The magnetic leakage field of the crack gets weaker rapidly as the lift-off value increases, which makes the detection more difficult, especially for many insulated pipes. On the other hand, for eddy current testing, the AC magnetic flux of the excitation coil decreases exponentially with respect to the probe lift-off value, meanwhile, the surface eddy current density decreases rapidly according to the law of electromagnetic induction. To achieve the inspection in a large lift-off, a large life-off NDT method based on the interaction between AC Magnetic field and MFL field is proposed. A series of simulations and experiments are conducted to verify its feasibility.
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