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Structure and Mechanical Behaviour of Wood-Fibre Composites
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
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: urn:nbn:se:uu:diva-229290ISBN: 978-91-554-8988-5 (print)OAI: oai:DiVA.org:uu-229290DiVA: diva2:736240
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
List of papers
1. Swelling of cellulose fibres in composite materials: Constraint effects of the surrounding matrix
Open this publication in new window or tab >>Swelling of cellulose fibres in composite materials: Constraint effects of the surrounding matrix
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2013 (English)In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 74, 52-59 p.Article in journal (Refereed) Published
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:uu:diva-188545 (URN)10.1016/j.compscitech.2012.10.006 (DOI)000314379700008 ()
Available from: 2012-10-17 Created: 2012-12-17 Last updated: 2017-12-06Bibliographically approved
2. Modelling of the hygroelastic behaviour of normal and compression wood tracheids
Open this publication in new window or tab >>Modelling of the hygroelastic behaviour of normal and compression wood tracheids
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.

Keyword
Compression wood, Reaction wood, Dimensional stability, Hygroelastic properties, Modelling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-219205 (URN)10.1016/j.jsb.2013.10.014 (DOI)000330162300010 ()
Available from: 2014-02-25 Created: 2014-02-24 Last updated: 2017-12-05Bibliographically approved
3. Effects of defects on the tensile strength of short-fibre composite materials
Open this publication in new window or tab >>Effects of defects on the tensile strength of short-fibre composite materials
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2014 (English)In: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 75, 125-134 p.Article in journal (Refereed) Published
Abstract [en]

Heterogeneous materials tend to fail at the weakest cross-section, where the presence of microstructural heterogeneities or defects controls the tensile strength. Short-fibre composites are an example of heterogeneous materials, where unwanted fibre agglomerates are likely to initiate tensile failure. In this study, the dimensions and orientation of fibre agglomerates have been analysed from three-dimensional images obtained by X-ray microtomography. The geometry of the specific agglomerate responsible for failure initiation has been identified and correlated with the strength. At the plane of fracture, a defect in the form of a large fibre agglomerate was almost inevitably found. These new experimental findings highlight a problem of some existing strength criteria, which are principally based on a rule of mixture of the strengths of constituent phases, and not on the weakest link. Only a weak correlation was found between stress concentration induced by the critical agglomerate and the strength. A strong correlation was however found between the stress intensity and the strength, which underlines the importance of the size of largest defects in formulation of improved failure criteria for short-fibre composites. The increased use of three-dimensional imaging will facilitate the quantification of dimensions of the critical flaws. 

Keyword
Strength, Agglomeration, X-ray microtomography, Composite materials
National Category
Mechanical Engineering
Research subject
Engineering Science with specialization in Solid Mechanics
Identifiers
urn:nbn:se:uu:diva-229212 (URN)10.1016/j.mechmat.2014.04.003 (DOI)000337983300011 ()
Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2017-12-05Bibliographically approved
4. X-ray micro-computed tomography investigation of fibre length degradation during the processing steps of short-fibre composites
Open this publication in new window or tab >>X-ray micro-computed tomography investigation of fibre length degradation during the processing steps of short-fibre composites
2014 (English)In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 105, 127-133 p.Article in journal (Refereed) Published
Keyword
Polymer-matrix composites (PMCs), . Microstructures, Injection moulding, X-ray tomography
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:uu:diva-229209 (URN)10.1016/j.compscitech.2014.10.011 (DOI)000346943400016 ()
Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2017-12-05Bibliographically approved
5. A method to measure moisture induced swelling properties of a single wood cell
Open this publication in new window or tab >>A method to measure moisture induced swelling properties of a single wood cell
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2016 (English)In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 56, 723-733 p.Article in journal (Refereed) Published
National Category
Applied Mechanics Computational Mathematics
Identifiers
urn:nbn:se:uu:diva-229210 (URN)10.1007/s11340-015-0119-9 (DOI)000375727600005 ()
External cooperation:
Projects
eSSENCE
Available from: 2016-01-05 Created: 2014-08-05 Last updated: 2017-12-05Bibliographically approved
6. A 3D in-situ investigation of the deformation in compressive loading in the thickness direction of cellulose fiber mats
Open this publication in new window or tab >>A 3D in-situ investigation of the deformation in compressive loading in the thickness direction of cellulose fiber mats
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2015 (English)In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 22, no 5, 2993-3001 p.Article in journal (Refereed) Published
Abstract [en]

Fiber mat materials based on cellulose natural fibers combines a useful set of properties, including renewability, stiffness, strength and dielectric insulation, etc. The dominant in-plane fiber orientation ensures the in-plane performance, at the expense of reduced out-of-plane behavior, which has not been studied as extensively as the in-plane behavior. Quantitative use of X-ray micro-computed tomography and strain analyses under in-situ loading open up possibilities to identify key mechanisms responsible for deformation. In the present investigation, focus is placed on the out-of-plane deformation under compressive loading of thick, high density paper, known as pressboard. The samples were compressed in the chamber of a microtomographic scanner. 3D images were captured before and after the loading the sample. From sequential 3D images, the strain field inside the material was calculated using digital volume correlation. Two different test pieces were tested, namely unpolished and surface polished ones. The first principal strain component of the strain tensor showed a significant correlation with the density variation in the material, in particular on the top and bottom surfaces of unpolished samples. The manufacturing-induced grooves generate inhomogeneities in the microstructure of the surface, thus creating high strain concentration zones which give a sensible contribution to the overall compliance of the unpolished material. More generally, the results reveal that, on the micrometer scale, high density fiber pressboard behaves as a porous material rather than a low density fiber network.

National Category
Mechanical Engineering Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-229211 (URN)10.1007/s10570-015-0727-7 (DOI)000361002000011 ()
Funder
Swedish Research Council
Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2017-12-05

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