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Moisture-Induced Strains and Stresses in Wood
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics. Univ Boras, Fac Text Engn & Business, SE-50190 Boras, Sweden.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

To design safe, reliable and durable timber structures subjected to varying natural outdoor or indoor climates, understanding the long-term behavior of wood when mechanically loaded or restrained to deform is crucial. The present thesis focuses on the numerical modeling of the long-term mechanical behavior of wood. The numerical analysis is divided in the moisture transport and the mechanical analyses. In the moisture analysis, the multi-Fickian moisture transport model is used to determine spatial and temporal moisture content fields over the analyzed domain due to changing relative humidity (RH) of the ambient air. The obtained moisture contents are taken into the mechanical analysis where a new mechanical model is applied for predicting rheological response of wood in three orthotropic directions simultaneously. Experimental results of different authors are used to support numerous numerical analyses performed for various wood species, deformation and loading modes in constant or changing RH conditions. The performed analyses show that the new mechanical model adequately predicts the viscoelastic behavior of hardwood and softwood species in two orthotropic directions simultaneously under a sustained load or deformation. A significant influence of grain orientation in relation to the applied mechanical load on the viscoelastic creep behavior of wood is observed. The mechanical model is also able to predict accurately the rheological behavior of hardwood subjected to a sustained compressive mechanical load and changing moisture content. Applying the moisture and the mechanical models to the glued-laminated timber specimens during wetting and drying shows good agreement with the experimental results. The magnitudes of moisture-induced stresses perpendicular to the grain indicate a possibility of crack initiation during drying. The influence of characteristic material parameters required in the models on the mechanical state of the analyzed specimens is also determined. A quantification of the viscoelastic and the mechanosorptive material parameters required in the mechanical model is the additional outcome of the performed numerical analyses. The mechanical model presented in this thesis in combination with the multi-Fickian moisture transport model enables a full two- or three-dimensional long-term mechanical analysis of timber members exposed to natural climate with RH variations.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 51
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1772
Keywords [en]
mechanosorption, multi-Fickian moisture transport model, orthotropy, rheology, three-dimensional mechanical model, viscoelasticity, wood
National Category
Applied Mechanics
Research subject
Engineering science with specialization in Applied Mechanics
Identifiers
URN: urn:nbn:se:uu:diva-375148ISBN: 978-91-513-0569-1 (print)OAI: oai:DiVA.org:uu-375148DiVA, id: diva2:1283119
Public defence
2019-03-22, Häggsalen Å10132, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2019-02-27 Created: 2019-01-28 Last updated: 2019-03-18
List of papers
1. Coupled two-dimensional modeling of viscoelastic creep of wood
Open this publication in new window or tab >>Coupled two-dimensional modeling of viscoelastic creep of wood
2018 (English)In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 52, no 1, p. 29-43Article in journal (Refereed) Published
Abstract [en]

Three coupled two-dimensional viscoelastic creep models for orthotropic material are analyzed. The models of different complexity are mathematically formulated and implemented in a finite element software. Required viscoelastic material parameters are determined by calibration procedure, where numerical results are compared against experimentally obtained viscoelastic strains caused by tensile or shear loading. Finally, a comparison method is used to evaluate the accuracy of strain predictions of each particular model. The analysis shows that all the models are able to accurately predict viscoelastic creep simultaneously in two perpendicular directions for various periods of time and wood species. Calculated numerical values of the viscoelastic material parameters suitable for the three models and wood species, i.e., Douglas fir (Pseudotsuga menziesii), Norway spruce (Picea abies), Japanese cypress (Chamaecyparis obtusa), and European beech (Fagus sylvatica L.), under constant tensile loading are also given.

Place, publisher, year, edition, pages
SPRINGER, 2018
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-341318 (URN)10.1007/s00226-017-0944-3 (DOI)000419587400002 ()
Available from: 2018-02-07 Created: 2018-02-07 Last updated: 2019-01-28Bibliographically approved
2. Rheological behavior of wood in stress relaxation under compression
Open this publication in new window or tab >>Rheological behavior of wood in stress relaxation under compression
2018 (English)In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 52, no 3, p. 793-808Article in journal (Refereed) Published
Abstract [en]

Rheological behavior of wood under uniaxial compression along and perpendicular to the grain in constant environment was examined. Tests with constant deformation rate until failure and stress relaxation tests with constant deformation applied stepwise were carried out. The experimental results of stress relaxation showed nonlinear material behavior over time that got more prominent under high deformation levels. Considerable amount of stress relaxed during applying the deformation. Wood experienced greater stress relaxation along the grain than perpendicular to it. Three rheological models for orthotropic material were calibrated to the experimentally determined stress-time curves in longitudinal and transverse directions simultaneously. Small deformation levels assuming linear strains were accounted for in the models. Required elastic material parameters were determined from the tests with constant deformation rate. A model including the highest number of viscoelastic material parameters was the most successful in predicting stress relaxation of wood under stepwise deformation. Modeling indicated that wood behavior was very close to linear viscoelastic in relaxation under small deformation. The obtained material parameters made the model suitable for predicting rheological behavior of wood comprehensively, under sustained deformation or load in constant conditions.

Place, publisher, year, edition, pages
SPRINGER, 2018
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:uu:diva-352579 (URN)10.1007/s00226-018-0993-2 (DOI)000430010200011 ()
Available from: 2018-08-07 Created: 2018-08-07 Last updated: 2019-01-28Bibliographically approved
3. Influence of grain direction on the time-dependent behavior of wood analyzed by a 3D rheological model: A mathematical consideration
Open this publication in new window or tab >>Influence of grain direction on the time-dependent behavior of wood analyzed by a 3D rheological model: A mathematical consideration
2018 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 72, no 10, p. 889-897Article in journal (Refereed) Published
Abstract [en]

A three-dimensional (3D) rheological model for an orthotropic material subjected to sustained load or deformation under constant climate has been mathematically formulated. The elastic and viscoelastic compliance matrices are symmetric, where the mathematical derivation of the latter is shown. The model is linear and requires constant numerical values for the elastic and viscoelastic material parameters. The model's ability to predict the natural time-dependent response in three material directions simultaneously is demonstrated on a Douglas fir (Pseudotsuga menziesii) specimen subjected to a constant uniaxial tensile load. The material extends in a longitudinal direction and contracts in the transverse directions with time. The required material parameters are taken from the literature when possible, otherwise they are assumed. Furthermore, the influence of misalignment between the directions of observation and wood material directions on induced time-dependent strains is analyzed. The analyses show that the misalignment has a large effect on the material behavior. In some cases, the specimen under constant uniaxial tension even extends in the perpendicular transverse direction with time. The obtained results clearly demonstrate the high importance of considering the alignment of material directions precisely in order to be able to interpret the time-dependent behavior of wood correctly.

Place, publisher, year, edition, pages
WALTER DE GRUYTER GMBH, 2018
Keywords
grain direction, linear elasticity, linear viscoelasticity, material orthotropy, symmetry of compliance matrix, three dimensional rheological model, uniaxial tension, wood
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:uu:diva-363211 (URN)10.1515/hf-2017-0180 (DOI)000445777000008 ()
Available from: 2018-10-17 Created: 2018-10-17 Last updated: 2019-01-28Bibliographically approved
4. Hygro-mechanical analysis of wood subjected to constant mechanical load and varying relative humidity
Open this publication in new window or tab >>Hygro-mechanical analysis of wood subjected to constant mechanical load and varying relative humidity
2018 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 72, no 10, p. 863-870Article in journal (Refereed) Published
Abstract [en]

A hygro-mechanical (H-M) analysis of a wooden specimen sustaining a mechanical load while subjected to varying relative humidity was performed to predict the long-term rheological behavior of wood. The numerical analysis was based on the experimental results of total strains, monitored in two orthotropic material directions on oak wood specimens under constant uniaxial compression and with moisture content (MC) variation. For the moisture analysis, a multi-Fickian moisture transport model (MFMTM) was used to obtain temporal and spatial MC fields, which were the input data in the mechanical analysis. The presented mechanical model assumed a decomposition of the total strains into the elastic, viscoelastic and mechanosorptive strains and the strains due to shrinkage and swelling. The moisture and mechanical analyses required material parameters, which were taken from the literature or were empirically obtained by a fitting procedure. The performed H-M analysis gave accurate numerical predictions of the experimentally obtained total strains in two orthotropic directions simultaneously. Thus, the analysis developed has a high potential for predicting the long-term rheological behavior of timber structures, assuming that the material parameters are determined previously, based on specific, extensive, multidimensional experimental analyses.

Place, publisher, year, edition, pages
WALTER DE GRUYTER GMBH, 2018
Keywords
compression perpendicular to grain, mechanical model, mechanosorption, multi-Fickian moisture transport model, varying climate conditions, viscoelasticity, wood
National Category
Wood Science Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-363210 (URN)10.1515/hf-2018-0035 (DOI)000445777000005 ()
Available from: 2018-10-17 Created: 2018-10-17 Last updated: 2019-01-28Bibliographically approved
5. Numerical analysis of moisture-induced strains and stresses in glued-laminated timber
Open this publication in new window or tab >>Numerical analysis of moisture-induced strains and stresses in glued-laminated timber
(English)In: Article in journal (Refereed) Submitted
National Category
Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-375147 (URN)
Available from: 2019-01-28 Created: 2019-01-28 Last updated: 2019-01-28

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