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  • 1.
    Blomqvist, Lars
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Johansson, Jimmy
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Sandberg, Dick
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Kifetew, Girma
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    The influence of individual veneer orientation on the shape stability of planar lamination2012In: Current and Future Trends of Thermo-Hydro-Mechanical Modification of Wood. Opportunities for new markets? / [ed] Mathieu Pétrissans and Philippe Gérardin, Nancy, France, 2012, p. 160-162Conference paper (Refereed)
  • 2.
    Bonarski, Jan T.
    et al.
    Polish Academy of Sciences, Poland.
    Kifetew, Girma
    Linnaeus University, Faculty of Technology, Department of Forestry and Wood Technology.
    Olek, Wieslaw
    Poznań University of Life Sciences, Poland.
    Effects of cell wall ultrastructure on the transverseshrinkage anisotropy of Scots pine wood2015In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 69, no 4, p. 501-507Article in journal (Refereed)
    Abstract [en]

    A hypothesis for explaining the differential anisotropicshrinkage behavior of wood has been proposed,and it was based on the differences in the cell wall ultrastructure.The starting point of the consideration is thatwood shrinkage is governed by its chemical composition,ultrastructure, and gross anatomy. It is also well knownthat the transverse shrinkage anisotropy of earlywood(EW) is more pronounced than that of the latewood (LW).In the paper, the cell wall ultrastructure and shrinkageanisotropy has been related to each other, and to thispurpose, a set of crystallographic texture descriptorswas applied. The descriptors are based on X-ray diffraction(XRD) experiments conducted on matched EW samplesfrom different growth rings of Scots pine. The rangeof the microfibril angle (MFA) was identified. The ratio ofthe maxima of inverse pole figures (IPFs) of both the tangential(T) and radial (R) directions was determined. Theratios quantify the inhomogeneity of the spatial arrangementof the ordered areas. The results of the study clearlyindicate that the transverse shrinkage of wood is governedmostly by a specific ultrastructural organization of moderatelyorganized cell wall compounds.

  • 3.
    Bonarski, Jan T.
    et al.
    Polish Academy of Sciences.
    Olek, Wiesław
    Poznań University of Life Sciences.
    Pospiech, Jan
    Polish Academy of Sciences.
    Kifetew, Girma
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Application of spatial distribution of cellulose crystallites for determining wood anisotropy2012In: Workshop micro-characterisation of wood materials and properties: 24–26 October, 2012 Edinburgh, Scotland, UK, 2012, p. 29-30Conference paper (Refereed)
    Abstract [en]

    The anisotropy of wood properties is related to the ultrastructural organization of wood cell walls. The mean microfibril angle (MFA) is the most obvious parameter quantifying the ultrastructure. Various methods for the MFA measurements have been developed. However, the direct microscopic techniques (both light and electron ones) as well as the indirect X-ray methods were dominating. However, the helical arrangement of cellulose fibrils in wood cells around the longitudinal anatomical direction results in spatial changes of orientations of the lattice planes. Such misorientation between the longitudinal anatomical direction and the microfibril axes has a spatial character and therefore, it can not be correctly described by a single parameter only, i.e. by MFA. The most comprehensive description of the spatial distribution of orientations of cellulose crystallites can be obtained when defining a set of parameters consisting of the rotating axis (given by the polar coordinates θ and ψ and referred to the sample framework) as well as the angle of rotation (ω) around the axis. In order to analyze of wood anisotropy a stereographic projection of the rotating axes on the base of the (010) plane of the lattice cell of cellulose is recommended. Regarding the crystallographic system of the monoclinic lattice of cellulose and the two-fold symmetry of the <010> axis, the projection plane corresponded to the a-c plane of the elementary cell [3]. An example of the projection and the distribution of the rotation axis characterizing the spatial organization of wood microfibrils is given in Fig. 1. The θ, ψ and ω parameters were determined with the original computer program SpaceWood. The parameters were determined regarding the crystallographic symmetry and the parameters of the unit-cell of cellulose given by Zugenmaier [3].

  • 4.
    Eliasson, Lars
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Kifetew, Girma
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Volume yield and profit in the production of clear finger-jointed Scots pine (Pinus sylvestrisL.) boards2010In: European Journal of Wood and Wood Products, ISSN 0018-3768, Vol. 68, no 2, p. 189-195Article in journal (Refereed)
  • 5.
    Johansson, Jimmy
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Kifetew, Girma
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    CT-scanning and modelling of the capillary water uptake in aspen, oak and pine2010In: European Journal of Wood and Wood Products, ISSN 0018-3768, Vol. 68, no 1, p. 77-85Article in journal (Refereed)
  • 6. Kifetew, G
    Application of the deformation field measurement method to wood during drying1996In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 30, no 6, p. 455-462Article in journal (Refereed)
    Abstract [en]

    A method for measuring the deformation field on small wood specimens during drying using a regular grid pattern is described. Preliminary results showed a large variation in surface deformation in the range 0% to 6.2%. To explain the transverse deformation field on the radial face, the different deformation ranges were related to the macrostructure of wood. The latewood region was responsible for the higher deformation field range, while the earlywood layer explained the lower strain held range. The method described could provide important information regarding the history of a specimen which has not so far considered in previous studies on the dynamic behaviour of wood.

  • 7. KIFETEW, G
    THE EFFECT OF CROSS GRAIN ON THE TENSILE-STRENGTH OF PINUS-SILVESTRIS1992In: European Journal of Wood and Wood Products, ISSN 0018-3768, E-ISSN 1436-736X, Vol. 50, no 10, p. 410-410Article in journal (Refereed)
  • 8. Kifetew, G
    The influence of the geometrical distribution of cell-wall tissues on the transverse anisotropic dimensional changes of softwood1999In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 53, no 4, p. 347-349Article in journal (Refereed)
    Abstract [en]

    The influence of the geometrical distribution of cell-wall tissues on the transverse anisotropic dimensional changes of softwood has been studied using two isotropic models. A finite element model has been used to calculate the thermal expansion of two aluminium plates without and with holes in the two perpendicular directions. Moisture expansion measurements in the two perpendicular directions were conducted using a polyamide (PA6) plate containing several holes. No differences were found in either the thermal expansion of the aluminium plates or the moisture expansion of the PA6 plate between the two perpendicular directions. Thus, the investigation suggests that the geometrical distribution of cell-wall tissues has no effect on the transverse anisotropic shrinkage of softwood.

  • 9. Kifetew, G
    et al.
    Lindberg, H
    Wiklund, M
    Tangential and radial deformation field measurements on wood during drying1997In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 31, no 1, p. 35-44Article in journal (Refereed)
    Abstract [en]

    In a previous study, the deformation field measurement method on wood during drying was described. This paper discusses the deformation field measurement results during drying to 8.2% moisture content on the radial and tangential surfaces. It also attempts to explain the observations by an approximate expression based on earlywood-latewood interaction theory. The deformation on the radial surface varied between -0.7% and 7.5%. The actual measurements on the radial surfaces support previous work. Deformation measurements on the tangential surfaces were between -0.5% and 9.0%. Although the investigations were carried out on gross wood specimens, the results provide an insight into the extent to which local density variation within the early- and latewood layers may influence the magnitude of surface deformation.

  • 10. Kifetew, G
    et al.
    Sandberg, D
    Material damage due to electron beam during testing in the Environmental Scanning Electron Microscope (ESEM)2000In: Wood and Fiber Science, ISSN 0735-6161, Vol. 32, no 1, p. 44-51Article in journal (Refereed)
    Abstract [en]

    This study describes the development of cell-wall damage, i.e., the creation of cracks across or in the vicinity of pits during the testing of twenty microtomed spruce (Picea abies karst.) samples in the Environmental Scanning Electron Microscope (ESEM). Samples were investigated both in an unloaded condition and under a constant tensile load and at different moisture levels. Regions of the moisture-cycled samples that had been exposed to an electron beam during image acquisition showed damage running through pits and their surroundings. Specimens loaded in the green condition and dried in the chamber for 2 h without beam exposure except during imaging showed no noticeable cell-wall damage. The results indicate that the electron beam may be a major source of damage initiation. Therefore, it is essential to note the circumstances of the rest when explaining the observations made in ESEM studies.

  • 11. Kifetew, G
    et al.
    Thuvander, F
    Berglund, L
    Lindberg, H
    The effect of drying on wood fracture surfaces from specimens loaded in wet condition1998In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 32, no 2, p. 83-94Article in journal (Refereed)
    Abstract [en]

    The study describes the effect of drying on fracture surfaces of Scots pine Pinus silvestris L. Microtomed specimens of isolated and combined early- and latewood, in green and oven-dried/resoaked state were loaded to failure in uniaxial tension parallel to the grain. The fracture surfaces were studied using scanning electron microscopy (SEM). Both green early-and latewood samples showed rough fracture surfaces, which in latewood was dominated by intrawall failure. In the resoaked state, transwall failure dominated and fracture surfaces were more flat, indicating a more brittle fracture process. Although variation in the data was large, the strength of the resoaked samples were generally lower than those of paired green samples. The observations support irreversible cell wall damage formed during drying which severely affects the failure mechanism.

  • 12. Kifetew, Girma
    Some aspects on the deformation behaviour of wood in relation to its structure1996Book (Other academic)
  • 13. Kifetew, Girma
    The effect of cross grain and knots on the tensile strength of Pinus sylvestris 1.1990Licentiate thesis, monograph (Other academic)
  • 14.
    Kifetew, Girma
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Larsson, Hanna
    Wood Technology.
    Axial Twist in Wood: a Comparison between Experimental result and Theoretical Calculation2001In: Proceedings of the First International Conference of the European Society for Wood Mechanics: April 19th-21st, 2001, Lausanne, Switzerland / [ed] P. Navi, Lausanne, Switzerland, 2001, p. 495-503Conference paper (Refereed)
  • 15.
    Kifetew, Girma
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Nilsson, Jonaz
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Sandberg, Dick
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Is it possible to constrain moisture movement of densified wood product mechanically?2011In: Mechano-chemical transformation of wood during THM processing / [ed] Navi, P. & Roth, A, Biel Switzerland: Bern University of Applied Sciencs, Architecture, Wood and Civil Engineering , 2011, p. 67-68Conference paper (Refereed)
    Abstract [en]

    Densification of wood in the transverse direction increases the density. One of the reasons for densifying wood is to produce high quality timber from timbers of low quality. However, densified wood shows an undesired behaviour, i.e. a tendency to return to its initial dimensions when it is subjected to heat and humidity, i.e. shape memory. There are several methods to overcome the problem of shape memory.

    This study uses a three-layered cross-laminated wood panel where one of the layers is of densified wood and the other two are of normal wood together these will mechanically restrain the shape memory of the densified layer.

    The study includes three stages:

    • Densification of clear wood in the radial direction
    • Manufacture of a three-layered cross laminated composite product with densified wood as a service layer
    • Testing of the shape stability when the composite was subjected to variations relative humidity (40-85 % RH at 20°)

    The result of this study reveals the significance of service to bottom layer thickness ratio on the shape stability of the cross laminated composite. Consequently, the performance and the shape stability of the cross laminated composite were significant when the service to bottom layer thickness ratio increases. Therefore, it appears feasible to disclose the appreciable degree of shape stability, hardness and wear resistance of the product. Accordingly, cross laminated composite can be considered as one of the promising mechanical methods for improving moisture movement of densified wood product.

  • 16.
    Nilsson, Jonaz
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Johansson, Jimmy
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Kifetew, Girma
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Sandberg, Dick
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Shape stability of modified engineering wood product (EWP) subjected to moisture variation2011In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 6, no 3, p. 132-139Article in journal (Refereed)
  • 17. Olek, Wiesław
    et al.
    Kifetew, Girma
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Bonanski, Jan T.
    Marklund, Erik
    Effects of cell wall ultrastructure on the transverse anisotropy of wood shrinkage2012In: Experimental and Computational Micro-Characterization Techniques in Wood Mechanics –COST Action FP0802, 2012Conference paper (Refereed)
  • 18.
    Sandberg, Dick
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Azoulay, Michel
    Baudin, Anders
    Blom, Åsa
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Carlsson, Bo
    Linnaeus University, Faculty of Science and Engineering, School of Natural Sciences.
    Eliasson, Lars
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Johansson, Jimmy
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Kifetew, Girma
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Nilsson, Bengt
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Nilsson, Daniel
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Nilsson, Jonaz
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Nordvall, Hans-Olof
    Thörnqvist, Thomas
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Utvändiga träfasader: Inverkan av materialval, konstruktion och ytbehandling på beständigheten hos fasader av gran och tall2011Report (Other academic)
    Abstract [en]

    The external façade must give expression to a building through both design and colour, and it must also protect the insulating layers in the wall from external influences. These functions can be fulfilled by almost all materials. If wood is to be competitive in this context, the wood material, the façade design and the surface treatment system must be chosen and interact in such a way that the façade is given a long life with little need for maintenance. A wooden façade will then in a broad sense be both economically and aesthetically attractive for the user.

    This study illustrates the state of knowledge regarding the outdoor use of pine (Pinus sylvestris L.) and spruce (Picea abies L. Karst.) facings above ground. Specifically, it deals with the use of wooden facings with regard to the choice of material, façade design, surface treatment and recycling. The market demands wooden facing systems, and the requirements emphasized by the actors on the market, e.g. the builders, real estate administrators, architects, designers, frame suppliers, contractors and representatives for the single-family timber housing industry can be summed up as follows:

    • There must be a specified life-time and given time intervals for maintenance of the wooden facings. (Shall be similar to those of competitive materials)
    • The supplier of the facing system should shoulder the long-term responsibility for its maintenance.
    • Flexibility, the supplier shall be able to replace or renovate the facings when necessary.
    • Building requirements, the wooden facing materials must be able to interact with other, specially fire-classified, materials.
    • The facing system shall have an attractive appearance.

    The primary market for the new facing systems should be multi-family houses but not necessarily multi-family houses of wood. The focus shall lie in the flexibility of the facing system in architectural expression, and in relation to other materials and systems. New building is important, but the million program, renovation and additions (ROT) and greater energy efficiency are also important spheres.

    The Swedish market is small (currently ca. 70 000 m3 wood for façades), but it should nevertheless be given priority before the Nordic countries, and thereafter Switzerland, Austria and Germany. The literature describes more or less well-founded recommendations for prolonging the life of wooden facing materials and extending their maintenance intervals, although some of the recommendations are directly conflicting.

    Many details relating to materials choice, façade design and surface treatment are important for the durability of wooden facings. It is difficult to separate the most important factors, but without taking into consideration aspects such as costs, availability and other factors of a practical nature, the following key factors can be identified as important for an environmentally correct and durable façade of pinewood or spruce:

    Choice of material

    • The wood shall have a high proportion of heartwood, preferably exclusively heartwood
    • The wood shall have vertical annual rings.

    Handling from forest to the façade

    • The wood shall be handled so that it does not suffer mechanical damage or microbial attack, or become wet or soiled, i.e. a rapid and correct handling with good packaging.

    Design

    • The façade shall start at least 30 cm above the ground level.
    • The façade shall be ventilated so that moisture can rapidly dry out. Ventilation of the space behind the facing is an easy way of achieving this.
    • Water run-off – no horizontal surfaces.
    • Flexibility –both in the construction and in the architectural design. There is a demand for facing systems which can be simply “hung onto” existing buildings.

    Surface treatment

    • Sealed end-grain sections – sealing of the end-grain surface to prevent moisture absorption into the wood is decisive for the life-time of the wood material. Nailing can open up new end-grain surfaces and should thus be carried out carefully and only after due consideration.
    • Rounded edges – increase the covering ability of paint and reduce the risk of mechanical damage to the facing boards.
    • Choice of surface treatment – vital for the performance of the facings. The wooden facings shall be delivered as part of the complete maintenance package.

    Nowadays there are many types of surface treatment where nano-technology is used to create an added value in a surface compared with what the more traditional products can offer. Nano-based surface-treatment products are already on the market, and they are said to make the surfaces dirt- and water-repellent, to prevent the growth of algae, fungi and moss, to improve UV- and temperature-resistance and colour permanence, to improve scratch- and abrasion-resistance, and to have anti-graffiti qualities etc. However, most of these products are new and for some of them there are still question marks with regard to their long-term performance and technical life-time, as well as their serviceability and thereto related economy seen from a life-cycle perspective for the product or system for which the surface treatment constitutes only a part.

    A cost analysis carried out as a part of the study makes the assessment that the new nano-technology-based surface-treatment systems could lead at most to a reduction of 15 %. in maintenance costs. The assumption is then that the façade needs to be cleaned every fifth or seventh year when a traditional painting system is used.

    According to the Swedish Standard, recovered wood from a wooden façade is defined as tree fuel and is generally designated recycled wood or, when the material is in a disintegrated form, as recycled chips,

    There is a major problem in recovering energy from recycled wood when a part of the material has been treated in some way, e.g. impregnated with a wood-protection agent or surface-treated, or when it contains other design components of e.g. plastic or metal. Recycled chips are a very good fuel for energy recovery provided the plant has adequate flue-gas cleaning and the ash is handled in a correct manner. Contaminated ash constitutes a problem, since it is classified as dangerous waste and cannot therefore be returned to the forest. If the content of heavy metals is not too high, the ash can be used as a covering and filling material. Otherwise, the ash must be deposited as landfill. A better sorting of household waste and an overhaul of the regulations would mean that the cleaned recycled wood could be burned in conventional biofuel boilers and that the contaminated portion could be burned separately.

     

  • 19. Thuvander, F
    et al.
    Kifetew, G
    Berglund, L A
    Modeling of cell wall drying stresses in wood2002In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 36, no 3, p. 241-254Article in journal (Refereed)
    Abstract [en]

    All applications of wood involve drying the material from the green state. The cell wall may be viewed as a laminate consisting of different layers. The layers have different orientations and therefore different moisture expansion characteristics. As a result, stresses will develop in the layers due to drying. Micromechanical models for fibre composite materials were used in combination with a laminate analogy in order to calculate these drying stresses in the cell wall layers S1, S2 and S3. Resulting stresses were very high. In reality viscoelastic effects will significantly reduce stresses at high moisture content. However, at lower moisture content irreversible cell wall damage is likely to form as a result of the stresses computed by the model.

1 - 19 of 19
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