Finite Element Simulations of a Creeping Wooden ... - WCCM 2016

Finite Element Simulations of a Creeping Wooden Structure Aiming for the Design of an Improved Support Structure: A Case Study of the 17th Century Man-of-war Vasa E. Kristofer Gamstedt / Alexey Vorobyev / Reza Afshar / Nico van Dijk / Ivón Hassel / Dan Wu Uppsala University Department of Engineering Sciences Box 534, SE-751 21 Uppsala, Sweden Old and culturally valuable wooden structures undergo creep deformation during decades or even centuries. To preserve these structures for future generations, it is first necessary to measure the increasing deformation with the aim to predict the overall creep. It would then be possible to engineer a support structure to mitigate further creep, and minimize the risk of structural failure. Due to the general complexity and interacting length scales, it must be carefully assessed which mechanisms should be included in the model and at which length scales. An engineering top-down approach is a natural starting point. It is however rarely possible to test larger archeological wood elements mechanically, since this can induce damage and necessitates partial dismantling of the structure. Instead, the constitutive behavior can be characterized on small samples that can be spared [1]. Up-scaling from the mechanical behavior of the material specimens to the structural response requires assessment of the effects of variation in density, grain orientation and wood treatment. A micromechanical model can be helpful in the scaling up from samples to beams. The next step upwards is to include the effects of joint movement, which could in principle contribute in the same order of magnitude as the material creep. Four length scales can be identified: Microstructure – material – members and joints – overall structure. Supporting measurements can be made on each of these scales, which would improve the reliability of the predictive model. Geodetic measurements on the global scale can be used in the validation process of the numerical model [2]. Although prediction for large structure is the goal, there is a role of micromechanics is addressing the natural variability of constitutive wood material. E.g. preservation compounds such as polyethylene glycol moisture content and microstructural properties such as local density affect the stiffness and creep behavior. An example is presented for the 17th century warship Vasa. In Figure 1 below, the Vasa museum ship is found and part of the present support structure. The approach outlined in this presentation is also considered to be applicable to other aging wood structures.

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Figure 1: (a) The Vasa warship in its museum in Stockholm, Sweden. (b) Part of the present support structure: steel cradles underneath the hull of the ship. References 1. Vorobyev, A., Arnould, O., Laux, D., Longo, R., van Dijk, N.P. and Gamstedt, E.K., “Characterisation of cubic oak specimens from the Vasa ship and recent wood by means of quasi-static loading and resonance ultrasound spectroscopy”, Holzforschung, 2015, doi: 10.1515/hf-2015-0073 2. van Dijk, N., Gamstedt, E.K. and Bjurhager, I., “Deformation analysis of wooden structures of cultural heritage: Translation of geodetic measurements into average strain fields”, Journal of Cultural Heritage, 2015, doi:10.1016/j.culher.2015.03.011