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Modelling of the hygroelastic behaviour of normal and compression wood tracheids
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
2014 (English)In: Journal of Structural Biology, ISSN 1047-8477, E-ISSN 1095-8657, Vol. 185, no 1, 89-98 p.Article in journal (Refereed) Published
Abstract [en]

Compression wood conifer tracheids show different swelling and stiffness properties than those of usual normal wood, which has a practical function in the living plant: when a conifer shoot is moved from its vertical position, compression wood is formed in the under part of the shoot. The growth rate of the compression wood is faster than in the upper part resulting in a renewed horizontal growth. The actuating and load-carrying function of the compression wood is addressed, on the basis of its special ultrastructure and shape of the tracheids. As a first step, a quantitative model is developed to predict the difference of moisture-induced expansion and axial stiffness between normal wood and compression wood. The model is based on a state space approach using concentric cylinders with anisotropic helical structure for each cell-wall layer, whose hygroelastic properties are in turn determined by a self-consistent concentric cylinder assemblage of the constituent wood polymers. The predicted properties compare well with experimental results found in the literature. Significant differences in both stiffness and hygroexpansion are found for normal and compression wood, primarily due to the large difference in microfibril angle and lignin content. On the basis of these numerical results, some functional arguments for the reason of high microfibril angle, high lignin content and cylindrical structure of compression wood tracheids are supported.

Place, publisher, year, edition, pages
2014. Vol. 185, no 1, 89-98 p.
Keyword [en]
Compression wood, Reaction wood, Dimensional stability, Hygroelastic properties, Modelling
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-219205DOI: 10.1016/j.jsb.2013.10.014ISI: 000330162300010OAI: oai:DiVA.org:uu-219205DiVA: diva2:698890
Available from: 2014-02-25 Created: 2014-02-24 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Structure and Mechanical Behaviour of Wood-Fibre Composites
Open this publication in new window or tab >>Structure and Mechanical Behaviour of Wood-Fibre Composites
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Wood fibres have several advantages compared to man-made synthetic fibres: they have high specific stiffness, are renewable, relatively inexpensive, available in industrial quantities and biodegradable. However, to increase and diversify their utilisation, it is necessary to increase the understanding on what controls their mechanical properties.

In this work, the hygroelastic behaviour of isolated wood fibres has been investigated using an analytical model and a finite element model based on three dimensional images obtained using synchrotron-based X-ray micro-computed tomography. It was thus possible to show how the cell wall responds to a mechanical load or a change in ambient relative humidity.

The wood fibres were then mixed with a biopolymer aiming to produce a cost-efficient, 100% renewable composite material. The microstructure of the produced composites has been characterised using X-ray microtomography and digital image processing. It was for instance possible to measure the moisture-induced swelling of fibres embedded in a polymeric matrix. The experimental results have then been successfully compared with prediction obtained with a finite element model. The length of the fibres inside the composite has also been measured from three dimensional images, aiming to understand how each step of the processing chain is affecting the degradation of the aspect ratio of the reinforcing fibres.

The presence of defects inside the composite has also been quantified using X-ray microtomography. The effects of the defects on the tensile strength have been predicted using an analytical model. The results have been compared with the measured tensile strength on each sample, showing that the size and orientation of the critical defect controls the tensile strength of the material.

Finally, wood-fibre mats without any matrix material were compressed in the chamber of a microtomographic scanner. Sequential images were taken during the test. Using digital volume correlation, it was possible to calculate the local strain field inside the material. The effects of heterogeneities on the strain field have then been investigated. The applied compressive load resulted in transport of material from high to low density regions.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 34 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1158
National Category
Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-229290 (URN)978-91-554-8988-5 (ISBN)
Public defence
2014-09-19, Ångström 4001, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2014-09-15 Created: 2014-08-05 Last updated: 2015-01-22

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Joffre, ThomasGamstedt, Kristofer

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