Study On The Magneto-Optic/Eddy Current Imaging Technology For Subsurface Micro-defects

In recent years, we have made great progress in the field of metallurgy, material, manufacture, assembly, etc. How to detect the fine surface and subsurface is paid more and more attention. A real-time, reliable, accurate and visual testing method is expected. That is a challenge to the conventional Nondestructive Testing (NDT).The application of photoelectric technology provides an effective path to test the subsurface micro-defects. In this paper, present a new Magneto-optic / Eddy current imaging (MOI), which not only preserves the merits of conventional eddy current testing, but also realizes the visual NDT for subsurface micro-defects. The result is real-time, and highly sensitive.Magneto-optic / Eddy current imaging is based on Faraday's electromagnetic induction and the law of magnetic optic effect. The principle is: alternating current excited from the excitation coil induces transient eddy current at the surface or subsurface of the mental sample. It can influence the induced magnetic field. When the laser penetrates the optical rotation crystal film, the polarization of the light will have a deflexion angle. If there are some defects in the sample, they will vary the distribution of eddy current field, and then change the whole magnetic field. Meantime, the magneto-optic sensor which is parallel to the surface of the sample converts this change to corresponding optical intensity change. Finally it is received by CCD to realize visual nondestructive test.Based on the basic theory, we focus on factors which influence magneto-opticeddy current imaging of the system, including the design of optical system, magneto-optic sensor, excitation coil, pulse exciting, image processing and so on.A semiconductor laser is used as the light source. Replaying measuring coil by laser, the testing efficiency and precision are increased, as well as the results are visual.Compared with conventional sinusoidal exciting current, MO1 uses the batch pluse excitation, which can dramatically increase the testing depth, improve the resolution of the system, and also avoid overheat of the excitation coil effectively.Faraday magneto-optic crystal plays an important role in the testing system. In this paper, we compare the performance and feature of two common magneto-optic sensors—magneto-optic glass and magneto-optic crystal film. The Bi: YIG magneto-optic film is preferred to meet the sensitivity need of the system.The optical intensity signal with defects information included is received by CCD. We compare the performance and feature of two image sensors—CCD and CMOS in the paper. Because of the predominance in sensitivity, resolution and low noise, CCD is more suitable for the system to increase the image quality.In order to sufficiently exploit the eddy effect, some related parameters including excitation frequency, the coil diameter, distance detected (lift-off), number of turns of coil, and so on, must be set reasonably. Whole of these are important factors to influence the eddy current density distribute and both influence the sensitive of the system.In experiments, we test samples of different material including copper, aluminum and iron, with different defects, such as aperture and fillister. Through multiple experiments, we have proofed the feasibility of the MOI system. Because there are many disturbing elements in the magneto-optic image, some image processing is used to remove noise and increase the reliability.In general, Magneto-optic / Eddy current imaging testing system is a mixture of photoelectric technology, circuit, data acquisition and processing and image display etc, which has realized the real-time and visual test of subsurface. It has the characteristics as follows:(1) The detected area can be covered rapidly and the testing speed is about 5-10 times of conventional eddy current testing.(2) The clearance of surface cladding is not needed before testing, because the image quality is not influenced on the whole, and the only thing is the better reflecting property of the measured surface.(3) The results are visible and pellucid, and can be easily preserved.