Influence of Density on the Out-of-Plane Mechanical Properties of Pressboard

A large number of power transformers rely on the mechanical properties of oil-impregnated cellulose. In particular, the deformation of windings in power transformer depends on the out-of-plane mechanical properties of pressboard insulation. Compressive loads are applied via clamping rings onto insulating paper and spacers. It is therefore relevant to define the out-of-plane stress-strain curves in compression for cellulose based materials. From a mere structural point of view pressboard can be regarded as a porous material in which the load bearing matrix consists of randomly connected fibers. It is expected that, by increasing the density, larger fiber-to-fiber contact areas are created and free span between fibers decreases. The combination of these two phenomena results in a material which is both stiffer and stronger. Furthermore cellulose is a polymer and its mechanical behavior also depends on temperature and moisture content. The present work investigates the relation between density and stiffness in the thickness direction, at different temperatures and moisture contents. Laboratory experiments were performed on a custom built set-up that allowed continuous measurements of load and deformation. The temperature and moisture content of the tested material were carefully controlled during the performing of the experiments.