Research On The Flaw Detection Technology Of Metal Plate Surface With Electromagnetic Ultrasonic

Many advantages of electromagnetic acoustic(EMAT), such as no couplant, high temperature resistant and easy operation, make it be widely studied. At present, electromagnetic acoustic has been used in the detection of train wheels, plate, aluminum and steel. Based on the principle of electromagnetic acoustic, electromagnetic surface wave was studied on in this paper. The electromagnetic surface wave is suitable for a wide range of health status detection on the surface of sheet metal because of its single pattern, energy concentration and propagating on the surface of the sample.This paper studied on the production principle of electromagnetic acoustic surface wave and propagation characteristics, based on which ANSYS software was used to conduct finite element simulations. The polygonal-shape coil and single magnet were combination to establish the 3D finite element model of the electromagnetic acoustic transducer. The electromagnetic acoustic surface wave produced by EMAT was divided into four parts, which were the electromagnetic field, eddy current, the Lorentz force and fluctuations, to make a analysis. And then the simulations of the aluminum plate were conducted to the production and propagation of the electromagnetic acoustic surface wave. The metal plate inspection detection device of the electromagnetic acoustic surface wave, which was consisted of the incentive, receiving and transducer, was put up based on the finite element simulation. The problem, which was the weak receiving single of the electromagnetic acoustic echo, was solved with the multi-stage amplification and the positive and negative two roads, the adjustable frequency and cycle of the pulse train generated by FPGA. The electromagnetic acoustic surface wave was used to detect the defects of the metal plate and the test of the sound field directivity was also conducted.The Lorentz force generated by 3D finite element model of the electromagnetic acoustic transducer made a displacement change of particle in the 30mm aluminum plate during the ANSYS simulation. The test was conducted on the aluminum plate and steel plate with 600mm long, 300mm width and 300mm thick. The test verified that the metal plate inspection detection device of the electromagnetic acoustic surface wave could generate a surface wave on the aluminum plate and steel plate, which could detect the defects in the plate. The test of the sound field directivity proved that the propagation of the surface wave had directivity. The echo single could reach the expected effect and the single could be displayed clearly by wavelet de-noising processing.