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Ultrastructural features affecting mechanical properties of wood fibres
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
2006 (English)In: Wood Material Science and Engineering, Vol. 1, 146-170 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this review is to re-examine some of the existing knowledge on the ultrastructure of softwood fibres and modelling of the hygroelastic properties of these fibres. The motivation is that the ultrastructure of wood fibres has a strong influence on fibre properties such as stiffness and hygroexpansion. This structure-property relationship can be modelled with, for instance, composite mechanics to assess the influence of ultrastructure on the fibre properties that in turn control the engineering properties of wood fibre composites and other wood-based materials. Comprehensive information about the ultrastructure is presented that can be useful in modelling the hygroelastic behaviour of wood fibres. Many attempts to model ultrastructure-property relationships that have been carried out over the years are reviewed. Even though models suffer from limiting approximations at some level, they have been useful in revealing valuable insights that can help to clarify experimentally determined behaviour of wood fibres. Still, many modelling approaches in the literature are of limited applicability, not the least when it comes to geometry of the fibre structure. Therefore, an example of finite element modelling of geometrically well-characterized fibres is given. This approach is shown to be useful to asses the influence of the commonly neglected irregular shape on elastic behaviour and stress state in wood fibres. Comparison is also made with an analytical model which assumes cylindrical fibre shape. Predictions of the elastic properties made with analytical modelling of cylindrical fibres and with finite element modelling of geometrically characterized fibres are in concert, but the stress state and failure predictions only show qualitative similarity. It can be concluded that calculations on fibres with the irregular and more realistic geometry combined with experiments on single fibres are necessary for a better and more quantitative understanding of the hygroelastic behaviour and particularly failure of wood fibres. It is hoped that this paper can provide a foundation and an inspiration for modelling, in combination with experiments and microscopy, for better predictions of the mechanical behaviour of wood fibres and wood fibre composites.

Place, publisher, year, edition, pages
2006. Vol. 1, 146-170 p.
Keyword [en]
elastic properties, hygroexpansion, modelling, structure_property relations, ultrastructure, wood fibres.
URN: urn:nbn:se:kth:diva-6099DOI: 10.1080/17480270701195374OAI: diva2:10713
Uppdaterad från manuskript till artikel: 20100914 QC 20100914Available from: 2006-09-12 Created: 2006-09-12 Last updated: 2010-09-14Bibliographically approved
In thesis
1. Hygroelastic behaviour of wood-fibre based materials on the composite, fibre and ultrastructural level
Open this publication in new window or tab >>Hygroelastic behaviour of wood-fibre based materials on the composite, fibre and ultrastructural level
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Wood fibres can be used as reinforcement in plastics for load carrying purposes. Some advantages compared with conventional man-made fibres are that wood fibres come from a renewable resource, have high specific stiffness and strength, are generally less hazardous to health, biodegradable, and can be manufactured at low cost and high volumes. A clear disadvantage with cellulose-based materials for structural use is their dimensional instability in humid environments. The hygroelastic properties are of high importance in materials development of improved wood-fibre composites. This work deals with the stiffness and hygroexpansion of wood fibres for composite materials. The long-term aim is to design engineered wood fibre composites based on better basic knowledge of wood fibres.

Mechanistic models have been used to link the fibrous microstructure with macroscopic composite engineering properties. The properties have been characterized experimentally for various wood-fibre composites and their fibre-mat preforms, by means of curvature measurements at various levels of relative humidity, as well as tensile and compressive tests. From these test results and microstructural characterization, the longitudinal Young’s modulus and transverse coefficient of hygroexpansion of wood fibres were identified by inverse modelling. Some effects of various pulp processes and fibre modifications on the elastic properties of the fibre were observed, illustrating how the mixed experimental-modelling approaches can be used in more efficient materials screening and selection.

An improved micromechanical analysis for wood-fibre composites has been presented. The model is more appropriate to combine with laminate analogy, to link fibre properties on the microscale to the macroscopic composite properties and vice versa. It also offers the possibility to include the effects of ultrastructure since it can account for an arbitrary number of phases. An approach to model ultrastructure-fibre property relations has been demonstrated. It includes analytical modelling of multilayered cylindrical fibres as well as finite element modelling of fibres with irregular geometry characterized with microscopy. Both approaches are useful and could be combined with experiments to reveal insights that can pave way for a firmer link between the wood fibre ultrastructure and wood fibre properties.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. xiii, 31 p.
Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472 ; 0414
wood fibres, ultrastructure, structure-property relations, microfibril angle, composites, characterization methods, hygroelastic properties, micromechanics, modelling, reinforcement potential
National Category
Engineering and Technology
urn:nbn:se:kth:diva-4098 (URN)
Public defence
2006-09-29, E2, Lindstedsvägen 3, Stockholm, 10:00

QC 20100914

Available from: 2006-09-12 Created: 2006-09-12 Last updated: 2013-01-14Bibliographically approved

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Neagu, Cristian R.Gamstedt, Kristofer E.
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