Selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers

In this paper, we describe a modal expansion approach for the analysis of the selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers （EMATs）. With the modal expansion approach for waveguide excitation, an analytical expression of the Lamb wave＇s mode expansion coefficient is deduced, which is related to the driving frequency and the geometrical parameters of the EMAT＇s meander coil, and lays a theoretical foundation for exactly analyzing the selective generation of Larnb waves with EMATs. The influences of the driving frequency on the mode expansion coefficient of ultrasonic Lamb waves are analyzed when the EMAT＇s geometrical parameters are given. The numerical simulations and experimental examinations show that the ultrasonic Lamb wave modes can be effectively regulated （strengthened or restrained） by choosing an appropriate driving frequency of EMAT, with the geometrical parameters given. This result provides a theoretical and experimental basis for selectively generating a single and pure Lamb wave mode with EMATs.

Mechanical stress in high-temperature superconducting ring-shaped bulk during the pulsed-field magnetization

The high-temperature superconducting(HTS)bulk has a higher critical current density and can trap the larger magnetic field.When the high-temperature superconducting bulk is magnetized by the pulsed-field,it will be subjected to electromagnetic stress and thermal stress.Furthermore,the bulk may be damaged under larger mechanical stress or strain during the pulsed-field magnetization.In this paper,the electromagnetic field,the temperature and the mechanical response of the ring-shaped bulk are simulated based on the electromagnetic and mechanical governing equations,and the simulated trapped field is consistent with the experimental results given in the reference.The stress distribution on the top surface of the ring-shaped bulk during pulsedfield magnetization is opposite to the case of field cold magnetization.Moreover,the inner edge on the middle plane of the bulk may be broken more easily than the outer edge during the pulsed-field magnetization.Afterward,the influences of the size of the inner radius,ambient temperature and rise time of the applied field are presented.Finally,the mechanical stress of the bulk with the cooling process is investigated,and the tensile stress on the surface can be enhanced by the cooling process.