EMAT design for minimum remnant thickness gauging using high order shear horizontal modes

Detection and sizing of corrosion are critical issues across many industries such as for the oil and gas industry or the petrochemical industry. Inspections may become difficult and time-consuming when the structures under inspection are only partially accessible such as for pipes under insulation or at pipe supports. It has been demonstrated in the literature that the cutoff frequency-thickness product of high order ultrasonic guided wave modes can be used in medium to long-range thickness gauging. As the thickness varies along an inspection line, the thickness variation acts as a low-pass filter for the high order ultrasonic guided wave modes. As the thickness drops below the cutoff frequency-thickness product of a given mode, this mode is filtered out of the propagating wave packet. The effectiveness of this technique depends on the number of excited modes and the width of the ultrasonic beam along the inspection line. Both of these parameters can easily be controlled using electromagnetic acoustic transducers (EMAT) for the excitation. Analytical and multiphysics finite element simulations were performed to optimize an EMAT that can excite enough modes to allow the measurement of the remnant thickness based on the number of modes propagating through a corroded area. The results were validated experimentally, and a thickness resolution of 2 mm was achieved in a 10 mm aluminum plate.