Development of a Helical Lamb Wave Electromagnetic Acoustic Transducer for Pipeline Inspection

The electromagnetic ultrasonic testing presents great potential in structural health monitoring due to its non-contact advantage. The electromagnetic acoustic transducers generating torsional and longitudinal guided wave have been extensively studied for pipeline structure. However, the transducer for helical wave and corresponding analysis still have not been adequately investigated. This paper proposes the novel transducer to generate helical Lamb wave in pipeline. Based on the Lorentz mechanism, the developed transducer contains the fan-shaped magnetic poles and a two-layer coil. The finite element model is built to simulate the wave generation and propagation from the transducer. The displacement distribution verifies the capacity of generating helical wave. The excitation and reception of helical wave are also carried out in the experiment. The actual frequency of the transducer measured from the frequency-sweeping coincides with the designed value. In addition, the defect detection experiment is conducted. The axial and circumferential location can be obtained according to the amplitude and time information. The further comparison and discussion indicate that the prior transducer with meander coil is not appropriate to generate helical wave. The developed transducer can generate wave with diverse incident angle which is superior than the transducer for torsional and longitudinal modes.