Due to the high magnetic flux density at the edge of the magnet, the narrow-magnet Electromagnetic Acoustic Transducer can excite the ultrasonic wave with a larger amplitude than the conventional EMAT, but the waveform of the former is distorted. This paper presents a new focused EMAT design with a narrow magnet, which can both enhance the ultrasonic A0-Lamb signal and correct the distorted waveform. A synthesis equation of the ultrasonic signals based on the contribution of the in-plane (IP) and out-of-plane (OP) components of the magnet field is proposed to analyze the mechanism of the waveform distortion. The distorted wave is corrected by an uneven split-wires arrangement strategy. The EMAT employs a sector-shaped narrow magnet and a set of focused meander-line coils containing uneven split wires, for generating a 1MHz focused A0-Lamb wave in an aluminum plate. A three-dimensional finite element model of the EMAT is established using COMSOL Multiphysics to perform numerical simulations, and the effectiveness of the EMAT design is verified. The experimental results show that the EMAT not only corrects the signal distortion due to the narrow magnet and suppresses its side lobes, but also enlarges the amplitude of the A0-Lamb wave by 55.6% under the same magnetic flux density when compared with conventional EMAT.

The conventional electromagnetic ultrasonic transducers (EMATs) rely on the static magnetic field created by magnets. The magnet increases the size of the EMATs, and the strong magnetic force of the magnet attracts the detected steel and even ferromagnetic particles. It can cause mechanical damage to the transducer and the detected objects. A new high-energy acoustic excitation system, without a static bias magnetic is designed, which does not include any magnets. As the core of the system, the high-energy pulse excitation power supplies a transient high voltage to the excitation coil by the LC oscillation circuit. The maximum amplitude of current can reach 1700 A, which is much larger than the current in the conventional EMATs. Compared with the conventional EMATs, the intensity of the ultrasonic signal is greatly strengthened and the size of the EMAT is effectively reduced. Therefore, it can detect high-temperature steel plates at a higher lift-off distance. In this paper, the transduction mechanism of high-energy pulse electromagnetic acoustic on ferromagnetic materials was studied, and the high-energy pulse excitation coil used for the A0 mode Lamb wave was designed. The interaction rule of the magnetic field, the force field, and the acoustic field, was obtained. Then, the EMAT lift-off characteristic experiment of the high-energy pulse excitation was carried out, and the defect detection experiment was conducted on a cracked steel plate. The results show that the A0 mode Lamb waves have caused a high signal-to-noise ratio and can accurately locate the crack, which has a great advantage in detecting the microcrack defects of ferromagnetic materials.

Internal defect, especially the parts of the cavity structure defect, is a challenging task in the field of non-destructive testing and often is falsely detected or undetected in the testing process. Creeping wave is a type of compressional wave, which propagates below the curved surface and is sensitive to surface and near-surface defects. Creeping wave detection provides a possibility to recognize internal defect. In this paper, the detection of cylindrical cavity defect by using the electromagnetic ultrasonic creeping wave was studied through the finite-element method. The sensitivity of creeping wave detection was improved through the design of coil parameters, which concentrate the energy and reduce the oblique incidence beamwidth of shear wave. Three common crack defects were analyzed in the structure of cavity. The results show that creeping wave detection can be used to recognize the defects which are more difficult to be detected in the cavity and the features of the defect are obvious, which provides a novel method for the internal cavity defect detection.

A plate with varying thickness (PWVT) is a common structure in engineering and the demand for its early damage detection is strong. In this paper, the distribution of plastic strain in PWVT subjected to uniaxial tension was studied and it proposed that the nonlinear parameter distributed linearly. Based on this assumption the wave equation in damaged PWVT was solved and the model of ultrasonic nonlinear response was established. The model showed that when the plastic deformation kept constant, A ₂ / A ₁ ² increased quadratically with propagation and when the propagation distance kept constant, A ₂ / A ₁ ² was proportional to the plastic deformation level. A ₂ / A ₁ ² carried the damage information of PWVT. A detection platform was built in simulation and laboratory respectively to investigate the ultrasonic nonlinearity in damaged PWVT. The simulation and experimental results were in good agreement with the model, which indicated that the model could accurately describe the behavior of ultrasound propagating in PWVT.