Influence of structural and materials properties on the flexural strength of laminated high-density cellulose fiberboards

Abstract: Laminated pressboard is a structural electrical insulation material used in power transformers. Typical applications are areas in the power transformers where high mechanical forces are present such as pressrings or winding tables and cleats which support the electrical cables connecting the transformer windings to tap-changes and bushings. The main constituent of the laminated pressboard is pressboard, which is a cellulosic material with anisotropic material properties as a result of the manufacturing process. Three-point bending tests are performed to measure the bending stiffness and flexural strength of the laminated pressboard. The objective of this study was to investigate several underlying factors that affect the result in a three-point bending test of laminated pressboard and to better understand the failure mechanism. The main factor investigated was the compressive strength of the pressboard. At the same time, the density variations in the pressboard and thickness variations of the laminae in the laminated pressboard were also considered. Furthermore, the methods used were a compression test of the pressboard, finite element simulations of the compression test and three-point bending test, microscope measurements, and laminate theory calculations. The compression test provided information about the compressive response. The failure mechanism in the form of a band of localized shear deformation through the thickness indicated that a kink band failure occurred. A kink band simulation of the compression test was successfully calibrated. A parametric study showed that the pressboard´s yield strength and fiber misalignment angle significantly influenced the compressive strength. The main finding of the study was that the compressive strength of the pressboard in the three-point bending analysis had a significant effect on the flexural strength of the laminated pressboard.