| 1. |
- Afshar, Reza, et al.
(author)
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Creep in oak material from the Vasa ship: : verification of linear viscoelasticity and identification of stress thresholds
- 2020
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In: European Journal of Wood and Wood Products. - : Springer Nature. - 0018-3768 .- 1436-736X. ; 78:6, s. 1095-1103
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Journal article (peer-reviewed)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 confrm 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 ftted 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 defne 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.
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| 2. |
- Öhlin, Jerry, et al.
(author)
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The association between cognition and gait in a representative sample of very old people - the influence of dementia and walking aid use
- 2020
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In: BMC Geriatrics. - : Springer. - 1471-2318 .- 1471-2318. ; 20
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Journal article (peer-reviewed)abstract
- Background: Cognition has been related with gait speed in older adults; however, studies involving the oldest age group, where many have mobility disability and cognitive impairment, are few. The aim was to investigate the association between global cognitive function and gait speed in a representative sample of very old people, and whether the association was affected by dementia, and walking aid use.Method: This cross-sectional study included 1317 participants, mean age 89.4 years, and 68% women, from the Umeå85+/Gerontological Regional Database. Self-paced gait speed was measured over 2.4 m, with or without walking aids, and global cognitive function with the Mini-Mental State Examination (MMSE). The association between cognition and gait speed was analyzed using multiple linear regression and stratified according to dementia. The influence of missing gait speed values was explored using multiple imputation. An interaction analysis was performed to investigate the influence of walking aid use.Results: In comprehensively adjusted analyses, MMSE associated with gait speed (unstandardized β (β) 0.011 m/s, 95% Confidence Interval [CI] = 0.009, 0.013, p < 0.001) in the total sample. No association was found in people with dementia (β 0.003 m/s, 95%CI = 0.000, 0.006, p = 0.058), until missing gait speed values were compensated for by multiple imputation (β 0.007 m/s, 95% [CI] = 0.002, 0.011, p = 0.002). In interaction analysis the use of walking aids attenuated the association between cognition and gait speed (β − 0.019 m/s, 95%CI = − 0.024, − 0.013, p < 0.001).Conclusion: Global cognitive function appears to associate with gait speed in very old people. However, in people with dementia selection bias was indicated since unless missing gait speed values were accounted for no association was observed. Walking aid use attenuated cognitive load, which may not apply to walking in daily activities, and requires further investigation.
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| 3. |
- Afshar, Reza, et al.
(author)
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Characterization of mechanical properties of Vasa oak and their application in a full-scale numerical model for support assessment
- 2019
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Conference paper (other academic/artistic)abstract
- The 17th century Vasa shipwreck is a well-known object of cultural heritage. According to geodetic measurements inside and outside of the ship as well as on the support structure, the ship is sinking onto its cradle. The analysis of measurements showed the ship undergoes continued deformation with increasing strain. Previous research projects on the Vasa ship have largely been focused on the chemical degradation of the Vasa oak, which concerns the waterlogged polyethylene glycol (PEG) impregnated oak wood. The main goal was to provide understanding of the degradation mechanisms and possible remedies to mitigate the chemical decay. In this paper, a review is presented of previous research in term of characterization of mechanical properties, and effects of PEG and moisture on the mechanical behaviour of the Vasa oak. In addition, a full-scale finite-element model of the Vasa ship has been developed to assess its current and future structural behaviour, as well as a tool to design an improved support structure. The mechanical properties, defined in the model in terms of orthotropic elastic engineering constants, have been determined in previous work. Moreover, creep properties of the archaeological wood material have been and are being characterized, so that the model can be extended by extrapolation to predict future deformation. Geodetic measurements have been used for validation of the static model. The approach undertaken in this project could hopefully be useful in design strategies of improved support for other aging and deforming wood structures in cultural heritage.
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| 4. |
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| 5. |
- Afshar, Reza, et al.
(author)
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A full-scale finite-element model of the Vasa ship
- 2017
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In: Proceedings of ECCOMAS Thematic Conference CompWood 2017.
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Conference paper (peer-reviewed)abstract
- A full-scale model of the 17th century Vasa shipwreck has been developed to assess its current and future structural stability as well as design an improved support structure. A wireframe model, consisting of only lines, points and curves to describe the geometry of the ship, has been provided by the Vasa museum. It has been developed based on geodetic measurements using a total station. From this wireframe model, a three-dimensional (3D) model comprising solid bodies for solid-like parts (i.e. hull and keel), surfaces for the shell-like components (deck planks) and lines for beam-like constituents (deck beams) has been developed in Creo Parametric 3D software. This geometric model has been imported in finite-element software, Ansys, for further development of the stiffeners (knees, riders, stanchions, masts, etc.), adjustment of the correct location of deck beams and, finally, structural analyses of the entire ship (Figure 1). The procedure for selection of the different types of elements in the finite-element (FE) model, the definition of orthotropic material properties for the timber structure and preliminary results are discussed in this paper. Experiences drawn from this engineering project may also be useful in development of finite element models for structural assessment of other complex wooden structures in cultural heritage.
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