| 1. |
- Bengtsson, Rhodel, et al.
(author)
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Comparison of measured creep in a wooden beam with finite element predictions based on orthotropic viscoelastic material model
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Other publication (other academic/artistic)abstract
- Creep is of concern for long-term deformations of wooden structures. Since wood is anisotropic and creeps in several material directions, it may not be sufficient to include only axial creep along the grain even for deformations in beam-like components. A bottle-neck is that creep characterisation in all material directions is both costly and complicated. Multiscale modelling from cell-wall creep including the main contributing features (density, ray content, microfibrillar angle) can contribute to fill to complete material models for wood creep. In the present study, we have chosen a four-point bending test of a Norway spruce beam to represent a loaded wooden component in a structure. Digital image correlation was used to gather data on strain and displacement fields during the creep test. The experimental results were compared with finite element predictions based on a 3D orthotropic viscoelastic model obtained by multiscale homogenisation. There was generally good agreement in the strain fields between the finite element simulations and experimental observations. However, the numerical predictions exhibits slightly greater stiffness in terms of displacement, suggesting the need for further refinement of the multiscale model or a combination of materials creep charactrisation and multiscale modelling.
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| 2. |
- Bengtsson, Rhodel, et al.
(author)
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Feasiblity of wooden towers for offshore wind turbines: Creep and fatigue predictions
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Other publication (other academic/artistic)abstract
- Long-term experiences with vertical axis wind turbines constructed with wood are positive, and show that wood towers are a viable alternative to conventional steel towers on land. Wood is a renewable material in contrast to steel and concrete and could steer a more sustainable use of raw material for future wind farms. The obvious drawback of moisture-induced softening and degradation in off-shore settings can be mitigated by efficiently sealing the tower using a barrier coating. In that case, fatigue sensitivity and creep deformations are the main design concerns. In this paper through finite element simulations of a floating tilted vertical-axis wind turbine, it was shown that fatigue issues can be resolved with proper design of the mast and the blade joints keeping the stress concentrations at bay. The numerical results also indicated that creep displacements are negligible. The review and calculations reinforce the assumption that the fast developments seen in timber high-rise building can also be expected for off-shore wind turbine towers.
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| 3. |
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| 4. |
- Florisson, Sara, et al.
(author)
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A numerical study of the effect of green-state moisture content on stress development in timber boards during drying
- 2019
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In: Wood and Fiber Science. - : Society of Wood Science and Technology. - 0735-6161. ; 51:1, s. 41-57
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Journal article (peer-reviewed)abstract
- Timber boards manufactured with a traditional sawing pattern often contain both heartwood andsapwood. In such boards, internal constraints can occur during drying because of a radial variation in greenstate(GS) MC between the heartwood (30-60%) and sapwood region (120-200%). Despite such knowledge,the initial MC is seldom considered when evaluating kiln-drying schedules. The effect of GS MC on thedevelopment of tangential tensile stress during drying is studied for four types of timber boards. A numericalmodel was developed that can simulate transient nonlinear orthotropic moisture flow and moisture–inducedstress and distortion in wood with the use of the finite element method. The stress analysis considers elastic,hygroscopic, and mechano-sorptive strain. The study shows that the GS MC does not significantly influencethe maximum stress state, but that it does influence the time at which the maximum tangential tensile stressoccurs at different exchange surfaces. This results in several periods in the drying schedule where unfavorablehigh stress situations in the tangential direction arise, which could lead to crack propagation.
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| 5. |
- Florisson, Sara, et al.
(author)
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A three-dimensional numerical analysis of moisture flow in wood and of the wood's hygro-mechanical and visco-elastic behaviour
- 2021
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In: Wood Science and Technology. - : Springer. - 0043-7719 .- 1432-5225. ; 55:5, s. 1269-1304
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Journal article (peer-reviewed)abstract
- A three-dimensional numerical model was employed in simulating nonlinear transient moisture flow in wood and the wood's hygro-mechanical and visco-elastic behaviour under such conditions. The model was developed using the finite element software Abaqus FEA, while taking account of the fibre orientation of the wood. The purpose of the study was to assess the ability of the model to simulate the response of wood beams to bending and to the climate of northern Europe. Four-point bending tests of small and clear wood specimens exposed to a constant temperature and to systematic changes in relative humidity were conducted to calibrate the numerical model. A validation of the model was then performed on the basis of a four-point bending test of solid timber beams subjected to natural climatic conditions but sheltered from the direct effects of rain, wind and sunlight. The three-dimensional character of the model enabled a full analysis of the effects of changes in moisture content and in fibre orientation on stress developments in the wood. The results obtained showed a clear distinction between the effects of moisture on the stress developments caused by mechanical loads and the stress developments caused solely by changes in climate. The changes in moisture that occurred were found to have the strongest effect on the stress state that developed in areas in which the tangential direction of the material was aligned with the exchange surface of the beams. Such areas were found to be exposed to high-tension stress during drying and to stress reversal brought about by the uneven drying and shrinkage differences that developed between the outer surface and the inner sections of the beams.
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| 6. |
- Florisson, Sara, et al.
(author)
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An overview of lab-based micro computed tomography aided finite element modelling of wood and its current bottlenecks
- 2023
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In: Holzforschung. - : Walter de Gruyter. - 0018-3830 .- 1437-434X. ; 77:11-12, s. 793-815
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Journal article (peer-reviewed)abstract
- Microscopic lab-based X-ray computed tomography (XµCT) aided finite element (FE) modelling is a popular method with increasing nature within material science to predict local material properties of heterogeneous materials, e.g. elastic, hygroexpansion and diffusion. This method is relatively new to wood and lacks a clear methodology. Research intended to optimise the XµCT aided FE process often focuses on specific aspects within this process such as the XµCT scanning, segmentation or meshing, but not the entirety of the process. The compatibility and data transfer between aspects have not been investigated to the same extent, which creates errors that propagate and negatively impact the end results. In the current study, a methodology for the XµCT aided FE process of wood is suggested and its bottlenecks are identified based on a thorough literature review. Although the complexity of wood as a material makes it difficult to automate the XµCT aided FE process, the proposed methodology can assist in a more considered design and execution of this process. The main challenges that were identified include an automatic procedure to reconstruct the fibre orientation and to perform segmentation and meshing. A combined deep-learning segmentation method with geometry-based meshing can be suggested.
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| 7. |
- Florisson, Sara, et al.
(author)
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Analysis of hygro-mechanical behavior of wood in bending
- 2021
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In: Wood and Fiber Science. - : International society of wood science and technology (SWST). - 0735-6161. ; 53:1, s. 27-47
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Journal article (peer-reviewed)abstract
- The empirical test developed as validation for a newbeamelementmodel that can account for bothmechanical and environmental load action in finite element analysis is presented. The testing protocol allowsfor the identification and analysis of contributing deflection components in bending under varying MCconditions, including mechano-sorption. The components of deflection in the shear-free span of a four-pointbending test and their responses to varyingmoisture are evaluatedwith an analytical procedure. The experimentwas conducted on clear, straight-grained sapwood and heartwood specimens of Norway spruce (Picea abies)(30 15 640 mm3). The program consisted of three phases: 1) long-term (LT) experiments under constanttemperature of 60°C and RH cycles between 40% and 80%, 2) a short-term static experiment to determine thevariation in the sample set and the load level of the LT experiment on end-matched specimens, and 3) creeptests at 60°C and constant humidity at either 40%or 80%to determine the effect ofmoisture on the viscoelasticcreep. Mass changes and hygro-expansion measured on matched specimens were used in the analyticalmethod. Constitutive models used for describing the material-level response to loads and moisture changeswere applied to the shear-free segment of the specimens disregarding actual moisture gradients and fiberorientation inside the test specimens. A successful identification of each deflection component and isolation ofmechano-sorption component was accomplished. In the 90 da of testing, the dominant component of the totaldeflection was the elastic component, followed by the mechano-sorptive component. Creep was found to benonnegligible and important in the correct description of mechano-sorption. The effect of moisture on theviscoelastic behavior showedmost important during loading and first stages of decreasing deflection rate phase.
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| 8. |
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| 9. |
- Florisson, Sara, et al.
(author)
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Macroscopic X-ray computed tomography aided numerical modelling of moisture flow in sawn timber
- 2022
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In: European Journal of Wood and Wood Products. - : Springer. - 0018-3768 .- 1436-736X. ; 80:6, s. 1351-1365
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Journal article (peer-reviewed)abstract
- Mathematical models are essential for the development of schedules for the air-circulation drying of timber in Swedish sawmills, but earlier models have been shown to be conservative leading to longer drying times than necessary. In the current study, macroscopic (macro) X-ray computed tomography (CT) has been used in both the development and validation of a finite element (FE) model, to enable the macro-CT aided FE modelling of the nonlinear transient moisture flow in wood. The model uses more advanced theory than has previously been used in Swedish sawmills, by incorporating a surface emission coefficient to simulate the surface resistance to moisture flow. A single piece of Norway spruce [Picea abies (L.) Karst.] timber was subjected to that part of a traditional kiln-drying schedule, which is associated with diffusion-driven moisture transport. The incorporation of macro-CT data into the FE model resulted in a more realistic representation of the board’s geometry, the initial moisture state, and the definition of material parameters. It also led to a better simulation of flow speed and moisture gradient, especially the asymmetric MC development within the cross section throughout the drying process.
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| 10. |
- Florisson, Sara, et al.
(author)
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Microscopic computed tomography aided finiteelement modelling as a methodology to estimatehygroexpansion coefficients of wood : a case studyon opposite and compression wood in softwoodbranches
- 2023
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In: Holzforschung. - : Walter de Gruyter. - 0018-3830 .- 1437-434X.
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Journal article (peer-reviewed)abstract
- Microscopic X-ray computed tomography (XµCT) aided finite element (FE) modelling is a popular method in material science to relate material properties to heterogeneous microstructures. Recently, a methodology was developed for the XµCT aided FE modelling of wood, which characterises the process from specimen preparation to estimation of material properties. In the current research, this methodology is tested on branches of Norway spruce (Picea abies (L.) Karst.) to estimate the hygroexpansion coefficients of opposite (OW) and compression wood (CW). These properties are largely unknown and have engineering implications. The study is complemented by measurements of density, moisture content (MC) and elastic moduli. Results showed that the methodology assisted in the design of an integrated process and the identification of bottlenecks. It was seen that the level of detail of the numerical model had a strong influence on the obtained hygroexpansion properties. CW from branches showed higher density and longitudinal shrinkage coefficients, and elastic moduli less affected by MC. These differences are unlikely caused by MC, but more likely by the characteristics of the microstructure.
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