| 1. |
- Bader, Thomas K., 1980-, et al.
(författare)
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Modeling the Mechanical Behavior of Wood Materials and Timber Structures
- 2023
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Ingår i: Springer Handbook of Wood Science and Technology. - : Springer. - 9783030813147 - 9783030813154 ; , s. 507-568
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Bokkapitel (refereegranskat)abstract
- This chapter aims at highlighting the benefit of numerical methods and their broad application in the field of wood, engineered wood-based products (EWPs), structural elements including glued-laminated and cross-laminated timber, and engineered timber structures. It focuses on the hygrothermo- viscoelastic material behavior of these elements and structures as a consequence of the behavior of wood materials. After motivating the need for models of wood, different types of numerical models and their application for determination of mechanical properties and dimensional stability of wooden boards, strand- and veneer-based engineered woodbased products, including glued-laminated and crosslaminated timber, as well as of connections in EWPs are reviewed and application examples are given. Methods and application examples are furthermore provided for moisturerelated stresses and deformations in timber structures, the influence of connections on the structural response, instability of structural systems, and modeling of prefabricated frame structures, before modeling of historical structures of wood is discussed. The chapter ends with discussing bottlenecks in modeling of wood materials and timber structures, which might be a starting point for further improvements and novel modeling strategies. © Springer Nature Switzerland AG 2023.
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| 2. |
- Blomqvist, Lars, 1962-, et al.
(författare)
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Stress distribution in veneers under lamination and simultaneously bending : an experimental and numerical investigation
- 2023
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Ingår i: Wood Material Science & Engineering. - : Taylor & Francis Group. - 1748-0272 .- 1748-0280. ; 18:3, s. 995-1002
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Tidskriftsartikel (refereegranskat)abstract
- Laminated veneer products (LVPs) are veneers glued together into a predetermined shape. Experimental and numerical investigations were performed under lamination and simultaneously bending of veneer laminate to study the stress distribution in the laminate. Laminates of different thicknesses were made of peeled veneers of European beech. The veneers were coated with adhesive, inserted in a mould which had the shape of a semicircle, and finally pressed at 20 degrees C to a laminate. Two Teflon-polymer films including sensors for measurement of the contact pressure were placed on both sides of the laminate to measure the local contact pressure (contact stress) between the laminate and the mould. At the beginning of the bending process, the contact stresses were locally distributed over the laminate in a similar pattern as in a three-point bending; after the laminate was further bent, the stress distribution rearranged to be as in four-point bending. In the end of the moulding, the local contact stresses increased over the entire laminate and reached a 'peak-value' over bent area in the middle part of the mould. A finite-element model was created to study the bending process. Regarding the overall development of the contact stress variations, the experimental and the numerical results agreed.
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| 3. |
- Florisson, Sara, et al.
(författare)
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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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Ingår i: Wood Science and Technology. - : Springer. - 0043-7719 .- 1432-5225. ; 55:5, s. 1269-1304
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Florisson, Sara
(författare)
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Moisture-induced stress and distortion of wood : A numerical and experimental study of wood's drying and long-term behaviour
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the current advances made in three-dimensional modelling of wood, it is possible to provide an overall picture of moisture flow, and moisture-induced stress and deformations, whereas previously, experiments only provided local measurements. The main aim of the doctoral thesis is to investigate the possibilities of the developed three-dimensional numerical model to predict the behaviour of wood when simultaneously exposed to a mechanical load and a particular climate. Three applications in the fields of wood drying and long-term behaviour of wood are considered: 1) the effect of green-state moisture content on the drying behaviour of timber boards, 2) the calibration of the numerical model based on a long-term four-point bending tests using small wood beams subjected to a constant temperature and systematic relative humidity (RH) changes, and 3) the validation of the numerical model by means of a long-term four-point bending test on solid timber beams subjected to Northern European climate. As part of the second application, an experimental methodology and analytical method were designed. The numerical model was developed in finite element software Abaqus FEA® and consists of several user-subroutines to include material orientation (i.e. annual ring pattern, conical shape and spiral grain), and the selected constitutive behaviour and required boundary conditions. To simulate the moisture flow, a nonlinear single-Fickian approach was combined with a nonlinear Neumann boundary condition, which describes the flux normal to the exchange surface based on a moisture and temperature dependent surface emission coefficient. A strain relation was used that accounts for hygro-expansion, and the elastic, creep and mechano-sorptive behaviour. The analytical method describes the elastic and creep deflection in the constant moment area of the four-point bending setup, and was used to isolate and assess the mechano-sorption deflection in the cumulative moisture content domain. The results show that the three-dimensional character of the numerical model contributed to the analysis and visualisation of the different stress states and deformations that are affected by material properties that vary (i.e. from pith to bark, between heartwood and sapwood, and due to temperature and moisture content), fibre orientation and climate. The simulations made on timber boards clarified phenomena, such as stress reversal and casehardening associated with wood drying, and showed that the green-state moisture content affected the time, size and frequency with which extremes in tangential tensile stress developed inside the timber during drying. The results of the calibration and validation indicated that the numerical model is able to describe moisture change and gradients in the considered temperature and relative humidity ranges (between -2-60℃ and 40-80% RH), as well as the deflection. The experimental methodology and analytical method led to a successful identification of each deflection component and isolation of the mechano-sorptive deflection curves. The experimental methodology benefitted the calibration of the numerical model. In conclusion, the presented three-dimensional numerical model compatible with Abaqus FEA® provides a powerful tool for scientists and timber engineers to study the combined effect of load and climate on stress state and deformations of various timber products in a wide field of applications.
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| 5. |
- Florisson, Sara, et al.
(författare)
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Numerical analysis of wood subjected to bending and northern European climate
- 2021
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Due to the natural process of sorption, wood constantly interacts with the surrounding climate to establish an equilibrium moisture content, even without the direct influence of rain, solar radiation or wind. However, when wood is subjected to a combination of a change in moisture content and a state of stress brought on by e.g. mechanical load, differential shrinkage or swelling, or differences in material properties, a continuous change in the level of stress and the occurrence of deformations and fracture can be the result.The techniques available for in-situ monitoring of changes in moisture content and hygro-mechanical and viscoelastic behaviour can only be employed in specific locations in or around the wood. Whereas, modern techniques employed in laboratories, such as digital image correlation and computed tomography are laborious and time consuming. With recent developments in three-dimensional modelling, endless predictions can be made both of moisture flow and of the hygro-mechanical and visco-elastic behaviour of wood in three-dimensional space, requiring experimental data only to calibrate and validate the model.A three-dimensional numerical model was created in finite element software Abaqus FEA® to simulate both the transient nonlinear moisture flow and the moisture-dependent distortion and stress, while account is being taken of the fibre orientation (annual ring pattern, conical shape and spiral grain). A nonlinear single-Fickian model in connection with a nonlinear Neumann boundary condition is used to describe the moisture flow. For a moisture-sensitive and visco-elastic material such as wood, it is common to describe the total strain rate as a summation of the elastic, hygro-expansion, visco-elastic creep and mechano-sorptive strain rates.The aim was to determine whether the model was able to simulate in an adequate way the beam bending that occurs under northern European climate conditions. To accomplish this, the following steps were taken: 1) on the basis of experimental data available in literature, a set of expressions was created to describe the moisture- and temperature-dependent diffusion coefficient and surface emission coefficient, 2) experimental results obtained for small beams tested under constant temperature and systematic relative humidity (controlled climatic) conditions were used to calibrate the numerical model, account being taken of the spiral grain that applied and the annual ring curvature, and 3) test results for solid beams tested in northern European (natural) climatic conditions were used to validate the numerical model, account being taken of the fibre orientation.The results obtained showed a clear distinction between the effect of moisture on the stress development caused by mechanical load and the stress development 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 a stress overturn brought about by wetting. The material orientation showed to have a strong effect on the estimated deflection, calibrated material parameters and normative stress states.
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| 6. |
- Florisson, Sara, et al.
(författare)
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Three-dimensional orthotropic nonlinear transient moisture simulation for wood : analysis on the effect of scanning curves and nonlinearity
- 2020
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Ingår i: Wood Science and Technology. - : Springer. - 0043-7719 .- 1432-5225. ; 54, s. 1197-1222
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Tidskriftsartikel (refereegranskat)abstract
- This paper introduces, with the development of user-subroutines in the finite-element software Abaqus FEA (R), a new practical analysis tool to simulate transient nonlinear moisture transport in wood. The tool is used to revisit the calibration of moisture simulations prior to the simulation of mechanical behaviour in bending subjected to climate change. Often, this calibration does not receive sufficient attention, since the properties and mechanical behaviour are strongly moisture dependent. The calibration of the moisture transport simulation is made with the average volumetric mass data experimentally obtained on a paired specimen of Norway spruce (Picea abies) with the dimensions 30x15x640 mm(3). The data, from a 90-day period, were measured under a constant temperature of 60 degrees C and systematic relative humidity cycles between 40 and 80%. A practical method based on analytical expressions was used to incorporate hysteresis and scanning behaviour at the boundary surface. The simulation tool makes the single-Fickian model and Neumann boundary condition readily available and the simulations more flexible to different uses. It also allows for a smoother description of inhomogeneity of material. The analysis from the calibration showed that scanning curves associated with hysteresis cannot be neglected in the simulation. The nonlinearity of the analysis indicated that a coherent set of moisture dependent diffusion and surface emission coefficient is necessary for the correct description of moisture gradients and mass transport.
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| 7. |
- Kuai, Le, et al.
(författare)
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A numerical and experimental investigation of non-linear deformation behaviours in light-frame timber walls
- 2022
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Ingår i: Engineering structures. - : Elsevier. - 0141-0296 .- 1873-7323. ; 252
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Tidskriftsartikel (refereegranskat)abstract
- In recent decades, there is a trend in Scandinavian countries to build multi-storey residential houses using prefabricated timber modules. It is a highly efficient construction process with less environmental impact and less material waste. A significant building element in the timber modules is the light-frame timber wall, which has to be carefully analysed and optimized in this process. This paper presents a new parametric Finite Element (FE) model that can simulate both in-plane and out-of-plane deformations in the light-frame walls. A new and flexible (Eurocode based) approach to define the properties of the mechanical connections is introduced. A numerical model is presented through simulations of several walls that were verified with full-scale experiments. The results indicate that the numerical model could achieve fairly reasonable accuracy with the new approach. Furthermore, several parametric studies are presented and discussed from global and local points of view, to investigate the effects of certain parameters that are not considered in the design method according to Eurocode 5.
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| 8. |
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| 9. |
- Kuai, Le, et al.
(författare)
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Modelling and experimental verification of light-frame timber modules loaded in shear
- 2021
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Ingår i: World Conference on Timber Engineering 2021, WCTE 2021. - : World Conference on Timber Engineering (WCTE).
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Konferensbidrag (refereegranskat)abstract
- Building with prefabricated light-frame modules is an innovative construction method whose market share is increasing in Sweden. It is often used to build low and mid-rise residential buildings where the modules are premanufactured in factories with high efficiency and accuracy. This building method has many advantages, though more research regarding wind stabilization of both individual and assembled modules is still needed. To study the racking stiffness and strength of such timber modules, a numerical model of their shear walls was created and verified based on experimental results from specially prefabricated test modules. A new test facility was built for this work, where several potentiometers and a digital image correlation system collected the data used to verify the simulation model. The results from the simulations agree well with the experimental findings, which indicate that the three-dimensional finite element model works with reasonable accuracy for this application. © WCTE 2021. All rights reserved.
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| 10. |
- Kuai, Le, et al.
(författare)
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Nonlinear FE-analysis and testing of light-frame timber shear walls subjected to cyclic loading
- 2023
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Ingår i: Construction and Building Materials. - : Elsevier. - 0950-0618 .- 1879-0526. ; 362
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Tidskriftsartikel (refereegranskat)abstract
- Light-frame timber shear walls have been used as load-bearing elements in buildings for several decades. To predict the performance of such structural elements under loading, numerous analytical and numerical models have been developed. However, little focus has been on the prediction of the plastic damage behaviour and unloading of the walls. In this paper, a parametric Finite Element (FE) model is further developed by introducing elasto-plastic connectors to simulate the mechanical behaviour of the sheathing-to-framing connections. To verify the accuracy of the elasto-plastic model, full-size walls were tested and compared with results from simulations. The numerical results, from a few loading cycles, indicate that the model achieves reasonable accuracy in predicting both the nonlinear elastic and plastic deformations. Both experimental and simulation results demonstrate the importance of opening locations relating to the external racking force. The results also indicate that for a double-layer wall, its racking strength can be achieved by summation of the separate contribution from each layer. Furthermore, the internal layer was observed to contribute significantly less than the external layer since its nail pattern was based on the sheathing pattern of the external layer. © 2022 The Authors
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