Experimental and simulation methods to study the Magnetic Tomography Method (MTM) for pipe defect detection

The Magnetic Tomography Method (MTM), a passive Non-Destructive Testing (NDT) technique based on the magneto-mechanical effect, has been claimed to be able to detect defects at large (>1 m) stand-off distances. In this study, the MTM signal was studied experimentally on a 4140-L80 pipe sample, in which a flat bottom defect was electrochemically generated. The Residual Magnetic Leakage Field (RMLF) signal was recorded using an annular array of AMR sensors. The experimental results show perturbations due to the defect (about 12 dB above noise level) only when the AMR sensors were positioned at a very low stand-off. The presence of ferromagnetic objects near the sensors could cause perturbations many times larger than that from a defect (about 18 dB above the noise level). A Finite Element (FE) model validated the experimental results. The model showed that there is a significant risk of false indications due to foreign ferromagnetic objects when measuring at a large distance from the pipe.