Effects of uneven wall thickness of steel pipes on magnetic permeability perturbation testing for internal defects

Magnetic Permeability Perturbation testing (MPPT) has the advantage of high sensitivity for deeply buried defects in steel pipes. However, in complex working conditions, variations in the partial wall thickness of the pipe can affect the defect detection results. The paper discusses the influence mechanism of uneven wall thickness on the MPPT of internal defects on steel pipes and analyzes the magnetic field distribution caused by uneven wall thickness which influences on the magnetic permeability perturbation (MPP) of internal defects. The MPP is enhanced at the thinning wall and weakened at the thickening wall. The effect of uneven wall thickness on MPPT signals is confirmed through simulation and experiment. The signal of defects in thinning wall is enhanced, while the signal of defects in thickening wall is weakened. The greater the wall thickness variation, the degree of signal distortion is more significant. Experimental results show that uneven wall thickness has an effect on the detection of the defects with different depths. Finally, a method using deep saturation magnetisation is proposed to mitigate the inconsistent signals caused by uneven wall thickness. The findings will contribute to quantitative evaluation of defects in MPPT.