Application of sub-surface imaging system for detecting defects of metal material

A sub-surface imaging method for detecting invisible and buried micro-flaws under fine metallic surface is described. This system, which has a laser source, a CCD camera, and an eddy current exciting device is developed for this work. A pulse generator supplies an intermittent square pulse to induce the exciting coil, which can intensify eddy currents yet reduce the working temperature of the exciting coil and specimens. The magnetic field variation produced by the imbedded defect causes a rotation of the polarization plane of the reflected beam, therefore the reflected beam carries an image of the defect, which is received by a CCD camera. Now the imperceptible flaws in subsurface of mental product are visually tested. Image edge processing approaches are designed to enhance the imaging effect. The method presented is proved to be feasible by the experiment results.