Analysis of signals for inclined crack detection through alternating current field measurement with a U-shaped probe

摘要

Alternating current field measurement (ACFM) is a technology widely applied in the petroleum industry and it is used to detect cracks that are perpendicular to the induced electric current. However, in the real world of NDT, neither the existence nor the orientation of cracks are known and it is impractical for operators to scan cracks in a strict parallel path. To improve detectability and eliminate the depth estimation error of an inclined crack with the ACFM device, the correlation between crack orientation and the disturbed electromagnetic field is investigated by simulation and experimental methods in this paper. The simulation results indicate that when a steel specimen is inspected by a U-shaped ACFM probe, the disturbed magnetic field consists of a perturbed magnetic field caused by the disturbed electric current and the magnetic flux leakage, which is verified by relevant experiments. Furthermore, in the experiment, the influence of crack orientation on the characteristic signal is analysed and it is found that with the orientation angle increasing from 0° to 90° the B_x curves of the steel specimen vary from dip to peak gradually, while the B_x curves of the aluminium specimen are all dips. Furthermore, the correlation between the crack orientation angle and the maximum variation of B_x (B_m) is established and the relative error of depth estimation caused by the crack orientation is studied. The results indicate that an inclined crack can be detected by the ACFM technology and that the crack orientation has a significant influence on the crack depth estimation. After discovering the crack orientation through use of the sensor array or grid scan, it is suggested that the B_m of 0° orientation calculated through the correlation between crack orientation and B_m established in this paper can be used to eliminate the error in crack depth estimation caused by the crack orientation through the ACFM device and promote the application field of the ACFM technigue.