VHS #1; **(b)** SWOSC-VHV #2; **(c)** SWOSC-VHS #3. The fundamental amplitude A1 and second harmonic amplitude A2 amplitude values can be derived by measuring the time domain signal of 60 mm intervals from three specimen for each material and then performing FFT. Substituting the derived A1 and A2 into Equation (2), nonlinear parameters can be obtained for each specimen. Figure 15 is the result of comparing the relative nonlinearity in each zone. In all three specimens of SWOSC-V, the relative nonlinearity increases with increasing propagation distance. The relative nonlinearity of all three specimens decreases at the propagation distance of 181-240 mm, which can be regarded as the material properties of the specimen. SWOSC-VHV also shows the same tendency of three specimens. However, SWOSC-VHS # 2 has a tendency to be different from the # 1 and # 3 specimens at 61-180 mm propagation distance. Three specimens with the same physical properties should show the same tendency toward in each section. However, since VWOSC-VHS #2 has a different tendency from the VWOSC-VHS #1 and #3 samples, it can be determined that there is some abnormality inside the material. Figure 15: Relative nonlinearity for each detection area (**a**) Results of the three-specimen test in SWOSC-V (**b**) results of the three-specimen test in SWOSC-VHV (**c**) results of the three-specimen test in SWOSC-VHS. Because of the tendency to suspect microscopic defects, additional experiments were conducted to ensure the reliability of the experimental data. Figure 16 is the relative nonlinearity with increasing distance, which was an additional experimental result. SWOSC-V and SWOSC-VHS could obtain a tendency for relative nonlinearity change as the distance increased. However, # 3 of SWOSC-VHV showed a large difference in relative nonlinear factor values from # 1 and # 2 at a propagation distance of 120 mm, but it was very difficult to be considered as a suspected defect region due to the agreement of the tendency for each specimen. At the propagation distance of 180 mm, SWOSC-VHH # 2 had the same tendency as the previous experimental results. A total of two experiments were used to consider suspected microdefects in SWOSC-VHV # 2 at a propagation distance of 180 mm. Figure 16: Relative nonlinearity variation of considering propagation distance (**a**) Results of the three-specimen test in SWOSC-V (**b**) results of the three-specimen test in SWOSC-VHV (**c**) results of the three-specimen test in SWOSC-VHS. Figure 17 is cross-sectional photographs of microdefect suspected areas and defective free area. Figure 17b shows a clear difference in the distribution of microdefects compared to Figure 17a. This is consistent with the experimental results and SEM photographs.