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Hygroelastic behaviour of wood-fibre based materials on the composite, fibre and ultrastructural level
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
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 [en]
wood fibres, ultrastructure, structure-property relations, microfibril angle, composites, characterization methods, hygroelastic properties, micromechanics, modelling, reinforcement potential
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-4098OAI: oai:DiVA.org:kth-4098DiVA: diva2:10715
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
List of papers
1. Influence of wood-fibre hygroexpansion on the dimensional instability of fibre mats and composites
Open this publication in new window or tab >>Influence of wood-fibre hygroexpansion on the dimensional instability of fibre mats and composites
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.

Keyword
A. Fibres, A. Preform, A. Wood, C. Micro-mechanics, Hygroinstability
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-6095 (URN)10.1016/j.compositesa.2004.10.023 (DOI)000229063300006 ()2-s2.0-15344347925 (Scopus ID)
Note
QC 20100920Available from: 2006-09-12 Created: 2006-09-12 Last updated: 2010-09-20Bibliographically approved
2. Characterization methods for elastic properties of wood fibers from mats for composite materials
Open this publication in new window or tab >>Characterization methods for elastic properties of wood fibers from mats for composite materials
2006 (English)In: Wood and Fiber Science, ISSN 0735-6161, Vol. 38, no 1, 95-111 p.Article in journal (Refereed) Published
Abstract [en]

 Wood fibers offer excellent specific properties at low cost and are of interest as reinforcement in composites. This work compares two alternative test methods to determine the stiffness of wood fibers from simple macroscopic tests oil fiber mats. One method is compression of the fiber trial in the thickness direction, which uses a statistical micromechanical model based oil first-order beam theory to describe the deformation. The other method is tensile testing of fiber trials and back calculation of the fiber stiffness with I laminate model. Experiments include compression tests and tensile stiffness index tests as well as determination of fiber content, orientation, and dimensional distribution. For trials with unbleached softwood kraft fibers, all effective value of the Young's modulus of 20.1 GPa determined by the compression method call be compared with values of 17.4-19.0 GPa obtained from tensile tests. These are ill agreement with values for similar cellulosic fibers found in literature. The compression method is more appropriate for low-density fiber mats, while the tensile test works better for well-consolidated high-density fiber mats. The two methods have different ranges of applicability and are complementary to one another. Limitations of the methods are also discussed. The main advantage of the methods is that they are quantitative. The potential as stiffening reinforcement of various types of Fibers can be systematically investigated, even if the fiber mat microstructures are different.

Keyword
wood fiber, stiffness, fiber trial, test methods, composites
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-6096 (URN)000235272600011 ()2-s2.0-33644673173 (Scopus ID)
Note
QC 20100902Available from: 2006-09-12 Created: 2006-09-12 Last updated: 2010-09-02Bibliographically approved
3. Stiffness contribution of various wood fibers to composite materials
Open this publication in new window or tab >>Stiffness contribution of various wood fibers to composite materials
2006 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Journal of Composites Materials, Vol. 40, no 8, 663-699 p.Article in journal (Refereed) Published
Abstract [en]

Wood pulp fibers can serve as useful reinforcement of plastics for increased stiffness. To assess the potential of various wood fibers as reinforcement, a method has been developed to determine the contribution of the fibers to the elastic properties of the composite. A micromechanical composite model and classical laminate mechanics are used to relate the elastic properties of the fibers to the elastic properties of the composite. A large variety of composites made of various wood pulp fibers in an epoxy vinyl ester matrix was manufactured. From the tensile test results of the composites, the contributing Young's moduli of the fibers in the longitudinal direction are back-calculated and summarized. One finding is that there is an optimum in fiber stiffness as a function of lignin content. It is also found that industrially pulped hardwood fibers have higher stiffness than the corresponding softwood fibers. One example is kraft-cooked Norway spruce fiber, for which a Young's modulus of 40 GPa is found. The effects of hornification, prehydrolysis, and sulfite processing are also investigated. The results indicate that mild defibration process should be used, that does not damage the cell wall structure so that the inherent high stiffness of the native fibers can be retained. It can be concluded that the proposed method works well to rank the wood fiber candidates in terms of their contribution to the composite stiffness.

Keyword
wood fiber composites, elastic properties, micromechanics, stiffness characterization
National Category
Wood Science Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-6097 (URN)10.1177/0021998305055276 (DOI)000237196900001 ()2-s2.0-33645744206 (Scopus ID)
Note
QC 20100914Available from: 2006-09-12 Created: 2006-09-12 Last updated: 2010-09-14Bibliographically approved
4. Stiffness of aligned wood fiber composites: Effects of microstructure and phase properties
Open this publication in new window or tab >>Stiffness of aligned wood fiber composites: Effects of microstructure and phase properties
2008 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 42, no 22, 2377-2405 p.Article in journal (Refereed) Published
Abstract [en]

The effect of wood fiber anisotropy and their geometrical features on wood fiber composite stiffness is analyzed. An analytical model for an N-phase composite with orthotropic properties of constituents is developed and used. This model is a straightforward generalization of Hashin's concentric cylinder assembly model and Christensen's generalized self-consistent approach. It was found that most macro-properties are governed by only one property of the cell wall which is very important in attempts to back-calculate the fiber properties. The role of lumen (whether it filled by resin or not) has a very large effect on the composite shear properties. It is shown that several of the unknown anisotropic constants characterizing wood fiber are not affecting the stiffness significantly and rough assumptions regarding their value would suffice. The errors introduced by application of the Hashin's model and neglecting the orthotropic nature of the material behavior in cylindrical axes are evaluated. The effect of geometrical deviations from circular cross-section, representing, for example, collapsed fibers, is analyzed using the finite element method (FEM) and the observed trends are discussed.

Keyword
Anisotropic, Composite, Self-consistent, Transverse properties, Wood fiber
National Category
Mechanical Engineering Wood Science
Identifiers
urn:nbn:se:kth:diva-6098 (URN)10.1177/0021998308095886 (DOI)000259686200004 ()
Note
Uppdaterad från manuskript till artikel: 20100914 QC 20100914Available from: 2006-09-12 Created: 2006-09-12 Last updated: 2010-09-14Bibliographically approved
5. Ultrastructural features affecting mechanical properties of wood fibres
Open this publication in new window or tab >>Ultrastructural features affecting mechanical properties of wood fibres
2006 (English)In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 1, no 3-4, 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
Taylor & Francis, 2006
Keyword
elastic properties, hygroexpansion, modelling, structure_property relations, ultrastructure, wood fibres.
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-6099 (URN)10.1080/17480270701195374 (DOI)
Note

Uppdaterad från manuskript till artikel: 20100914 QC 20100914

Available from: 2006-09-12 Created: 2006-09-12 Last updated: 2016-12-12Bibliographically approved
6. Modelling of effects of ultrastructural morphology on the hygroelastic properties of wood fibres
Open this publication in new window or tab >>Modelling of effects of ultrastructural morphology on the hygroelastic properties of wood fibres
2007 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 42, no 24, 10254-10274 p.Article in journal (Refereed) Published
Abstract [en]

Wood fibres constitute the structural framework of e.g. wood, paper, board and composites, where stiffness and dimensional stability are of importance. An analytical modelling approach has been used for prediction of hygroelastic response, and assessment of the stresses in thick-walled cylinder models of wood fibres. A wood fibre was idealised as a multilayered hollow cylinder made of orthotropic material with helical orientation. The hygroelastic response of the layered assembly due to axisymmetric loading and moisture content changes was obtained by solving the corresponding boundary value problem of elasticity. A simple solution scheme based on the state space approach and the transfer matrix method was employed. This was combined with an analytical ultrastructural homogenisation method, used to link hygroelastic properties of constituent wood polymers to properties of each layer. Predicted hygroelastic response captured experimentally measured behaviour. Fibres that were constrained not to twist showed a stiffer response than fibres allowed twisting under uniaxial loading. It was also shown that the ultrastructure, i.e. the microfibril angle, will control the hygroexpansion in the same way as it controls the compliance of the cell wall. Qualitative failure trends comparable with experimental observations could be established with stress analysis and a simple plane-stress failure criterion.

Keyword
norway spruce tracheids, carbon nanotube arrays, state-space formalism, southern pine fibers, mechanical-properties, cell-wall, composite-materials, moisture-content, elastic-modulus, part i
National Category
Wood Science Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-6100 (URN)10.1007/s10853-006-1199-9 (DOI)000249980100050 ()2-s2.0-35148830728 (Scopus ID)
Note
Uppdaterad från manuskript till artikel: 20100914 QC 20100914Available from: 2006-09-12 Created: 2006-09-12 Last updated: 2010-09-14Bibliographically approved

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