Non-destructive investigation of material properties of the Vasa warship: a pilot study

Paper in proceeding, 2012

The Vasa warship is one of the most important national treasures of Sweden. In 1956, the pride of the Swedish navy was located on the seabed, before the ship was raised in 1961. In order to maintain the integrity of the Vasa warship, conservation treatment was carried out during a period of 17 years after salvage. This PEG (Poly Ethylene Glycol) treatment prevented the ship from serious shrinkage and distortion that would have caused collapse of the cell walls. These degradation processes have diminishing effects on the strength and stiffness of the ship structure. During the last 10 years, interest has focused on the mechanical properties of Vasa oak and the influence of PEG penetration in oak. Global non-destructive testing (NDT) of the mechanical properties, combined with semi-destructive tests (SDT) to predict both strength and stiffness parameters of the Vasa oak as input information, is of great importance when it comes to defining the real structural behaviour. The main aim of the paper was to compare the applicability of property prediction through non-destructive and semi-destructive testing. In particular, portable X-ray equipment was used in this study to obtain accurate density measurements on site which are valuable in establishing a relationship between the influence of stiffness parameters and PEG and X-ray density. It was possible to obtain accurate estimates and a strong correlation with the density of Vasa oak structural components using X-rays in combination with digital image processing. The proposed procedure can be used in situ with satisfactory results for the evaluation of timber properties.