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STATE OF DAMAGE IN VASA OAK – MECHANICAL PROPERTIES AND MOLAR MASS OF CELLULOSE I Bjurhager 1, T Iversen 2, LA Berglund 1,3 1 - KTH, Royal Inst of Technology, Sweden, Dept of Fiber and Polymer Technology 2 - Innventia AB, Box 5604, SE-114 86 Stockholm 3 - Wallenberg Wood Science Center, Stockholm In all considerations of degradation of Vasa oak, the practical consequences of degradation mechanisms have to be assessed. The mechanical integrity of the ship is perhaps the most critical question. This integrity depends strongly on the mechanical properties of the oak material itself. What is the degradation state of the mechanical properties of Vasa oak? If the properties are degraded; what type of structural degradation is causing this property degradation? Then follows; what is the mechanism for reduced mechanical properties? The early work was focused on Vasa oak damage not from any environmental effects but from the support structure itself. Analysis of the ship demonstrated the insufficiency of the current structure and the consequences for Vasa oak damage by transverse compressive loading in primarily the radial direction of the oak (Ljungdahl and Berglund, 2007). Effects from moisture and polyethyleneglycol (PEG) impregnation were also discussed. The environmental degradation effect on mechanical properties is a more difficult problem. The anisotropic structure of wood means that one can easily define more than 50 different mechanical properties, depending on type of loading, direction of loading and property of interest. The goal was from the very early stages to connect any biological or chemical degradation of the wood cell wall to a measurable mechanical property. It means strength properties are more meaningful than stiffness characteristics. Furthermore, the crucial load-bearing component of the wood cell wall is the cellulose microfibril. Although this was questioned at some of the early meetings in the research program, the literature is clear on this point. The cellulose microfibril component is essentially a tensile material in plant structures. The cell wall tensile strength is therefore a good candidate as the mechanical property to focus on. It is furthermore likely to be the most critical Vasa oak property with respect to the safety and mechanical integrity of the Vasa ship. The large beams and other structures in Vasa are subjected to bending, and again, tensile strength is likely to be the critical event in wood beam failure due to bending moments. 104