Research On The Influence Factors Of The Normal Beam EMAT Nondestructive Testing

Electromagnetic ultrasonic detection technology as an emerging technology, withthe advantage of non-contacting、no coupling and don’t require good surface condition，can be applied to high temperature, insulation layer and other special occasions, so itattracted much attention. Face the normal beam EMAT nondestructive testing that iswidely used for industrial applications, considering the principle of EMAT, anequivalent circuit model can be build, which including the coupling of the bias magneticfield, the excitation coil and metal specimen. This paper uses a spiral EMAT coil andpermanent magnet consisting a electromagnetic acoustic transducer, buildelectromagnetic ultrasound test platform, test and determines the optimal frequency andthe excitation signal cycle of detection coil.This paper studies the paramagnetic materials、anti-magnetic and ferromagneticmaterial, the surface state is detected,the lift-off effect, the changes of magnitude in thecharacteristics of ultrasonic EMAT generated at different temperatures. The resultsshowed that the non-magnetic material in the anti-ferromagnetic bias magnetic field abody will stimulate the opposite direction of the external magnetic field and additionalmagnetic field, reducing the static magnetic field, and the paramagnetic material willproduce an additional magnetic field in the static magnetic field, and with the staticmagnetic field in the same direction, increasing the static magnetic field, the magneticfield would add Lorentz force impact, leading to different amplitudes of the echosignal.With the lift-off distance increasing, the equivalent inductance decreases beforeslowly rising, the equivalent inductance increasees sharply after slowly increasing, theresonance frequency of the coil increases sharply then decreased slowly. The skineffect and proximity effect between the coil and tested sample become weak, and theecho signal amplitude is reduced. Surface conditions of sample effect the radiationdistribution of the spiral type electromagnetic ultrasonic transducer,and ultrasonicscattering characteristics lead to variation of the amplitude of a signal. Temperature inferromagnetic materials increases conductivity decreases and the permeability, Lorenzforce is reduced, the magnetostrictive force increases, between100and300℃, signalis in state of decay, the increment decrement Lorenz force greater than themagnetostrictive force increment, decrement of temperature higher than300℃magnetostrictive force greater than Lorenz the ultrasonic signal amplitude and scattering,resulting in changes.Results of this research can be used to guide electromagnetic acoustic (EMAT) detection technology developed, non-contact, with no coupling and the test specimensurface required on-line rapid detection of low and high temperature detection, has abroad prospect in the field of non-destructive testing of metal.