Abstract
To maintain the integrity of the Vasa warship after salvage in 1961, conservation treatment with polyethylene glycol was carried out to prevent the collapse of cell walls. This treatment had negative effects on the strength and stiffness of the oak and the hull structure has been found to slowly deform over time. It is of interest to construct a three-dimensional numerical computer model to model and predict the deformation of the warship. This creates difficulties related to the complexity of measuring the detailed material properties that are required as input. In this context, a non-destructive methodology to predict the stiffness of the Vasa oak in terms of moduli of elasticity in the three principal directions of timber at critical positions in the ship would be useful. The twofold aim of the paper is to propose a strategy for a support system and to conduct an on-site assessment of the warship to predict the mechanical properties of the Vasa oak material. This paper also contains an up-to-date review of all essential mechanical data measured on the Vasa oak. The preliminary investigation using an X-ray technique to investigate the density properties produced promising results for future use in the evaluation of the mechanical performance. Based on these results, a procedure to establish the stiffness properties of the Vasa oak in terms of MOE was suggested, using a combination of data from previous measurements, in combination with extended tests on Vasa oak specimens and an X-ray-based density calibration procedure. The general complexity of the Vasa warship can be mainly attributed to large variations in the properties of Vasa oak due to surface degradation, chemical treatment and the disintegration of the cell-wall structure originating from centuries of waterlogged conditions. That causes difficulties when assessing mechanical and physical properties on a structural level. A combination of visual inspection together with X-ray investigation is of great importance to evaluate those properties and to obtain more accurate estimates. The results from evaluating mechanical properties can serve as input in a numerical model and serve as a foundation for decision-making relating to the modification of the support system.
Highlights
The Vasa warship is one of the most important national treasures of Sweden
The overall twofold aim of this paper is to develop an on-site assessment strategy to predict the mechanical properties of the Vasa oak material at structural level and to propose a strategy for developing a new support system that distributes the loads from the hull structure to the cradle
On-site assessment of the Vasa warship we propose a detailed methodology for the on-site assessment
Summary
The Vasa warship is one of the most important national treasures of Sweden. After the launch in Stockholm Harbour, the “jewel” of the Swedish Navy experienced difficulty in terms of stability and manoeuvrability and sank on her maiden voyage in 1628. (1) The general complexity of the Vasa warship can be mainly attributed to large variations in Vasa oak; to the natural variability of the wood and to the surface degradation, chemical treatment and the disintegration of the cell-wall structure originating from centuries of waterlogged conditions This might cause difficulty in the assessment and evaluation of the mechanical and physical properties at structural level. (4) The large variations in all the mechanical properties from the oak in the Vasa warship require more detailed investigations of these properties at the various locations in the ship’s structure This variation is mainly due to the content of PEG in and the origin of the test specimen (sapwood or heartwood), but it is probably due to the natural morphological variation, which is especially pronounced in oak. Author details 1Department of Civil and Environmental Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden. 2Department of Engineering Sciences, Uppsala University (UU), Box 534, SE-751 21 Uppsala, Sweden
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