*3.3. Experiment Results*

Figure 11 shows the scan results of the MFLT module on the Tube-1. Magnetic field distribution around the damages could help to recognize the presence of the damages. All the taper-type wears (t11, t12, t13, and t14) could be detected, and the magnetic field intensity increases as the size of the wear increases. In addition, the slit-type damages on the membrane (m11, m12, and m13) could also be detected, but the damages on the welding line (w11, w12, and w13) were out of the sensing area. The smallest slit-type damage (m11) has a length of 20 mm, depth of 0.9 mm, and 7 mm width that could be detected. Similarly, the taper-type wears on Tube-4 (t41, t42, t43, and t44) could be detected, as shown in Figure 12. However, the smallest size of slit-type damage on Membrane-3 (m31) having a length of 60 mm, depth of 0.3 mm, and width of 7 mm could not be detected; this is because the damage has a smallest depth of 0.3 mm. The damages (m32 and m33) which have depths of 1.2 mm and 2.4 mm, could be detected. The slit-type damages on the Weld-4 (w41 and w42) were out of the sensing area, but a part of the w42 signal could be measured because the damage has the deepest depth of 1.8 mm.

**Figure 11.** Distribution of magnetic field on the Tube-1, Membrane-1, and Weld-1 having artificial taper-type wear (single side) and slit-type damages.

**Figure 12.** Distribution of magnetic field on the Tube-4, Membrane-3, and Weld-4 having artificial taper-type wear (double side) and slit-type damages.

Figures 13 and 14 show the detection result of the slit-type damages. All defects (t21~t28, t31~t38) with a depth of 0.3 mm or more located in Tube-2 and Tube-3 could be detected. In addition, damages of 0.9 mm in depth and 30 mm in length (w11) or more were detected in Weld-1 could also be detected. However, damages (w21, w22, w31, w32, w33) located in Weld-2 and Weld-3 were difficult to detect. Nevertheless, damages with a depth of 0.9 mm or more in Weld-4 (w41, w42) and damages with a depth of 1.2 mm or more in Membrane-3 (m32, m33) could be detected. This is because that when the sensor for magnetic flux density measurement scans Tube-4, it is skewed toward Membrane-3, and the lift-offs of Membrane-2 and Membrane-3 are not the same.

From the above results, the depths of damages detectable in the tube and membrane are 0.3 mm and 0.9 mm, respectively. In addition, some damages having a depth of 0.9 mm or more could be detected in the welded part due to the influence of the welding beads.

**Figure 13.** Distribution of magnetic field on the Tube-2, Membrane-1 (Weld-1, 2) having slit-type damages.

**Figure 14.** Distribution of magnetic field on the Tube-3, Membrane-3, and Weld-4 having slit-type damages.

Figure 15 shows the graph showing the relationship measured data with the depth of the defect on Tube-3. The damages have the same width of 7 mm and length of 60 mm. The measured data is the minimum data points selected from the numbers of Hall sensor array that are on the damages during the scan. There are 30 sensors, and 15 sensors data plotted on Figure 15a,b. The data in Figure 15a has more noise than in Figure 15b because some sensors are located far from the damages. Then, data of 15 sensors is used for further evaluation of the damages' depth. The average data of the sensors are used to reduce noise that may occur rather than a single sensor. Also, the relationship between the measured data with the damages' depth is expressed in Equation (13). This form of the experimental equation is same as the theoretical analysis by the dipole model of the previous section (Equation (9)). The factors *c*1, *c*2, and *a*<sup>2</sup> are 121.08, 6.65, and 2.52, respectively.

$$d = \sqrt{121.08 \, V\_H + 6.65} + 2.52 \,\tag{13}$$

**Figure 15.** Relationship between the depth of flaw and the measured data with magnetic flux density method: (**a**) data of 30 sensors and (**b**) data of 15 sensors.

From Equation (13), the depth of damages on the tube (), membrane (-), and weld (Δ) were estimated, as shown in Figure 16. The standard deviations of the depth estimation are 0.329, 0.269, and 0.523 mm for the damages on the tube, membrane and weld, respectively. The best estimation result is for damages on the membrane because the surface specimen is flat. The worst case for the damages on weld were due to the roughness of the weld surface, the sensor lift-off variation due to welding bead, and the edge effect at the terminal of the magnetizer.

**Figure 16.** Estimation of depth of damages on the tube, membrane, and weld.

Figure 17 shows the B-scan result measured by the FUP after filling the acoustic medium and wrapping it with PET to the damage of Tube-1 (t11~t14). The horizontal axis represents time (*Ti*), and the vertical axis represents the moving distance of the FUP. The position of *T*<sup>1</sup> for each movement distance was about 20 μs before the start of the scan, but after 550 mm, it appeared at 17–18 μs. This is because of the variation of the inclined angle of the FUP and deformation of the membrane due to variation of the FUP lift-off. Therefore, it is necessary to shift the flying time on specimen surface (*T*1) for each scan position, as expressed in Equation (14). Furthermore, it can be seen that in the vicinity of 100, 300, 525, and 725 mm, the delay of the FUP signal is longer than that of other locations, and near-side damage occurs in the corresponding region. It can also be determined from the delay of the signal that the shape of the damage is inclined to one side, and the depth can be estimated.

[

$$\begin{bmatrix} \overrightarrow{T}\_1 \end{bmatrix} = \begin{bmatrix} \overrightarrow{0} \end{bmatrix} \tag{14}$$

Figure 18 shows the B-scan results of the FUP measured from the back surface of the damages (m31–33) of Membrane-3 and (w21, w22) of Weld-2 using FUP. Similar to the previous experimental results. The position of T1 for each movement distance was about 22 μs before the start of the scan, but after 450 mm past m32, it is back to 21 μs. Unlike the case of the near-side damage in Figure 17, it is possible to recognize that there is no near-side damage because the FUP signal appears continuously. On the other hand, it is observed that m31, m32, and m33 damages occur around 130 mm, 460 mm, and 750 mm, respectively. In addition, signal attenuation appears in the range of 625–700 mm. This is because the ultrasonic wave attenuates at the edge of the weld defect w42 located in Membrane-3. A similar phenomenon occurred near the weld defect (w41) at 325–380 mm. The depth of the damages was estimated, as shown in Figure 19. The standard deviation of the depth estimation is about 0.089 mm, which is much more accurate than using the magnetic flux leakage testing method.

**Figure 17.** B-Scan results with FUP on the near-side damages of Tube-1 (**a**) before and (**b**) after *T*<sup>1</sup> adjustment.

**Figure 18.** B-Scan results with FUP on the far-side damages of Membrane-3 and Weld-2 (**a**) before and (**b**) after *T*<sup>1</sup> adjustment.

**Figure 19.** Estimation of damages' depth using the FUP.

#### **4. Conclusions**

In this paper, a nondestructive inspection system was proposed to detect defects on the near-side and far-side of the boiler water-cooled wall tube, membrane, and weld and to quantitatively evaluate the size of the defects. A magnetizer manufactured in a curved shape according to the cross-sectional shape of the tube, membrane, and welding part magnetizes a portion of the water-cooled wall in the axial direction. In addition, the shape of the surface defect can be qualitatively determined from the magnetic flux density distribution measured by the magnetic sensor array deflected from the center of the magnetizer to one side. The minimum depths of surface defects that can be measured are 0.3 mm, 0.9 mm, and 1.2 mm in each case of the tube, membrane, and weld. The depth of defects located in the tube, membrane, and weld can be quantitatively evaluated with a standard deviation of 0.329, 0.269, and 0.523 mm. A method of scanning with a flexible ultrasonic probe (FUP) after applying an acoustic medium to the defect surface of the water-cooled wall, covering a thin film of PET (polyethylene terephthalate), and applying a separate acoustic medium was proposed. According to the FUP arranged in a direction perpendicular to each cross-section of the tube, membrane, and weld, the location and shape of the surface defect and the back defect can be distinguished. Furthermore, the depth of the defect can be quantitatively evaluated with a standard deviation of 0.089 mm.

By combination of the magnetic flux leakage testing and ultrasonic testing, both the near-side and far-side defects could be detected and a quantitative evaluation of the depth could be made. Furthermore, the system is also expected to detect and evaluate the internal surface defects. For instance, if the defect is shallow in the near-surface, then the magnetic flux leakage testing is efficient for detection; otherwise, if the defect is deep to near the far-side, then the ultrasonic is more efficient. The further development of the proposed system should quantitatively evaluate different sizes of the defect such as length and width, or recognize the shape of the defects.

**Author Contributions:** Conceptualization, M.L. and J.L.; methodology, M.L.; software, M.L. and C.-T.P.; validation, J.L., M.L. and E.C.; formal analysis, E.C.; investigation, J.L. and M.L.; resources, J.L. and M.L.; data curation, E.C. and M.L.; writing—original draft preparation, J.L.; writing—review and editing, M.L.; visualization, C.-T.P. and E.C.; supervision, J.L. and M.L.; project administration, M.L. and J.L.; funding acquisition, M.L. and J.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2019.342, and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.NRF-2019R1A2C2006064). **Conflicts of Interest:** The authors declare no conflict of interest.
