An EMAT for inspecting heat exchanger tubes of stainless steel using longitudinal guided waves based on non-uniform static magnetic fields

For guided wave inspection of pipes, EMAT systems have been built and used for generating guided waves of many different modes. The EMATs are usually located on the outer surfaces of pipes during the inspection process of many kinds of pipes. The inspection of heat exchanger tubes is an exception in that the EMATs can only be placed inside the tubes. Moreover, the inner diameters of heat exchanger tubes are small, ranging from several millimetres to dozens of millimetres, which limits the maximum size of the transducers. Existing papers describe specially-developed EMATs for generating and receiving torsional waves when inspecting stainless steel heat exchanger tubes. However, torsional waves are easily affected by the baffle plates that support the heat exchanger tubes. In this paper, in order to avoid the influence of baffle plates, a new EMAT is proposed to inspect stainless steel heat exchanger tubes using longitudinal guided waves. Both simulation and experimental results show that an unwanted mode exists in the received signal of the L(0,2) mode due to the existence of non-uniform static magnetic fields. Methods of suppressing the unwanted mode are presented by improving the structure of the EMAT based on the spatial distribution of the static magnetic field. Further defect detection experiments identify the detection performance of the proposed EMAT.