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Studies of the fibre direction and local bending stiffness of Norway spruce timber: for application on machine strength grading
Linnaeus University, Faculty of Technology, Department of Building Technology.
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Machine strength grading is a production process in the sawmill industry used to grade sawn timber boards into different strength classes with specific characteristic values of the bending strength, modulus of elasticity (MOE) and density. These properties are called grade determining properties. Each of these is predicted on the basis of a statistical relationship between the property and a so-called indicating property (IP), which is based on non-destructively assessed board properties. In most cases, the prediction of strength is crucial for the grading. The majority of commercial grading machines rely on a statistical relationship of strength to an IP, which is either a global dynamic MOE or an averaged flatwise bending MOE measured over a board length of about one meter. The problem of today’s machine strength grading is that the accuracy of the strength prediction is rather poor with a coefficient of determination of about R2 ≈ 0.5 − 0.6. One consequence of this is that much of the strength potential of timber is unused.

The intention of this research is to contribute to a long-term goal, which is development of a method for prediction of bending strength that is more accurate than the methods available today. The research relies on three hypotheses. First, accurate prediction of bending strength can be achieved using an IP that is a localized MOE value (determined over a short length) that represents the lowest local bending stiffness of a board. Second, knowledge of the local bending stiffness with high resolution along a board’s longitudinal direction can be established on the basis of fibre direction within the board in combination with dynamic MOE. Third, fibre directions in the interior of a board can be determined by application of fibre angle models utilizing data of fibre directions on the board’s surfaces obtained from tracheid effect scanning. Following these hypotheses, this work has included laboratory investigations of local material directions, and development of models for fibre directions of the interior of boards. The work also included application of one-dimensional (1D) analytical models and three-dimensional (3D) finite element models of individual boards for the mechanical behaviour, analysis of mechanical response of boards based on experiments and based on the suggested models. Lastly, the suggested models were evaluated by comparisons of calculated and experimentally determined local bending stiffness along boards, and of predicted and experimentally determined bending strength.

The research contributes with in-depth knowledge on local fibre directions close to knots, and detailed information on variation of the local bending stiffness in boards. Moreover, fibre angle models for fibre directions in the interior of boards are presented. By application of the fibre angle models in the 3D model of the whole board, the local bending stiffness along timber boards can be determined over a very short length (l < 50 mm). A comparison with results determined on an experimental basis show a very close similarity implying that the applied models are sufficient to capture the variation of local bending stiffness, caused by knots and fibre distortions, with very high accuracy. Furthermore, it is found that by means of IPs derived using the suggested models, bending strength can be predicted with high accuracy. For a timber sample comprising 402 boards, such IPs results in coefficient of determination as high as R2 = 0.73. However, using IPs based on the 3D finite element model did not improve the R2 value achieved when using the IPs based on the 1D model.

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2018.
Series
Linnaeus University Dissertations ; 307/2018
Keyword [en]
digital image correlation, diving angle, fibre angle, grain angle, indicating property, laser scanning, modulus of elasticity, tracheid effect
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
URN: urn:nbn:se:lnu:diva-69636ISBN: 978-91-88761-13-2 (print)ISBN: 978-91-88761-14-9 (electronic)OAI: oai:DiVA.org:lnu-69636DiVA, id: diva2:1172057
Public defence
2018-02-01, N1017, Hus N, Växjö, 10:00
Opponent
Supervisors
Available from: 2018-01-10 Created: 2018-01-09 Last updated: 2018-01-17Bibliographically approved
List of papers
1. Evaluation of in-plane and out-of-plane fibre direction in Norway spruce using a laboratory scanner utilising the tracheid effect
Open this publication in new window or tab >>Evaluation of in-plane and out-of-plane fibre direction in Norway spruce using a laboratory scanner utilising the tracheid effect
(English)Manuscript (preprint) (Other academic)
National Category
Wood Science
Identifiers
urn:nbn:se:lnu:diva-69632 (URN)
Available from: 2018-01-09 Created: 2018-01-09 Last updated: 2018-02-09
2. Growth layer and fibre orientation around knots in Norway spruce: a laboratory investigation
Open this publication in new window or tab >>Growth layer and fibre orientation around knots in Norway spruce: a laboratory investigation
Show others...
2018 (English)In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 52, no 1, p. 7-27Article in journal (Refereed) Published
Abstract [en]

The strength of structural timber largely depends on the occurrence of knots and on the local material directions in the surroundings of such knots. There is, however, a lack of methods for establishing a full dataset of the local material directions. The present research aims at the development and application of a laboratory method to assess the geometry of growth layers and the orientation of fibres in a high-resolution 3D grid within wood specimens containing knots. The laboratory method was based on optical flatbed scanning and laser scanning, the former resulting in surface images and the latter, utilizing the tracheid effect, resulting in in-plane fibre angles determined in high-resolution grids on scanned surfaces. A rectangular solid wood specimen containing a single knot was cut from a tree in such a way that it could be assumed that a plane of symmetry existed in the specimen. By splitting the specimen through this plane through the centre line of the knot, two new specimens with assumed identical but mirrored properties were achieved. On one of the new specimens, the longitudinal-radial plane was subsequently scanned, and the longitudinal–tangential plane was scanned on the other. Then, by repeatedly planing off material on both specimens followed by scanning of the new surfaces that gradually appeared, 3D coordinate positions along different growth layers and 3D orientation of fibres in a 3D grid were obtained. Comparisons between detected fibre orientation and growth layer geometry were used for the assessment of the accuracy obtained regarding 3D fibre orientation. It was shown that the suggested method is well suited to capture growth layer surfaces and that it provides reliable information on 3D fibre orientation close to knots. Such knowledge is of great importance for understanding the properties of timber including knots. The quantitative data obtained are also useful for calibration of model parameters of general models on fibre orientation close to knots.

Place, publisher, year, edition, pages
Springer, 2018
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-69631 (URN)10.1007/s00226-017-0952-3 (DOI)000419587400001 ()
Available from: 2018-01-09 Created: 2018-01-09 Last updated: 2018-01-18Bibliographically approved
3. Assessment of a Three-Dimensional Fiber Orientation Model for Timber
Open this publication in new window or tab >>Assessment of a Three-Dimensional Fiber Orientation Model for Timber
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2016 (English)In: Wood and Fiber Science, ISSN 0735-6161, Vol. 48, no 4, p. 271-290Article in journal (Refereed) Published
Abstract [en]

Wood is an orthotropic material with very different properties along and across fibers, and every board has its own pattern of knots and fiber deviations. Therefore, detailed knowledge of the three-dimensional (3D) fiber orientation of individual boards would enable more accurate assessment of properties such as stiffness, strength, and shape stability. This paper presents a method for modeling 3D fiber orientation of side boards of Norway spruce. The method is based on dot laser scanning and utilization of the tracheid effect, and it is verified by a comparison between strain fields calculated on the basis of the fiber orientation model and corresponding strains determined using digital image correlation (DIC) technique. By means of the method, it is possible to identify knots and to reproduce the fiber orientation in clear wood in the vicinity of knots. Fiber orientation models of side boards including traversing edge knots were established and integrated in finite element models of boards used for simulation of four-point bending tests. The same boards were also tested in laboratory and displacement fields of the wide faces were recorded at different load levels using DIC technique. Comparisons of strain fields from measurements and simulations showed close agreement, regarding both strain patterns and strain levels. Local strain concentrations caused by very small defects were detected using the models and also found from the laboratory test results. The modeling approach may be used both to achieve improved accuracy of existing machine strength grading methods and, after further development, also for more advanced analysis of eg crack propagation and strength of timber.

National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-57467 (URN)000388430500006 ()
Available from: 2016-10-19 Created: 2016-10-19 Last updated: 2018-01-09Bibliographically approved
4. Local variation of modulus of elasticity in timber determined on the basis of non-contact deformation  measurement and scanned fibre orientation
Open this publication in new window or tab >>Local variation of modulus of elasticity in timber determined on the basis of non-contact deformation  measurement and scanned fibre orientation
Show others...
2015 (English)In: European Journal of Wood and Wood Products, ISSN 0018-3768, E-ISSN 1436-736X, Vol. 73, no 1, p. 17-27Article in journal (Refereed) Published
Abstract [en]

During the last decade, the utilization of non-contact deformation measurement systems based on digital image correlation (DIC) has increased in wood related research. By measuring deformations with DIC systems, surface strain fields can be calculated. The first aim of this study concerns the possibility to detect detailed strain fields along the entire length of a wooden board subjected to pure bending and the potential of using such strain fields to determine a bending modulus of elasticity (MOE) profile along a board. Displacements were measured over 12 subareas along a flat surface of the board. For each such area, a separate local coordinate system was defined. After the transformation of locally measured coordinates to a global system, high resolution strain fields and a corresponding bending MOE profile were calculated. A second method in establishing bending MOE profiles is to use fibre angle information obtained from laser scanning and a calculation model based on integration of bending stiffness over board cross sections. Such profiles have recently been utilized for accurate strength grading. A second aim of this study was to investigate the accuracy of the bending MOE profiles determined using the latter method involving fibre angle information. Bending MOE profiles determined using the two described methods agree rather well. However, for some patterns of knot clusters, the local bending MOE, calculated on the basis of fibre angles and integration of bending stiffness, is overestimated. Hence, this research adds knowledge that may be utilized to improve the newly suggested strength grading method.

Place, publisher, year, edition, pages
Springer, 2015
National Category
Construction Management
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-38089 (URN)10.1007/s00107-014-0851-3 (DOI)000347688100002 ()2-s2.0-84920552031 (Scopus ID)
Available from: 2014-11-07 Created: 2014-11-07 Last updated: 2018-01-09Bibliographically approved
5. Modelling local bending stiffness based on fibre directions in sawn timber
Open this publication in new window or tab >>Modelling local bending stiffness based on fibre directions in sawn timber
(English)Manuscript (preprint) (Other academic)
National Category
Wood Science
Identifiers
urn:nbn:se:lnu:diva-69634 (URN)
Available from: 2018-01-09 Created: 2018-01-09 Last updated: 2018-02-09
6. Local bending stiffness for prediction of bending strength: Evaluation of models and concept
Open this publication in new window or tab >>Local bending stiffness for prediction of bending strength: Evaluation of models and concept
(English)Manuscript (preprint) (Other academic)
National Category
Wood Science
Identifiers
urn:nbn:se:lnu:diva-69635 (URN)
Available from: 2018-01-09 Created: 2018-01-09 Last updated: 2018-02-09

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