Evaluation of Geometric Parameters of Volumetric Defects in Dielectric Coatings by the Electrospark Method of Nondestructive Testing (High-Voltage Spark Testing)

Abstract

The paper examines the feasibility of detecting nonthrough defects (internal defects and interfacial defects at the metal-coating interface) in dielectric coatings using the electrospark method of nondestructive testing. It is shown that partial discharges, which induce a characteristic distortion of the test-voltage pulse, serve as an informative indicator of a coating defect. Based on an equivalent capacitive model of a coating with a defect, a relationship between the apparent charge and the geometry of the air gap is derived. The results are validated by numerical simulation of the electric-field distribution in COMSOL and by experiments on plexiglas specimens with blind holes of varying depth. An algorithm is proposed for extracting the partial-discharge signal using a weighted sum of increments in the root-mean-square deviations of the second derivative of the voltage. The obtained dependences make it possible to estimate the residual coating thickness in the defect region.