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A 3D in-situ investigation of the deformation in compressive loading in the thickness direction of cellulose fiber mats
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
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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.

Place, publisher, year, edition, pages
2015. Vol. 22, no 5, 2993-3001 p.
National Category
Mechanical Engineering Applied Mechanics
URN: urn:nbn:se:uu:diva-229211DOI: 10.1007/s10570-015-0727-7ISI: 000361002000011OAI: oai:DiVA.org:uu-229211DiVA: diva2:736192
Swedish Research Council
Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2015-11-30
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.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1158
National Category
Applied Mechanics
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)
Available from: 2014-09-15 Created: 2014-08-05 Last updated: 2015-01-22

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Joffre, ThomasGamstedt, E. Kristofer
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