Measurements of the magnetic field variations related with the size of V-shaped notches in steel pipes

Abstract

Gas and oil pipeline networks require periodic inspections to detect cracks or notches that can cause industrial accidents and environmental contamination. For these inspections, the metal magnetic memory (MMM) method could be used as a non-destructive testing (NDT) technique, which does not need expensive equipment and high-skilled operators. However, more investigations are required to quantify the size and shape of defects in ferromagnetic pipes using the MMM signals. We present experimental measurements of MMM signals around five small V-shaped notches of an ASTM-A36 steel pipe using a three-axis magnetoresistive sensor. The V-shaped notches have different values of depth (500 µm, 1000 µm, 1500 µm, 2000 µm and 2500 µm) and width (1000 µm, 1500 µm, 2000 µm, 3000 µm and 3500 µm). We measured the variations of tangential and normal MMM signals around these defects and their relationships with the size of each defect. The first V-notch defect (500 µm depth and 1000 µm width) registers variations of the tangential and normal MMM signals of 14.32 µT ± 1.62 µT and 27.95 µT ± 1.14 µT, respectively. On the other hand, the fifth V-notch defect (2500 µm depth and 3500 µm width) has variations of the tangential and normal MMM signals of 68.75 µT ± 1.10 µT and 71.37 µT ± 0.72 µT, respectively. The MMM method could be used for real-time monitoring of V-shaped notches in steel pipes. This method does not require special treatment of steel pipes.