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Structure and properties of Vasa oak
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
2006 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

The Vasa ship is not adequately supported. Measurements of the hull show that the ship deforms and rotate towards the port side. In addition, damages on the hull at support areas have been observed. The damages are due to high compressive loads. At damaged zones the support has been removed and the loads are thus transferred to adjacent support stanchions. In order to design an improved support, knowledge of the mechanical behaviour of the material is needed. In particular, radial modulus, strength and deformation mechanisms are of interest. In the present study, the mechanical behaviour of recent oak and oak from Vasa is studied. Furthermore, effects of PEG content, degradation and moisture on the properties of Vasa oak are investigated.

Oak is characterized by a very abrupt change from earlywood to latewood, where the latewood is much denser than earlywood. Also present in oak are large rays in the radial direction of the wood.

Small specimens were tested in compression using Digital Speckle Photography (DSP) in order to obtain strain fields of the whole specimen surface. This technique also provided data on failure mechanisms. Dynamic mechanical thermal analysis (DMTA) was performed to establish differences in moisture softening.

In radial compression, modulus and strength of Vasa oak are reduced by 50% compared with recent oak. A significant change of failure mechanism is observed for Vasa oak. In recent oak, failure in radial compression is by continuous folds of rays in the earlywood followed by continued plastic collapse of the earlywood layer. In Vasa oak rays show a more brittle fracture in each earlywood region. DMTA results indicate no effect on moisture softening of Vasa oak from presence of PEG although more work is needed to confirm this. Moisture adsorption for PEG-extracted Vasa oak is not significantly higher than for recent oak below 60% RH, suggesting that the extent of degradation of Vasa oak is limited. Vasa oak containing PEG is much more hygroscopic than PEG-extracted Vasa oak already at 50%. This difference is increasing with increasing relative humidity.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , 11 p.
Series
Trita-AVE, ISSN 1651-7660 ; 2006:31
Keyword [en]
oak, archaeological wood, European oak, transverse anisotropy
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4016ISBN: 91-7178-386-5 (print)OAI: oai:DiVA.org:kth-4016DiVA: diva2:10398
Presentation
2006-06-14, D41, Lindstedtsvägen 17, Stockholm, 15:15
Opponent
Supervisors
Note
QC 20101118Available from: 2006-06-01 Created: 2006-06-01 Last updated: 2010-11-18Bibliographically approved
List of papers
1. Transverse anisotropy of compressive failure in European oak: A digital speckle photography study
Open this publication in new window or tab >>Transverse anisotropy of compressive failure in European oak: A digital speckle photography study
2006 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 60, no 2, 190-195 p.Article in journal (Refereed) Published
Abstract [en]

The mechanical behaviour of European oak (Quercus robur L.) was studied in radial and tangential compression. Young's modulus and the yield strength were approximately 1.7- and 1.6-fold higher, respectively, in the radial direction. Strain fields were determined by digital speckle photography (DSP). Strains and the effective Poisson ratio could be determined separately in earlywood and latewood during deformation and failure events. In radial compression, strain data showed that rays contributed significantly to the high modulus. In addition, multiseriate ray microbuckling was observed to control yield strength. The microbuckling was localised in the low- density earlywood. In tangential compression, yield strength was controlled by vessel collapse in the low- density regions of the latewood. The strain field data provide direct evidence that the rays are the main microstructural factor controlling transverse anisotropy in European oak.

Keyword
compressive failure, digital speckle photography, European oak, ray microbuckling
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-5872 (URN)10.1515/HF.2006.031 (DOI)000236203100013 ()2-s2.0-33644923540 (Scopus ID)
Note
QC 20101005Available from: 2006-06-01 Created: 2006-06-01 Last updated: 2017-12-14Bibliographically approved
2. Transverse mechanical behaviour and moisture adsorption of waterlogged archaeological wood from the Vasa ship
Open this publication in new window or tab >>Transverse mechanical behaviour and moisture adsorption of waterlogged archaeological wood from the Vasa ship
2007 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 61, no 3, 279-284 p.Article in journal (Refereed) Published
Abstract [en]

Damage on the hull of the 17th century Swedish warship Vasa has been observed recently. Damage in the form of indentations in the wood is caused by high compressive loads from the support structure. In the process of developing an improved support structure, radial mechanical properties and the deformation mechanisms of Vasa oak are particularly important. Causes of differences in PEG content and oak degradation are also of interest. The radial modulus and compressive strength of Vasa oak are 50% lower than for recent oak. Furthermore, a significant change in failure mechanism is observed. More brittle separation fracture of the rays of Vasa oak is observed compared to the continuous folds of rays in recent oak. Tangential stiffness and strength are also 30% and 50% lower, respectively. Comparably small differences in moisture absorption between PEG-extracted Vasa oak and recent oak indicate a low extent of degradation of the Vasa oak.

Keyword
archaeological oak, mechanical properties, PEG, rays
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-5873 (URN)10.1515/HF.2007.048 (DOI)000247006400010 ()
Note
QC 20101005. Uppdaterad från Manuskript till Artikel (20101005).Available from: 2006-06-01 Created: 2006-06-01 Last updated: 2017-12-14Bibliographically approved

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