# Inspection of Semi-Elliptical Defects in a Steel Pipe Using the Metal Magnetic Memory Method
## 3 Results and Discussion  
Five semi-elliptical shape defects were made on the surface of an ASTM A36 ferromagnetic pipe [50] with a length of 113.55 mm, an outer diameter of 89 mm, and a thickness of 5.45 mm. The defects were generated with the vertical cutter tool Ball Noise End Mills series 1725 through a computer numerical control (CNC) machine (see Figure 5). The resulting shape dimensions of the five defects are depicted in Figure 6.  
Figure 4: Flowchart of the measurement system of semi-elliptical defects in ferromagnetic pipes. Figure 5: Generation of semi-elliptical shape defects on the pipe surface using the CNC machine. The tangential (\(H_{t}(x)\)) and normal (\(H_{n}(x)\)) MMM signals around semi-elliptical defects on the pipe surface were obtained using the measurement system. Six continuous tests were reported for each of the five semielliptical defects S1-S5. The scan length was 12 mm with the defect located in the center. The tangential and normal MMM signals for the five defects are shown in Figures 7 and 8, respectively. As it is observed, the resulting MMM signal shapes for all the samples agree with the theoretically expected values; see Figure 1. The tangential component maximum \(H_{t}\_max(x)\) and the tangential component mean value along the scanning length \(H_{t}\_mean(x)\) are shown in Table 1. In addition, Table 2 shows the maximum and minimum values of the MMM signal component \(H_{n}(x)\_max\) and \(H_{n}(x)\_min\), the distance between the normal component maximum and minimum values \(\Delta_{n}(x)\), and the normal component mean value along the scanning length \(H_{n}(x)\_mean\). \begin{table}
\begin{tabular}{c c c} \hline
**Defect Sample** & \
\begin{tabular}{c} \(H_{t}(x)\_max\) \\ **(mT)** \\ \end{tabular} &
\begin{tabular}{c} \(H_{t}(x)\_mean\)** \\ **(mT)** \\ \end{tabular} \\ \hline S1 & 83.28 \(\pm\) 0.354 & 43.49 \(\pm\) 0.335 \\ S2 & 91.3 \(\pm\) 0.328 & 46.77 \(\pm\) 0.265 \\ S3 & 92.55 \(\pm\) 0.243 & 56.66 \(\pm\) 0.288 \\ S4 & 86.12 \(\pm\) 0.160 & 55.77 \(\pm\) 0.237 \\ S5 & 80.27 \(\pm\) 0.207 & 57.42 \(\pm\) 0.380 \\ \hline \end{tabular}
\end{table} 
Table 1: Tangential MMM signals of the five semi-elliptical defects. \begin{table}
\begin{tabular}{c c c c} \hline
**Defect Sample** & \begin{tabular}{c} \(H_{n}(x)\_min\) \\ **(mT)** \\ \end{tabular} & \begin{tabular}{c} \(H_{n}(x)\_max\)** \\ **(mT)** \\ \end{tabular} & \
\begin{tabular}{c} \(\Delta_{n}(x)\) \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} \(H_{n}(x)\_mean\)** \\ **(mT)** \\ \end{tabular} \\ \hline S1 & 160.75 \(\pm\) 0.596 & 217.15 \(\pm\) 0.493 & 3.806 \(\pm\) 0.242 & 183.72 \(\pm\) 0.304 \\ S2 & 194.2 \(\pm\) 1.018 & 263.18 \(\pm\) 0.564 & 3.526 \(\pm\) 0.411 & 225.16 \(\pm\) 0.448 \\ S3 & 193.92 \(\pm\) 0.728 & 260.67 \(\pm\) 0.589 & 3.814 \(\pm\) 0.885 & 223.16 \(\pm\) 0.545 \\ S4 & 188.55 \(\pm\) 0.468 & 244.17 \(\pm\) 0.516 & 2.115 \(\pm\) 0.152 & 216.42 \(\pm\) 0.127 \\ S5 & 196.57 \(\pm\) 0.398 & 237.15 \(\pm\) 0.423 & 2.423 \(\pm\) 0.128 & 220.15 \(\pm\) 0.266 \\ \hline \end{tabular}
\end{table} 
Table 2: Normal MMM signals of the five semi-elliptical defects. Figure 6: Dimensions of the five semi-elliptical defects machined on the surface of the ASTM A36 ferromagnetic pipe. The maximum tangential component \(H_{t}(x)\_max\) has an increasing behavior for the defect samples S1, S2, and S3 and a decreasing behavior for S4 and S5. The tangential component mean \(H_{t}(x)\_mean\) has an increasing value tendency from S1 to S5. In this sense, the measuring of \(H_{t}(x)\_max\) and \(H_{t}(x)\_mean\) might be further researched to be used to quantitatively determine the defect severity level. As it is observed in Figure 7, the location of the maximum tangential values are nearby the defect centers. Hence, such values might be used to locate a defect. As can be seen from Table 2, no significant difference in the normal component mean value \(H_{t}(x)\_mean\) was obtained for the considered defect samples.