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Influence of grain direction on the time-dependent behavior of wood analyzed by a 3D rheological model: A mathematical consideration
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.
Univ Boras, Fac Text Engn & Business, SE-50190 Boras, Sweden.
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. Vol. 72, no 10, p. 889-897
Keywords [en]
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: urn:nbn:se:uu:diva-363211DOI: 10.1515/hf-2017-0180ISI: 000445777000008OAI: oai:DiVA.org:uu-363211DiVA, id: diva2:1256616
Available from: 2018-10-17 Created: 2018-10-17 Last updated: 2019-01-28Bibliographically approved
In thesis
1. Moisture-Induced Strains and Stresses in Wood
Open this publication in new window or tab >>Moisture-Induced Strains and Stresses in Wood
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
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:nbn:se:uu:diva-375148 (URN)978-91-513-0569-1 (ISBN)
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

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Huc, Sabina

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