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Effects of barrelling during axial compressive tests of cubic samples with isotropic, transversely isotropic and orthotropic elastic properties
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.ORCID iD: 0000-0001-5855-2260
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
2016 (English)In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 137, 1-8 p.Article in journal (Refereed) Published
Abstract [en]

For scarce materials, such as archaeological wood, cubic samples are often used instead of standardised prisms for mechanical tests, since the elasticity can be determined in all three directions within a single sample, but with such samples barrelling makes it difficult to identify the elastic properties. The purpose of the present study is firstly to numerically investigate the effects of barrelling in cubic samples during compressive testing; secondly to numerically investigate and compare barrelling on isotropic and transversely isotropic material parameters; thirdly to compare four strain measurement techniques using digital image correlation, strain gauges and direct readings from the testing machine and finally to estimate the error due to barrelling by implementing the experimentally obtained orthotropic material parameters to the numerical model. The presented relative errors provide information when the perturbation caused by barrelling is negligible or significant for various materials and strain measurements. As an example, the results of compressive tests on waterlogged archaeological oak impregnated with polyethylene glycol are discussed.

Place, publisher, year, edition, pages
2016. Vol. 137, 1-8 p.
Keyword [en]
Cubic samples, Compressive testing, Barrelling formation, Wood
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:uu:diva-306193DOI: 10.1016/j.compscitech.2016.10.015ISI: 000389292600001OAI: oai:DiVA.org:uu-306193DiVA: diva2:1040053
Projects
Vasa support structure
Funder
Swedish Research Council FormasVINNOVASwedish Research Council
Available from: 2016-10-26 Created: 2016-10-26 Last updated: 2017-02-01Bibliographically approved
In thesis
1. Static and time-dependent mechanical behaviour of preserved archaeological wood: Case studies of the seventeenth century warship Vasa
Open this publication in new window or tab >>Static and time-dependent mechanical behaviour of preserved archaeological wood: Case studies of the seventeenth century warship Vasa
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Wooden objects have been widely used in the history of humanity and play an important role in our cultural heritage. The preservation of such objects is of great importance and can be a challenging task. This thesis investigates the static and time-dependent mechanical behaviour of archaeological oak wood from the Vasa warship. Characterisation of mechanical properties is necessary for the formulation of a numerical model to design an improved support structure. The ship was impregnated with polyethylene glycol (PEG) for dimensional stabilisation. All elastic engineering constants of the Vasa oak have been identified and compared with those of recent oak by means of the static and dynamic testing. The experiments were done on samples with cubic geometry, which allowed obtaining all elastic constants from a single sample. The usage of cubic samples with orthotropic mechanical properties during compressive experiments was validated with finite-element simulations. The Young's moduli of the Vasa oak in all orthotropic directions were smaller than those for the recent oak. The shear moduli of Vasa oak was determined and verified with the resonant ultrasound spectroscopy. The time-dependent mechanical behaviour of the Vasa oak has been studied. Creep studies were performed in uniaxial compression on the cubic samples in all orthotropic directions. The samples loaded in the longitudinal direction were subjected to different stress levels. A stress level below 15% of the yield stress in the longitudinal direction did not result in non-linear creep with increasing creep rates within the time frame of the tests. The results of the studies in radial and tangential directions showed that creep was dominated by the effect of annual fluctuations in relative humidity and temperature. The weight changes based on annual fluctuations of relative humidity were measured for Vasa oak and recent oak. The Vasa oak showed higher variations due to an increased hygroscopicity which is the result of the impregnation with PEG. In conceiving a full-scale finite-element model of Vasa ship, not only the stress-strain relations of the material but also those of the structural joints are needed. Since the in-situ measurement of joints is not an option, a replica of a section of the ship hull was built and tested mechanically. The load-induced displacements were measured using 3D laser scanning which proved to have advantages to conventional point displacement measurements. The mechanical characteristics of the Vasa oak and joint information presented in this work can be used as input for a finite-element model of the Vasa ship for simulation of static and time-dependent behaviour on a larger scale.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 52 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1474
Keyword
archaeological wood, compression test, cubic samples, elastic constants, oak wood, barrelling formation, quasi-static loading, resonant ultrasound spectroscopy (RUS), Vasa ship, creep, 3D laser scanning, Time-dependent behaviour
National Category
Other Natural Sciences
Identifiers
urn:nbn:se:uu:diva-314266 (URN)978-91-554-9810-8 (ISBN)
Public defence
2017-03-31, Polhemssalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Projects
Stötta Vasa
Available from: 2017-03-01 Created: 2017-02-01 Last updated: 2017-03-06

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