| With the rapid development of manufacture processing, material, metallurgy and assembly technologies in recent years, there are much higher requirements for the quality of surface and subsurface of materials and components, in addition, comprehensive quality-inspection methods are required for the merchandises manufacturing with large quantity. Surface defects with potential damage are one of the dangerous elements affecting the quality and safety of materials and components, and the defects at the subsurface which are more secluded and dangerous are always the reasons for the fatal ones. One of the important functions of nondestructive testing (NDT) is to detect the defects at the surface and subsurface of materials and components.In the early development stage of NDT, obtaining the information of the defects and damnification of the materials and components without damaging there intrinsic characters is emphasized. With the development of technology, visual NDT instead of the traditional testing methods is highly required which is used to "see" the inner state intuitionisticly and visually. It is at this requirement background that the visual NDT of testing the defects at the subsurface comes into being and develops very fast.The surface nondestructive testing technology has been mature nowadays, while there is no ideal testing method for detecting the subsurface defects. There are some limitations for the conventional testing methods including ultrasonic, x-ray and eddy current methods. Ultrasonic testing, for example, requires a coupling liquid between the transducer and the specimen, which makes nondestructive test slow and tedious. Radiography testing involves the use of expensive and cumbersome equipment. In transmission of x-ray the source and the receiver have to be on opposite sides of the inspection surface, which complicates the adjustment of the test setup. In addition to being time consuming, radiography techniques require specific precautions to NDT personnel against radiation hazards. So to study on the visual inspection technology and to develop the visual instruments for the surface & subsurface defects have not only academic significance, but also practical value.The combination of photovoltaic technology and no-contact nondestructive testing technology becomes the effective solution for detecting the subsurface defects. Integrating the eddy effect fully with Faraday's magneto-optic effect, this paper raises a new NDT technology, which is called magneto-optic microscopy technology, to realize the visual NDT for the subsurface defects. The operation principle is analyzed deeply in this paper. The coil excited by the impulse signal induces eddy current in the metal specimen. If there are defects at the subsurface of the specimen, the magnetic field excited by the eddy current will be varied with the change of eddy current distribution, and then so does the vertical part of the magnetic field. By the change of this magnetic field, the magneto-optic sensor (magneto-optic garnet film) will induce the magneto-optic effect to change the polarized direction of linear polarized light. After being reflected by the polarization beamsplitter, the linear polarized light with defect information can be received by the CCD to show the image, so the real-time and visual imaging inspection for the subsurface defects of the specimen is realized.On the basis of the Principe, in this paper, various factors which influence the quality of the magneto-optic images, including the optical imaging system, magneto-optic sensor, image sensor, excited coil and excited frequency etc. are researched systematically, the optimal parameters are determined based on the experiment results, and the magneto-optic microscopy system are improved and consummated gradually to meet the anticipated requirements. The following is the main achievements.1. This research is the first one that has detected the flaws at the subsurface of the specimen directly and correctly.2. Using the semiconductor laser as the light source for its strong polarization character. The reflected light in the optical system not only enhances the ability of the magneto-optic images by making full use of the magneto-optic effect (the Faraday rotation angle is twice as original one), but also realizes the low noise optical route by keeping the reflected light from entering the laser.3. Utilizing the intermittent pulse excitation method to obtain the eddy current magnetic filed for this testing device. Compared with the conventional testing methods, this design can not only induce the excitation coil to achieve the eddy current, but also weaken the overheating of the coil effectively.4. Using annular Mn-Zn ferrite as the magnetic core of the eddy current excitation device. With low eddy current wastage, this ferrite magnetic core is featured with enhancement and focalization of the magnetic filed. It is easy to exert the magneto-optic effect and eddy current effect of the coil with high frequency to ensure the precision and sensitivity of the testing system.5. The influence to the eddy current excitation by the parameters including the detect distance (lift-off), dimension of the coil and input frequency are deduced in this paper and is proved by experiments to obtain the evidence of designing the eddy current excitation device for the magneto-optic imaging system.6. Choosing garnet magneto-optic film (Bi: YIG) as the magneto-optic sensor in this device. Based on the comparison and analyse of the magneto-optic performance and characteristics of the magneto-optic films and the actual requirements in the experiments, the garnet film (Bi:YIG) is chosen as the magneto-optic sensor. Compared with common magneto-optic glass, the garnet film with features of low optical absorption, high Faraday rotation angle and high magneto-optical figure of merit can improve the testing sensitivity effectively.7. Specimen with flaw at the subsurface are designed, and plentiful experiments analyse and research have proved the feasibility of the magneto-optic micro imaging detection. The following conclusion can be obtained by analyzing the images of subsurface defects: with the excitation at the same frequency, the closer the distance to the surface is, the clearer the image is; with the different material at the same testing condition, the different definition of the magneto-optic images are found; the approximate figure of the defects can be determined by the histogram of the images.8. Designing image processing software to capture and optimize the magneto-optic images. Many factors, such as the variety of light, magnetic domain changes of garnets and various noises disturb to the received magneto-optic image, the images are always with low resolution. With the Matlab toolbox, the magneto-optic image is optimized with high resolution in this paper, and the reliability of this testing system is improved.In conclusion, the magneto-optic imaging detection technology integrating the optics imaging, electric-circuit designing, image capturing and saving with image processing is featured with not only strong practicability, contactless coupling, portable device as conventional eddy current testing technology, but also high efficiency, high precision and easy control. It realizes the visual real-time detection of the defects at the surface and subsurface, and the testing results are visual and understandable, easy to be downloaded and saved.
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