# Application of Metal Magnetic Memory Testing Technology to the Detection of Stress Corrosion Defect
## 3 Results and Discussion
### Experimental Results  
The distributions of MFL signals (\(H_{\text{n}}\) and \(H_{\text{t}}\)) during experiment 1 (static tension) under different loads were plotted in Figure 5. It is found that the normal component \(H_{\text{n}}\) and the tangential component \(H_{\text{t}}\) presented different signal characteristics at the defect during the load process from Figure 5. As can be seen in Figure 5, the demagnetization signals at the defect presented prominent distortion characteristics. \(H_{\text{n}}\) presented a \"trough-peak\" shape, while \(H_{\text{t}}\) presented a \"peak\" shape. Compared with the demagnetization signals, the MFL signals at the defect occurred significant changes after loading. The normal component \(H_{\text{n}}\) changed from \"trough-peak\" to \"peak-trough,\" while the tangential component \(H_{\text{t}}\) changed from \"peak\" to \"trough\"; and the baseline amplitude value of \(H_{\text{n}}\) and \(H_{\text{t}}\) besides the defect increased dramatically. In addition, with the increase of the applied load, the abnormal peak amplitude of \(H_{\text{n}}\) and \(H_{\text{t}}\) increased. The theory of the magnetic charge can explain the change of MFL signals during the load process [39; 41]. The distributions of MFL signals (\(H_{\text{n}}\) and \(H_{\text{t}}\)) during experiment 2 (corrosion without loading) at different corrosion degrees were plotted in Figure 6. It is found that \(H_{\text{n}}\) and \(H_{\text{t}}\) of the demagnetization  
Figure 5: MFL signals distributions of normal and tangential components during experiment 1 (static tension) at different loads measured on Line 3 (**a**) and (**b**), respectively. signal presented other characteristics. \(H_{\text{n}}\) showed an approximately linear change along Line 3 with a shallow gradient, while \(H_{\text{t}}\) kept a nearly constant value. As can be seen in Figure 6, \(H_{\text{n}}\) and \(H_{\text{t}}\) presented different distortion characteristics at the defect during the corrosion process. \(H_{\text{n}}\) presented a \"trough-peak\" shape, while \(H_{\text{t}}\) presented an apparent \"peak\" shape. With the increase of corrosion degree, the abnormal peak amplitude of \(H_{\text{n}}\) and \(H_{\text{t}}\) increased. The results indicated that both \(H_{\text{n}}\) and \(H_{\text{t}}\) presented abnormal peak features during the corrosion process, showing the location of corrosion defect accurately. The distributions of MFL signals (\(H_{\text{n}}\) and \(H_{\text{t}}\)) during experiment 3 (corrosion after loading) at different corrosion degrees were plotted in Figure 7. It is found that \(H_{\text{n}}\) and \(H_{\text{t}}\) of the demagnetization signal also presented different characteristics. \(H_{\text{n}}\) showed an approximately linear change along Line 3 with a shallow gradient, while \(H_{\text{t}}\) kept an almost constant value. After the specimen was loaded, the gradient and amplitude of \(H_{\text{n}}\) increased significantly, while changes only in amplitude could be found for \(H_{\text{t}}\) still almost horizontal linear distribution. Such a phenomenon is due to the piezomagnetic effect [43]. It is known that applied stress can lead to the reorientation of magnetic domains along the tensile direction [44]. As can be seen in Figure 7, \(H_{\text{n}}\) and \(H_{\text{t}}\) presented different distortion characteristics at the defect during the corrosion process. \(H_{\text{n}}\) presented a visible \"trough-peak\" shape, while \(H_{\text{t}}\) presented an apparent \"peak\" shape. With the increase of corrosion degree, the abnormal peak amplitude of \(H_{\text{n}}\) and \(H_{\text{t}}\) increased significantly. At the same time, the gradient of \(H_{\text{n}}\) and the baseline amplitude value of \(H_{\text{n}}\) and \(H_{\text{t}}\) decreased. The results of experiment 3 (corrosion after loading) indicated that both \(H_{\text{n}}\) and \(H_{\text{t}}\) presented abnormal signal characteristics during the corrosion process, showing the location of the corrosion defect accurately.