Characterizing the internal strain in composite insulation under dry and wet conditions

Abstract

This paper investigates the internal mechanical strain, developed within composite and polyester resin dielectrics, under dry and wet conditions. The paper characterizes this strain by a set of colored images, caused by relative retardation orthogonal components of the polarized white light used to illuminate the specimens in the microscope. An image-editing tool is used to analyze the structure of the strain-generated colors by assigning an intensity value to each pixel of the Red, Green and Blue components of the image. Each point in the image is identified by a set of three numbers ranging from 0 to 255. The Microsoft Excel is employed to process the results and to provide a simple comparison among various cases of the analyzed data. The change in the color map between dry and wet conditions for each examined specimen is used as an indicator of strain pattern variation, and consequently, as a tool to characterize the impact of water absorption on composites. This approach offers a quantitative data from strain images without a need for a complex mathematical procedure. On the other hand, the paper focuses on the resin as an absorber and transmitter of moisture, and tries to verify this hypothesis by subjecting the specimens of polyester resin, cast in different ways, to the same tests as that for the composite ones. Despite the dependence of the obtained results on the materials used and conditions applied, there is a clear effect of the moisture as an aging element on reducing the mechanical stress developed within the dielectric material.