Study On The Magneto-optic/Eddy Current Real-time Imaging Detecting Techniques For Sub-surface Defects

Nondestructive testing (NDT) of metal deep flows has been an important research field for a long term. From 20 century 90s, some Aeronautic and Astronautic countries such as America have searched for a fast, accurate, visual nondestructive method to detect aircrafts. The Magneto-optic Imaging (MOI) is such method they have found. It has made notable progress after efforts of years. This dissertation is based on two national funded projects, one is Sichuan key scientific project named "Research on Pulsed eddy current flaw detector ", another is Sichuan fundamental research project named "Research on visual NDT based on magneto-optic/ eddy current effect ". The research deals with Pulsed eddy current (PEC) inspect and magneto-optic/ eddy current imaging (MOI) detect. Base on the PEC technique, we focus on the design of optical, mechanical and electronic parts of MOI technique. In this dissertation, several features of MOI have been in-depth researched, which are included operation principle, experimental set-up, confirming parameters , experiment of the defected specimen, and image manipulation etc. The main work we finished is described as follow.1. Study on the light rotation phenomena and Faraday's magneto-optic effect The light rotation mainly relates to crystal microcosmic helix structure, but Faraday's magneto-optic effect not only relates to crystal structure, but also relates to parameters of magnetism, wavelength, intensity and frequency of the magnetic field applied, magnetization intensity etc. otherwise, the medium of the light rotation has dispersive character, which declines with the increasing of the light wavelength. In the static magnetic field, Faraday's effect of diamagnetism light rotation medium presents linear relation with the applied magnetic field. Faraday's effect of sub-ferreous optical rotation medium have a direct proportion to applied magnetic field magnetization intensity . In the condition of alternating magnetic field, the rotary angle changes with excitation frequency . In order to obtain the best inspect result using Faraday's magneto-optic/eddy current effect, we should choose proper magneto-optic transducer, light wavelength, applied magnetic field intensity and frequency etc according to the specimen attribute.2. Pulsed eddy current (PEC) inspection technique. PEC is a potential method for quantitatively determination of deep surface cracks, for there is a strong relationship between the peak signal amplitudes and crack depths. This method can exactly locate the position of the crack,although the resolution is poor; when pulsed eddy current is excited, the eddy current induced in the metal is a oscillatory extinct signal, it is created and extincted in the pulse rising (dropping) edge, In conclusion , choosing proper pulse frequency will be capable of increasing test resolution.3. Design the MOI inspection system. We propose three kinds of MOI inspection system and divide the MOI system into two kinds according the excitation, one is relation eddy current ,another is sheet eddy current. The principle of MOI is: alternating current excited in the excitation coil induces transient eddy current of metal surface or sub-surface can vary the distribution of eddy current field, thus influence the induced magnetic field , when the laser penetrates an optical rotation crystal which integrates into the coil, the light polarize direction will deflexion a rotary angle, and then analyzer can detect the deflexion angle to realize flaws visual nondestructive test. If the thickness of optical rotation crystal , material, incident light strength and direction are constant, the value of rotary angle of light will only be relevant to magnetization intensity vector M. So by the PEC principle, the evenly distributed, layered eddy current in the inspect area will induce magnetic field which is perpendicular the specimen surface. If there are some defects in this area, and then the defects can vary the distribution of eddy current, at last can change the magnetic field, meantime, a magneto-opt...