Design Of Bulk Wave Pulsed Electromagnet EMAT Using Forrail Detection

The safety of rail is very significant. The piezoelectric transducers are widely applied to the rail inspection in the world. However, the detecting speed and accuracy are limited by the dependence on acoustic liquid, which can’t guarantee the safe and fast operation requirement of the rail inspection. Electromagnetic acoustic transducer(EMAT) needs no couplant and are able to remedy those weaknesses of the piezoelectric transducers. However, the EMAT with permanent magnets would attract iron particles, move difficultly and wear detectors on ferromagnetic material(such as the steel rail). These disadvantages limit the application of EMAT on rail inspection. Therefore, in order to avoid these disadvantages, this article designs a bulk wave pulsed electromagnet EMAT(PE-EMAT).The mechanism of electromagnetic ultrasonic bulk wave and the testing methods of PE-EMAT have been studied. Based on the actual detection requirements, the working process, structure, type and materials of the bulk wave PE-EMAT are designed. Then the overall scheme of the whole paper is given.By analyzing the physical process of the bulk wave PE-EMAT, the mathematical model is proposed. A 2-D finite element model for bulk wave transmitting, propagating, detecting and a 3-D finite element model for pulsed magnetic field generating have been presented. The models are optimized by analyzing the ultrasonic wave propagation and the characteristics of the defect and the spatial distribution of the pulsed magnetic field. The simulation time of models is reduced by 42%. Both an inspection system and a Hall-effect sensor are established to verify the correctness of models. The establishment of the models is the foundation of the PE-EMAT design.Aiming at decreasing the parameters and simplifying the optimization method of bulk wave PE-EMAT, the parameters effects upon the performances of bulk wave PE-EMAT are analyzed and compared based on the finite element models. Aiming to optimize the amplitude of echoes of the ultrasonic signal, orthogonal tests are conducted to obtain the optimal coil and pulsed electromagnet. Then the optimal detection performance of the bulk wave PE-EMAT on rail inspection has been verified by a contrast experiment. The experiment shows that the ultrasonic signal amplitude of the bulk wave PE-EMAT designed increases by 95%, and the signal to noise ratio increases by 6 d B.A rail inspection system is established in order to test the inspection ability of the bulk wave PE-EMAT working on the GTS-60 standard rail block. The experiment result indicates that the bulk wave PE-EMAT designed is effective in detecting common defects of the rail. The bulk wave PE-EMAT can be moved easily on rail and almost does not wear detectors.