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| 2. |
- Bogren, Karin M., et al.
(författare)
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Dynamic-mechanical properties of wood-fibre reinforced polyactide : experimental characterization and micro-mechanical modelling
- 2006
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Ingår i: Journal of Thermoplastic Composite Materials. - : SAGE Publications. - 0892-7057 .- 1530-7980. ; 19:6, s. 613-638
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Tidskriftsartikel (refereegranskat)abstract
- Wood-fiber reinforced polylactide is a biodegradable compositewhere both fibers and matrix are from renewableresources. When designing new materials of this kind, itis useful to measure the influence of fiber–matrixinterface properties on macroscopic mechanicalproperties. In particular, a quantitative measure of thedynamic stress transfer between the fibers andthe matrix when the material is subjected tocyclic loading would simplify the development of wood-fibercomposites. This is obtained by comparing themechanical dissipation of the composite with avalue predicted by a viscoelastic micromechanical model basedon perfect interfacial stress transfer. Theloss factors predicted by the model are 0.12 and 0.16 at dryand humid conditions, respectively, which amountto 63 and 66% of the experimentally determinedvalues. For Young's moduli the predicted values are 1.01 and0.88 GPa, which correspond to 92% of the experimentallydetermined values. The mismatch between thepredicted and experimental values may be attributed toimperfect interfaces with restrained stress transfer.Loss factors are also determined for specificmolecular bonds using dynamic Fourier transform infrared(FT-IR) spectroscopy. These values show the sametrends with regard to moisture content as themacroscopically determined loss factors.
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| 3. |
- Joffre, Thomas, et al.
(författare)
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Modelling of the hygroelastic behaviour of normal and compression wood tracheids
- 2014
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Ingår i: Journal of Structural Biology. - : Elsevier BV. - 1047-8477 .- 1095-8657. ; 185:1, s. 89-98
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Marklund, Erik, et al.
(författare)
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Stiffness of aligned wood fiber composites: Effects of microstructure and phase properties
- 2008
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Ingår i: Journal of composite materials. - : SAGE Publications. - 0021-9983 .- 1530-793X. ; 42:22, s. 2377-2405
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Neagu, Cristian R., et al.
(författare)
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Modelling of effects of ultrastructural morphology on the hygroelastic properties of wood fibres
- 2007
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Ingår i: Journal of Materials Science. - : Springer Science and Business Media LLC. - 0022-2461 .- 1573-4803. ; 42:24, s. 10254-10274
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Neagu, Cristian R., et al.
(författare)
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Ultrastructural features affecting mechanical properties of wood fibres
- 2006
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Ingår i: Wood Material Science & Engineering. - : Taylor & Francis. - 1748-0272 .- 1748-0280. ; 1:3-4, s. 146-170
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Neagu, R. Cristian, et al.
(författare)
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The potential of wood fibers as reinforcement in cellular biopolymers
- 2012
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Ingår i: Journal of cellular plastics (Print). - 0021-955X .- 1530-7999. ; 48:1, s. 71-103
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Tidskriftsartikel (refereegranskat)abstract
- Wood fiber-reinforced polylactic acid composite foams have been successfully produced using supercritical carbon dioxide. The addition of fibers had a strong effect on microstructure of the foams. An increase in wood fiber content implied smaller average cell size and higher average cell wall thickness as estimated from image analysis of scanning electron microscopy micrographs. Addition of 10 wt% wood fibers seemed to be a limit to obtain foams, with the used processing conditions. The stiffness properties of the foams in compression improved upon addition of wood fibers. A significant increase of specific stiffness was achieved by adding 5-10 wt% wood fibers. It was shown that the stiffness was about 50% higher in the transverse direction for reinforced foams. The strength in the transverse direction increased for foams with unmodified wood fibers but decreased for foams with two types of treated wood fibers as compared with the strength of the pure polylactic acid foam of similar density. A butyl tetracarboxylic acid treatment followed by an additional surfactant treatment results in reduced wood fiber network-forming ability and reduced fiber-matrix adhesion. This contributes to the inferior observed strength properties in this study. The experimental stiffness was comparable with a superposed micromechanical model for a three-phase fiber-reinforced foam. The model shows that increasing the relative density, that is, the ratio of the density of the foam to the density of the composite material, by adding wood fibers results in a noteworthy increase in the transverse compression stiffness of the foams but only at relative density values above 0.2 for the used processing conditions in this study. The key factor for reinforcement is the relation between foam relative density and fiber volume fraction in the preform. The foaming conditions have to be adapted for each wood fiber content to obtain foams with the desired relative density.
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| 8. |
- Tavares da Costa, Marcus Vinicius, 1989-, et al.
(författare)
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Experimental assessment of micromechanical models for fragmentation analysis of thin metal oxide coatings on polymer films under uniaxial tensile deformation
- 2019
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Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 370, s. 374-383
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Tidskriftsartikel (refereegranskat)abstract
- The barrier properties of metal oxide coatings deposited on polymer substrates is of interest in packaging applications for beverages and food. The durability of the coatings is compromised mechanically due to brittleness of the coatings, leading to multiple cracking when deformed. This cracking behavior has been investigated in-situ for atomic layer deposited coatings of TiO2 (6 and 20 nm thick) and mixed oxides of TiO2 and Al2O3 (4 and 20 nm thick) on polyethylene terephthalate substrate films by observing multiple cracking under uniaxial tension. Two key models to identify cohesive and adhesive properties from the fragmentation data have been implemented, and their differences are discussed in light of the new experimental data. Such material properties include the Weibull strength distribution of the coating, the interfacial shear strength, the strength at crack saturation, and fracture toughness at crack onset and at saturation. These properties can be useful for materials selection and possibly for design simulations, when the unaccounted effects like finite deformation and substrate yielding do not take over. A significantly weaker interface and cohesive strength was observed for the thinner mixed-oxide coating, where the dimensions of the material heterogeneities were of the same order as the coating thickness. These dimensions seem to be the limiting factor of how thin coatings can be made without a major loss in structural integrity.
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