| Magnetic flux leakage (MFL) methods are used widely in the nondestructive evaluation (NDE) of natural gas transmission pipelines. In this technique, the pipe wall is magnetized in the axial direction and a circumferential array of Hall sensors is used to detect any leakage flux caused by the presence of defects in the pipe wall. In general, this tool generates an axially oriented magnetic field, which is sensitive to the presence of circumferential cracks. However, these inspection tools are insensitive to stress corrosion cracking (SCC) which are oriented largely in the axial direction.; A possible solution to this problem is to utilize the fields associated with the circumferential currents generated in the pipe-wall by the movement of the magnetizer relative to the pipe-wall. Since the motion of the tool inside the pipe is along the pipe axis, the motional emf due to the term is negligible between the poles of the magnet. However, at the poles, the radially oriented magnetic fields generate a significant amount of circumferentially directed currents. The intersection of these motion-induced currents with the axial SCC results in a perturbation of the current distribution. The fields associated with the perturbation currents carry information relating to the presence of axial SCC.; The finite element modeling of the interaction between axial stress corrosion cracks and circumferential currents is a significant challenge in terms of both the computation time and memory requirements. The challenges arise due to the nonlinearity of material properties, the size of tight cracks relative to that of the magnetizer, and also time stepping involved in modeling velocity effects. An approach is presented to avoid these problems by decomposing the overall task into four simpler subtasks that can be performed sequentially. The simulation results demonstrate the feasibility of detection of both single and symmetric SCC and SCC colonies of arbitrary orientation. |
