Nondestructive Visualization and Quantitative Characterization of Defects in Silicone Polymer Insulators Based on Laser Shearography

Effective defects detection method for silicone polymer insulators is desperately needed to eliminate defective insulators in factory examination and make early warnings for accidents of in-service insulators. This paper investigates a laser shearography method that reveals surface deformation anomalies induced by internal defects after applying thermal excitation. Samples with artificial defects of different sizes and depths in the silicone rubber sheath were tested. Both fringe patterns and phase distribution maps were generated after the experiments for defects identification by indicating the displacement derivative with respect to the shearing direction. The relationship between the defect parameters and the relative phase difference was discussed, and a parameter named detection sensitivity was defined to evaluate the system detection ability on different defects. Finally, Canny edge detection algorithm and proposed image gradient method were employed to quantitatively estimate the defect size, and the ability of identifying defects was also improved. With its noncontact testing, full-field measurement, real-time imaging, and visual results, laser speckle imaging would have quite promising applications in nondestructive testing of silicone polymer insulators and it can be extended for other composite materials insulating equipment.