# Application of Metal Magnetic Memory Testing Technology to the Detection of Stress Corrosion Defect
## 3 Results and Discussion
### Analysis and Discussion  
The magnetic charge is an equivalent model, which has been developed to interpret the magnetic field leakage of ferromagnetic materials. For a ferromagnet, its external magnetic field would be considered to originate from the magnetic charge [41]. For the convenience of discussion, it is assumed that the magnetic flux leakage was caused by the equivalent magnetic charge on the two interfaces of the defect zone. Then, the MFL \(\mathbf{H}\) at the measuring point caused by equivalent magnetic charge \(q\) is given as  
\[\mathbf{H}=\frac{q}{4\pi\mu_{0}\pi^{3}}\mathbf{r} \tag{4}\]  
where \(\mu_{0}\) is the permeability of air, \(\mathbf{r}\) is the position vector from equivalent magnetic charge to measuring point. The schematic diagram of MFL signals in experiment 1 (static tensile) is shown in Figure 12a. The initial leakage magnetic field \(H_{\rm i}\) is formed by the magnetic polarity generated due to the accumulation of a small number of magnetic charges at both sides of the defect after the specimen was demagnetized [39; 41; 46], which coincides with the initial magnetic field \(H_{\rm e}\). The demagnetization signals are the vector sum of \(H_{\rm e}\) and \(H_{\rm i}\). Thus, \(H_{\rm n}\) presents a \"trough-peak\" shape, and \(H_{\rm t}\) gives a \"peak\" shape at the defect. After being loaded, the magnetization of the specimen changes under the combined action of the geomagnetic field and stress [43; 44], results in the formation of leakage magnetic field \(H_{\rm L}\). The accumulation of more magnetic charges at both sides of the defect due to the stress concentration at the defect after the specimen was loaded, resulting in a strong leakage magnetic field \(H_{\rm m}\), which is opposite to the leakage magnetic field \(H_{\rm i}\). In addition, the leakage magnetic field \(H_{\rm m}\) is stronger than \(H_{\rm i}\), causing the reversal of MFL signals at the defect. Under such conditions, the MFL signals are the vector sum of the initial magnetic field \(H_{\rm e}\), leakage magnetic field \(H_{\rm L}\), \(H_{\rm m}\), and \(H_{\rm i}\). Thus, the normal component \(H_{\rm n}\) of MFL signals at the defect presents a \"peak-trough\" shape, and the tangential component \(H_{\rm t}\) shows a \"trough\" shape. With the increase of the load, the stress concentration on both sides of the defect increases, increasing the number of magnetic charges, which leads to the strengthening of leakage magnetic field \(H_{\rm m}\), causing the increase of the MFL signals at the defect (see Figure 5). Figure 11: SEM and EDX observed the cross-section of the sample corroded for 8 h: (**a**) Cross-section SEM image; (**b**) analytical line; (**c**) corresponding X-ray maps of Fe and O. The schematic diagram of MFL signals in experiment 2 (corrosion without loading) is shown in Figure 12b. The defect formed on the surface of the specimen due to corrosion, results in the formation of an initial leakage magnetic field \(H_{\rm i}\). In addition, magnetic charges accumulated at both sides of the defect due to the effect of corrosion, result in the formation of leakage magnetic field \(H_{\rm c}\), which is opposite to the initial magnetic field \(H_{\rm i}\). Under such conditions, the MFL signals are the vector sum of the initial magnetic field \(H_{\rm e}\), leakage magnetic field \(H_{\rm c}\) and \(H_{\rm i}\). Thus, the normal component \(H_{\rm n}\) of MFL signals presents a \"trough-peak\" shape, and the tangential component \(H_{\rm t}\) presents a \"peak\" shape. With the increase of corrosion degree, the leakage magnetic field \(H_{\rm c}\) is gradually enhanced, causing the MFL signals increasing at the defect (see Figure 6). The schematic diagram of MFL signals in experiment 3 (corrosion after loading) is shown in Figure 12c.