Creep in oak material from the Vasa ship: verification of linear viscoelasticity and identification of stress thresholds

R Afshar,A Ahlgren,E K Gamstedt,G Almkvist,M Cheylan

doi:10.1007/s00107-020-01566-1

Abstract

Creep deformation is a general problem for large wooden structures, and in particular for shipwrecks in museums. In this study, experimental creep data on the wooden cubic samples from the Vasa ship have been analysed to confirm the linearity of the viscoelastic response in the directions where creep was detectable (T and R directions). Isochronous stress–strain curves were derived for relevant uniaxial compressive stresses within reasonable time spans. These curves and the associated creep compliance values justify that it is reasonable to assume a linear viscoelastic behaviour within the tested ranges, given the high degree of general variability. Furthermore, the creep curves were fitted with a one-dimensional standard linear solid model, and although the rheological parameters show a fair amount of scatter, they are candidates as input parameters in a numerical model to predict creep deformations. The isochronous stress–strain relationships were used to define a creep threshold stress below which only negligible creep is expected. These thresholds ranges were 0.3–0.5 MPa in the R direction and 0.05–0.2 MPa in the T direction.

Highlights

The creep properties of conserved archaeological wood material in large artefacts have not been much investigated despite the importance for the dimensional stability and long-term preservation
In addressing creep of polyethylene glycol (PEG)-treated waterlogged wood in large wooden structures, like shipwrecks, some key questions present themselves. Do these materials show linear creep within the stress levels anticipated in the structure? In a finite element model of the ship, which can potentially be used in the design of improved support structures, one would like to implement a relatively simple material model accounting for creep
It is well known that the theory of linear viscoelasticity is valid for the low stress levels, which is the condition of this study

Summary

Introduction

The creep properties of conserved archaeological wood material in large artefacts have not been much investigated despite the importance for the dimensional stability and long-term preservation. In addressing creep of PEG-treated waterlogged wood in large wooden structures, like shipwrecks, some key questions present themselves. Do these materials show linear creep within the stress levels anticipated in the structure? In a finite element model of the ship, which can potentially be used in the design of improved support structures, one would like to implement a relatively simple material model accounting for creep. This would imply a linear viscoelastic model accounting for creep deformation, where the creep strains depend linearly on the applied stresses within a given range. If the support structure can be designed in such a way that the stresses in main load carrying components are relieved down to a level beyond this threshold, creep deformation would effectively not be an issue

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