共查询到20条相似文献，搜索用时 31 毫秒
1.
《Wood material science & engineering》2013,8(3-4):146-170
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. 相似文献
2.
Graham A. Ormondroyd S. K. Källbom S. F. Curling B. K. Stefanowski B. K. Segerholm M. E. P. Wålinder 《Wood material science & engineering》2017,12(4):203-210
During fibre processing, wood fibres are subjected to a range of physical and chemical conditions sufficient to slightly alter their chemical composition and hence their ultimate performance when used in the manufacture of wood fibre-based composites. In order to better understand the effects of refiner conditions on material performance, wood fibres were subjected to processing at different refiner pressures (4, 6, 8 and 10 bar) and subsequently dried in a flash drier. The fibres were analysed for changes in surface area, surface energy, surface structure and water vapour sorption characteristics. The methods applied were nitrogen adsorption utilising the Brunauer–Emmett–Teller theory, inverse gas chromatography, scanning electron microscopy and dynamic vapour sorption. It was found that increasing refiner pressure resulted in fibres of lower surface area, accompanied by increasing dispersive surface energies up to operating refiner pressures of 8 bar. It was found with fibres refined at different pressures that as the refiner pressure increased the equilibrium moisture content of the fibre decreased at the set relative humidities. However, it was also noted that the hysteresis was not significantly different between each of the refiner pressures. The results suggest that different refiner pressures can be used to tune the surface characteristics which may be beneficial to product development and the improvement of the environmental profile of the wood fibre composites. 相似文献
3.
4.
We compared the chemical composition of wood fibres and fibre surfaces of several eucalypt species and hybrids originating
from various growth sites in South Africa. The objective was to test for differences in chemical surface composition due to
genetics or site with the ultimate aim to facilitate a tailor-made supply of wood for pulping that results in an optimal blend
of fibres that can be pulped together with similar yields. This, however, requires a sound knowledge of the fibre properties.
The surface functionality on the single fibre level is a key property, because it determines how good inter-fibre bonding
will be when paper is formed, which depends amongst other fibre properties on the amount of free hydroxyl groups that are
available and therefore on the cellulose content on the fibre surface. The cellulose and lignin content on the fibre surface
were determined with chemical force microscopy, a variation of atomic force microscopy. Since the general bulk composition
of the fibre and the surface composition might differ, both parameters were determined. We found significant differences in
the cellulose and lignin content on fibre surfaces, with regard to genotype and site, respectively. In some, but not all,
cases, the surface composition of wood fibres followed the bulk composition, and differences were generally more pronounced.
Differences due to genotype were significant, especially with regard to the surface lignin content—but variation due to site
was also distinctly recognisable. This variation in surface functionality could be the reason why some pulpwood blends result
in a lower pulp yield and different quality. 相似文献
5.
Anne-Mari Olsson Lennart Salmén Michaela Eder Ingo Burgert 《Wood Science and Technology》2007,41(1):59-67
In order to investigate the way in which fibre properties affect the mechano-sorptive creep phenomenon in paper, single wood
fibres were exposed to tensile stresses at a constant humidity of 80% relative humidity (RH) and in a cyclic humidity environment
varying between 80 and 30% RH. Contrary to earlier claims, it was demonstrated that single wood fibres exposed to a cyclic
RH show a considerably higher creep than that corresponding to the highest RH experienced in the cycle, i.e., a mechano-sorptive
behaviour. The creep strain rate at cyclic humidity was shown to be a function of the creep rate at constant climate, and
to be an apparent linear function of the applied stress. 相似文献
6.
In recent years there has been interest in using flax fibres to produce composites because of a number of attributes, including low density, biodegradability and high mechanical properties. It was found that treatment of flax fibres may be required to improve the bond quality with a resin. These treatments also have an impact on the properties of the fibres themselves. The objective of this project was to evaluate the impact of three treatment methods on the mechanical properties of flax fibres. The three treatment methods were alkaline, enzyme and steam-heat. After treatment, flax fibres were tested in tension using a universal test machine. Results showed that tensile strength and Young's modulus of flax fibre can be enhanced significantly by the three treatment methods, compared with untreated flax fibres. Enzyme treatment was shown to be the best approach to improve mechanical properties of flax fibre than alkaline and steam-heat treatment. 相似文献
7.
Summary The applicability of models for describing the short-term properties of chipboard were investigated using the results from experiments conducted on samples of a laboratory-made chipboard (comprised of Scots pine chips and urea formaldehyde resin), and sections of urea formaldehyde and Scots pine strips.The rule of mixtures equations, modified for short fibre composites, were employed to predict the strength and stiffness of chipboard. They were found to provide a good estimate of both the chipboard strength and modulus of elasticity, with the assumptions being made that the chips were aligned parallel to the direction of applied stress and that the chips could be treated as having cylindrical geometry.The results of this investigation revealed that chipboard is a highly unusual material. Compared to conventional fibre-reinforced composites, chipboard has a very high volume fraction of fibres, and the similar moduli of elasticity of the fibres and matrix can result in the concentration of stress in the matrix rather than in the fibres. However, these factors did not prevent the assessment and development of appropriate short fibre composite models to predict the strength and stiffness of chipboard. 相似文献
8.
During refining of mechanical pulp, a process occurring at high speed at temperatures of 140–160°C, the flexibility and bonding
ability of wood fibres are increased. To understand the mechanical behaviour of the fibres in this operation, deformations
at low speed of wet wood specimens at 50°C were studied under two different combinations of shear and compression loadings.
The results were compared with the behaviour of wet wood in pure compression. Some features of the deformation that occurred
in earlywood were analysed using an image analysis procedure. During pure compression the cell walls bend independently of
the shape of the fibre cells and their cell wall thickness. Under combined shear and compression, however, mainly the corners
of the fibre cells get deformed. In a second deformation performed in compression, the fibre cells follow the same deformation
pattern as given by the first deformation type whether in compression or in combined shear and compression. The interpretation
was that permanent defects in the cells themselves were introduced already in the first load cycle of the wood samples. The
load combination with lower shear gave the same permanent strain as the case of pure compression but using less energy. 相似文献
9.
Michaela Eder Olivier Arnould John W. C. Dunlop Joanna Hornatowska Lennart Salmén 《Wood Science and Technology》2013,47(1):163-182
The properties of wood and wood-based materials are strongly dependent on the properties of the fibres, that is, the cell wall properties. It is thus highly important to be able to mechanically characterise cell walls in order to understand structure–property relationships. This article gives a brief overview of the state of the art in experimental techniques to characterise the mechanical properties of wood at both the level of the single cell and that of the cell wall. Challenges, opportunities, drawbacks and limitations of single fibre tensile tests and nanoindentation are discussed with respect to the wood material properties. 相似文献
10.
Ulrich Müller Thomas Jost Christian Kurzböck Alexander Stadlmann Wolfgang Wagner Stefan Kirschbichler 《Wood material science & engineering》2020,15(5):312-324
ABSTRACT Multi material mix is a promising approach to reduce weight and the carbon footprint in automotive engineering at competitive costs. As a result, automotive industry is getting more venturous, exploring and applying other materials than metals and plastics – e.g. fibre reinforced plastics (FRPs). In this context, engineered wood products (EWPs) and wood composites should be considered: Wood composites provide high stiffness, strength, excellent damping, high resistance against fatigue and a very low density at low material costs. It is hypothesized that modern wood composites are competitive to metals and artificial fibre-reinforced materials when designed and applied properly. The application of wood and wood composites in automotive engineering calls for precise and reliable material data, required for initial material selection and later in numerical simulation. In this study, a material model normally used for modelling FRPs was adopted. A material database was generated for three hardwood species, to establish the required material parameters and validate material model. Results prove that wooden components can be simulated in crash situations and the selected material model is applicable, even in full vehicle simulation. 相似文献
11.
The fracture behaviour of single wood fibres is governed by geometrical constraints: in situ ESEM studies on three fibre types 总被引：1，自引：1，他引：0
In situ tensile tests were performed in an environmental scanning electron microscope (ESEM) on earlywood, transition wood
and latewood cells of Norway spruce (Picea abies [L.] Karst.). In order to examine the single wood fibres in a wet state, a specially designed tensile testing stage with
a cooling device was built. The fracture behaviour of the cell types was studied at high resolution while straining. Different
failure mechanisms were observed for the three tissue types. The thin-walled earlywood fibres showed tension buckling which
gave rise to crack initiation and resulted in low tensile strength, whereas thick-walled latewood fibres predominately failed
by transverse crack propagation without fibre folding. 相似文献
12.
Stress model of a wood fibre in relation to collapse 总被引：1，自引：0，他引：1
T. C. Innes 《Wood Science and Technology》1995,29(5):363-376
A wood fibre cell from a Tasmanian Eucalypt is typically cylindrical in shape with a length to diameter ratio of approximately 501. Early in the process of seasoning for solid timber, when the fibre lumens are still saturated, internal tension within a fibre can rise to a value high enough to cause it to physically flatten, or collapse. A stress model of a fibre cell has been developed which predicts the stress and strain distributions within the fibre wall as a function of temperature, moisture content, and fibre wall strength properties and size in the early stages of drying. This model will be used together with measurement of the behaviour of collapse prone timbers to determine conditions which will avoid collapse during seasoning.The author is pleased to acknowledge the assistance of Emeritus Professor A. R. Oliver, Associate Professor P. E. Doe, University of Tasmania, and the Australian Furniture Research and Development Institute 相似文献
13.
QIN Te-fu HUANG Luo-hua LI Gai-yun 《林业研究》2005,16(3):241-244
对以铝酸酯为偶联剂对木粉进行表面改性处理后制备的木粉/聚丙烯复合材料的力学性能和形态学特征进行了研究。结果表明:铝酸酯偶联剂可以增加木塑复合材料的抗冲击强度,但会对复合材料的抗拉强度和抗弯强度造成负面的影响。对木塑复合材料的动态力学性能和微分扫描热量分析研究表明,以铝酸酯作为偶联剂,对木塑复合材料的储存模量和损失模量有少许增加,同时可降低材料的熔点和熔解热。利用扫描电镜观察木塑复合材料的木材与塑料界面发现,经铝酸酯处理过的木材与聚丙烯复合界面之间具有更好的相容性。这些研究结果表明,在木塑复合材料制造过程中利用廉价的铝酸酯作为木材化学改性剂,对改善复合材料的性质同样起作良好的作用。图6 表2 参16。 相似文献
14.
The fibrillar orientation in the S2-layer of wood fibres as determined by X-ray diffraction analysis
Summary It is the fibrillar orientation in the S2-layer which to a great extent determines the mechanical properties of the wood fibre, with regard both to strength and stiffness and to swelling properties. Measurements of the average fibril angle of fibres are not however easy and the results differ between the methods used. In order to evaluate in more detail how the fibril angle varies in spruce wood, an X-ray method based on diffraction from the 040-plane was developed. By comparison with microscopic examination it is concluded that reliable results relating to the fibrillar orientation in the S2-layer are obtained with the X-ray technique. It is shown that the fibril angle of mature wood is rather constant with regard to both age of the annual ring and its position in the height of the tree. The fibril angle of the earlywood is found to be only slightly higher than that of latewood fibres. It is also shown that compression wood may be easily identified by virtue of the fact that its fibril angle is much higher than that of normal mature wood.The authors thank Ms Ulla Jonsson for the microscopic measurements and Dr Anthony Bristow for the linguistic revision 相似文献
15.
A coupled experimental and numerical modelling approach was used to investigate the mechanism of softwood fracture at the fibre level. First, a three-dimensional mixed lattice-continuum fracture model was developed to investigate the mechanism of wood fracture, taking into account the porosity of its structure and its heterogeneities at the fibre level. The critical volumes in the specimen where crack propagation was more probable were modelled by a lattice that could show the alternation of earlywood and latewood fibres, and the other regions were considered as the continuum medium. The proposed model was used to investigate the mode I fracture of a small softwood sample in RL orientation. Secondly, a method was developed for microscopic observation of the crack trajectory and investigating the mechanism of initiation and propagation of cracks. This approach was used for microscopic investigation of the fracture behaviour of spruce specimens in mode I and RL orientation. The results of the numerical study were compared with the experimental results. The prepeak and postpeak behaviour of the obtained stress-displacement curve and also the crack opening trajectory in cross-section and longitudinal section in the model and experiments were in good agreement. Both the model and the microscopic observation showed that in mode I fracture and RL orientation, the main trajectory of the crack propagates in the earlywood ring. 相似文献
16.
圆筒形单板层积材(简称圆筒LVL)是一种新型木质工程材料，它是以螺旋缠绕的方式将单板加工成交错层积的构造，从而抑制弹性模量的下降。圆筒LVL具有很好的力学性能和优良的工程性能，可广泛应用于建筑等领域。本文就圆筒LVL的制造方法、研究状况、特点及用途等进行了较为详细的归纳总结。 相似文献
17.
R. P. Kibblewhite 《Wood Science and Technology》1980,14(2):143-158
Summary A requirement of long-term research on pulp fibres in that the material for study be stored for prolonged periods without deterioration and without changes in properties. In this paper effects of pulp freezing and thawing and of frozen pulp storage on fibre, wet web, and handsheet properties are discussed. A variety of radiata pine kraft pulps, a radiata pine sodium bisulphite pulp, and silver beech and hard beech (Nothofagus species) kraft pulps are examined.The expanded walls and diameters of beaten fibres were contracted by pulp freezing. This behaviour made fibres less flexible and less able to collapse during papermaking operations. The freezing treatment also caused fibre kinks and other fibre configurations which existed in a pulp before freezing to be fixed into position and made somewhat resistant to straightening when in strained wet webs. It was found that extents of fibre kink can be varied depending on the degree to which fibre configurations are forced into a pulp network before freezing. Increasing periods of frozen storage caused the intensity and distribution of bonds redeveloped by the freezing treatment to be progressively modified. Fibre walls were, however, not contracted further by increasing periods of frozen storage.The technical assistance of Miss D. Brookes is gratefully acknowledged 相似文献
18.
19.
《Wood material science & engineering》2013,8(3-4):118-129
Abstract Twist in wood, being closely related to spiral grain, may cause serious problems in building structures, furniture and joinery. It is therefore of great interest to sort out, at an early stage in the manufacturing process, trees, logs and boards that have an excess of spiral grain. The spiral grain pattern is described by a helical deviation of the fibre direction in relation to the longitudinal direction of a living tree or a log and seems to be an indicator for other defects such as compression wood. Remote microwave sensing of spiral grain has received a lot of interest during the past two decades. Its development has been impeded by the large variation with moisture content of the microwave properties of wood and by the complexity in modelling the electromagnetic field in a log with spiral grain. A review is presented of a direct method with no requirement for information on moisture content for boards. This procedure has recently been generalized to cylindrical logs and trees having a constant slope of the grain. A further generalization is presented here to allow for the normal spiral grain pattern with radially changing slope of grain in wood under bark. Based on this theory, a measurement procedure is proposed for the detection of wood grain angle with radial dependence, requiring no information on moisture content in the sapwood, which is also applicable to completely or partially frozen wood. A suitable application would be an instrument to use in the forest for measurements on living trees or logs. 相似文献
20.
《Wood material science & engineering》2013,8(2):52-58
Abstract A coupled experimental and numerical modelling approach was used to investigate the mechanism of softwood fracture at the fibre level. First, a three-dimensional mixed lattice–continuum fracture model was developed to investigate the mechanism of wood fracture, taking into account the porosity of its structure and its heterogeneities at the fibre level. The critical volumes in the specimen where crack propagation was more probable were modelled by a lattice that could show the alternation of earlywood and latewood fibres, and the other regions were considered as the continuum medium. The proposed model was used to investigate the mode I fracture of a small softwood sample in RL orientation. Secondly, a method was developed for microscopic observation of the crack trajectory and investigating the mechanism of initiation and propagation of cracks. This approach was used for microscopic investigation of the fracture behaviour of spruce specimens in mode I and RL orientation. The results of the numerical study were compared with the experimental results. The prepeak and postpeak behaviour of the obtained stress–displacement curve and also the crack opening trajectory in cross-section and longitudinal section in the model and experiments were in good agreement. Both the model and the microscopic observation showed that in mode I fracture and RL orientation, the main trajectory of the crack propagates in the earlywood ring. 相似文献