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Influence of wood-fibre hygroexpansion on the dimensional instability of fibre mats and composites
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
2005 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, Vol. 36, no 6, 772-788 p.Article in journal (Refereed) Published
Abstract [en]

Wood fibres are gaining increased use as reinforcement in plastics. One of the disadvantages of cellulose-based fibres is their propensity to absorb water and swell. This inevitably leads to undesired dimensional instability of the composite and its fibre-mat preform. A measure of the hygroexpansion behaviour of the fibres could serve to rank the suitability of different kinds of cellulosic fibres with regard to dimensionally stable composites. A method has been developed to determine the hygroexpansion coefficient of wood fibres. Since fibre mats manufactured with conventional techniques generally have a thickness gradient of fibre orientation, fibre mats and composites will curl if the moisture content varies. Models based on laminate mechanics and micromechanics of hygroexpansion have been used to estimate the transverse hygroexpansion coefficient of the fibres from experimental results of curvature and thickness changes, and tensile stiffness of fibre mats based on unbleached sulphate cooked softwood. The fibre orientation distribution through the fibre-mat thickness was characterized with a tape-splitting technique and image analysis. The transverse hygroexpansion coefficient of the wood fibres was found to depend on the assumed ratios of the elastic parameters of the transversely isotropic fibres. Values in the range of 0.10-0.15 strain per relative change in moisture content were found for plausible degrees of elastic anisotropy of the fibres. Means of how to suppress dimensional instability of cellulose based composites and fibre mats are addressed.

Place, publisher, year, edition, pages
2005. Vol. 36, no 6, 772-788 p.
Keyword [en]
A. Fibres, A. Preform, A. Wood, C. Micro-mechanics, Hygroinstability
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-6095DOI: 10.1016/j.compositesa.2004.10.023ISI: 000229063300006Scopus ID: 2-s2.0-15344347925OAI: oai:DiVA.org:kth-6095DiVA: diva2:10709
Note
QC 20100920Available from: 2006-09-12 Created: 2006-09-12 Last updated: 2010-09-20Bibliographically 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.
Series
Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472 ; 0414
Keyword
wood fibres, ultrastructure, structure-property relations, microfibril angle, composites, characterization methods, hygroelastic properties, micromechanics, modelling, reinforcement potential
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-4098 (URN)
Public defence
2006-09-29, E2, Lindstedsvägen 3, Stockholm, 10:00
Opponent
Supervisors
Note

QC 20100914

Available from: 2006-09-12 Created: 2006-09-12 Last updated: 2013-01-14Bibliographically approved
2. Hygroelastic properties of wood fibres for composite applications
Open this publication in new window or tab >>Hygroelastic properties of wood fibres for composite applications
2004 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

Wood pulp fibres are gaining increased use for other applications than paper and board. Wood fibres can also 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 composite materials. Mechanistic models have been used to link the fibrous microstructure with these macroscopic engineering properties. The properties have been characterised 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, the contribution of the fibres to the stiffness and hygroexpansion of composites were determined. These approaches can be used to rank different candidate fibres according to their potential as reinforcement in dimensionally stable and stiff composites. With straightforward macroscopic test methods 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, which illustrate how the mixed experimental-modelling approaches can be used in quantitative and more efficient materials development.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2004
Series
Trita-HFL, ISSN 1104-6813 ; 0381
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-479 (URN)
Presentation
2004-12-16, Q2, 13:00 (English)
Opponent
Supervisors
Available from: 2004-11-08 Created: 2004-11-08 Last updated: 2012-03-22Bibliographically approved

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