A micro-CT investigation of density changes in pressboard due to compression

Abstract: Pressboard, a high-density cellulose-based product, is used both as load bearing structures and dielectric insulation in oil-filled power transformers. During transformer operation, mechanical forces and vibrations are applied on the material. In particular, this investigation focuses on pressboard sheets placed between the turns of transformer windings, called the spacers, which during short circuit are subjected to high compressive forces of impulse nature. As a result of these forces, remaining deformations are created in the components. One step to reduce the negative consequences that come from the deformations is to understand how the fiber structure of the material changes by these forces, thus finding the week link. Understanding these changes could lead to future modification of the material to better withstand short circuits. To see the fiber structure and its changes in the material, pressboard has been investigated with a micro-CT at the Division of Applied Mechanics at Uppsala University. The scanned images have been reconstructed and analyzed in NRecon, CTAnalyser, and Matlab to investigate the density distribution changes and to identify the densification patterns. The study shows that pressboard initially has an inhomogeneous density distribution through the thickness and after mechanical indentations, the densification tends to begin in the more porous parts of the material. The project also included a prediction of the densification pattern, performed by finite element analysis (FEA) using a simplified material model. The results show that a material model with varying Young’s modulus through the thickness, based on a stiffness and porosity relationship, can produce similar densification patterns as in the experiments.