In-Line Inspection for Ferromagnetic Bent Pipe Using 3-D-Printed Flexible MFL Sensor Array

Abstract

Ferromagnetic steel pipes play a critical role in industries such as energy, transportation, and manufacturing, where they are often exposed to extreme environmental conditions that can lead to corrosion and structural defects. Periodic inspection is, therefore, crucial to prevent potentially catastrophic failures. Magnetic flux leakage (MFL) is a commonly adopted nondestructive testing technique due to its cost-effectiveness and high sensitivity; however, conventional rigid MFL probes suffer from severe lift-off variations when inspecting bent pipe sections, leading to substantial degradation in detection performance. To overcome this limitation, this work proposes a novel flexible MFL sensor array probe based on a 3-D-printed flexible framework. The proposed technique uses independent miniature magnet and sensor arrays integrated with a flexible structure, allowing the probe to bend and conform to the inner surface of the bend pipeline while maintaining a small and stable lift-off. The impact of structural flexibility was systematically investigated with 3-D finite-element simulations. Results reveal that asymmetry predominantly affects the radial magnetic field, with a 5-mm asymmetry producing a radial field shift 6.84 times larger than the axial shift. Based on these insights, a prototype probe was fabricated with axial-oriented sensing and tested on a bent carbon steel pipe with a 15-mm inner radius and 100-mm bending radius. The probe successfully detected hole defects with a radius as small as 1.5 mm. Defects on the lateral side exhibited stronger signals than those on the extrados, consistent with simulation predictions. These results demonstrate that the proposed flexible MFL probe offers a promising solution for reliable in-line inspection of bent pipelines, overcoming the limitations of state-of-the-art rigid designs.