Influence of the crack angle on the magnetic flux leakage detection signal in pipelines

In this paper, the influence of the crack angle on the detection signal is analyzed so as to detect the crack defects of the oil and gas pipelines effectively. Based on the principle of magnetic flux leakage testing and Maxwell equations, the crack detection structures of 90°, 67.5°, 45°, 22.5° and 0° outside the pipelines were simulated and analyzed using the finite element software Comsol. The dragging experiment which is special for the crack angles was performed on the steel plate in the conditions of permanent-magnet axial excitation. It is shown that the signal amplitude reaches the maximum when the magnetization direction is perpendicular to the crack. The amplitude of crack leakage magnetic signal declines with the decreasing of the angle between the crack orientation and the magnetization direction. When the angle is less than 30°, the effective flux leakage signal could not be detected.