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Strength grading of structural timber and EWP laminations of Norway spruce: Development potentials
SP Sveriges Tekniska Forskningsinstitut.ORCID iD: 0000-0002-8513-0394
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Strength grading of structural timber is a process by which value is added to sawn products. It is to the greater part carried out using machine grading based on statistical relationships between so called indicating properties and bending strength. The most frequently applied indicating property (IP) on the European market is the stiffness in terms of average modulus of elasticity (MOE) of a timber piece, although MOE is a material property that varies within timber.

A major limitation of today’s grading methods is that the described relationships are relatively poor, which means that there is a potential for more accurate techniques. The main purpose of this research has been to initiate development of more accurate and efficient machine grading methods.

Strength of timber is dependent on the occurrence of knots. At the same time, knot measures applied as indicating properties until today have shown to be poor predictors of strength. However, results from this research, and from previous research, has shown that not only size and position of knots but also fibre deviations in surrounding clear wood are of great importance for local stiffness and development of fracture under loading. Thus, development of new indicating properties which take account of knots as well as properties of surrounding fibres, determined on a very local scale, was considered as a possible path towards better strength grading.

In the research, results from contact-free deformation measurements were utilized for analysis of structural behaviour of timber on both local and global level. Laser scanning was used for detection of local fibre directions projected on surfaces of pieces. Scanned information, combined with measures of density and average axial dynamic MOE, was applied for calculation of the variation of local MOE in the longitudinal board direction. By integration over cross-sections along a piece, a stiffness profile in edgewise bending was determined and a new IP was defined as the lowest bending MOE along the piece.

For a sample of Norway spruce planks, a coefficient of determination of 0.68 was achieved between the new IP and bending strength. For narrow side boards to be used as laminations in wet-glued glulam beams, the relationship between IP and tensile strength was as high as 0.77. Since the intended use of the narrow boards was as laminations in wet-glued beams, the possibility of grading them in a wet state was also investigated. Grading based on axial dynamic excitation and weighing gave just as good results in a wet state as when the same grading procedure was applied after drying.

It was also found that the relationship between the new IP and strength was dependent on what scale the IP was determined. Optimum was reached for moving average MOE calculated over lengths corresponding with approximately half the width of investigated pieces.

Implementation of the new IP will result in grading that is more accurate than what is achieved by the great majority of today’s grading machines. The new method will probably also be particularly favourable for development of engineered wood products made of narrow laminations.

Abstract [sv]

Hållfasthetssortering av konstruktionsvirke innebär att värdet på sågade produkter ökar. Sorteringen genomförs oftast med maskinella metoder baserade på statistiska samband mellan s.k. indikerande egenskaper och böjhållfasthet. Den indikerande egenskap (indicating property, IP) som är vanligast på den Europeiska marknaden är styvhet uttryckt som ett medelvärde för elasticitetsmodulen (modulus of elasticity, MOE) i ett virkesstycke, trots att MOE är en materialegenskap som varierar i virket.

En betydande begränsning med dagens sorteringsmetoder är att de beskrivna sambanden är förhållandevis svaga, vilket innebär att det finns en potential för metoder med högre noggrannhet. Det huvudsakliga syftet med detta doktorandprojekt har varit att initiera en utveckling mot sådana metoder.

Hållfasthet hos virke är beroende av förekomst av kvistar. Samtidigt har de kvistmått som fram till idag kommit till användning visat sig vara dåliga prediktorer av hållfasthet. Resultat från såväl denna som tidigare forskning har dock visat att inte bara kvistars storlek och läge, utan också variationen i fiberriktning i omgivande träfibrer, är av stor betydelse för lokal styvhet och brottförlopp under inverkan av last. Utveckling av nya IP som tar hänsyn till såväl kvistar som omgivande träfibrers egenskaper fastställda på mycket lokal nivå bedömdes vara en möjlig väg för att uppnå bättre hållfasthetssortering.

I detta doktorandprojekt användes beröringsfri deformationsmätning för analys av det strukturella beteendet hos virkesstycken på såväl lokal som global nivå. Laserskanning utnyttjades för detektering av lokala fiberriktningar projicerade på virkesstyckenas ytor. Med utgångspunkt från skannad information, virkesdensitet och medelvärde för axiell dynamisk elasticitetsmodul kunde variationen i lokal elasticitetsmodul i virkesstyckenas längdriktning bestämmas. Genom integration över tvärsektioner längs ett virkesstycke kunde en profil över hur böjstyvheten i styva riktningen varierade i virkesstyckets längdriktning beräknas. En ny IP definierades som den lägsta elasticitetsmodulen i böjning utmed virkesstyckets längd.

För ett urval av granplankor erhölls en förklaringsgrad på 0.68 mellan den nya indikerande egenskapen och böjhållfasthet. För smala sidobrädor avsedda att användas som lameller i våtlimmade limträbalkar var motsvarande förklaringsgrad mellan samma IP och draghållfasthet så hög som 0.77. Eftersom sidobrädorna var avsedda att användas som lameller i våtlimmade balkar genomfördes en studie avseende möjligheten att hållfasthetssortera i vått tillstånd med hjälp av axiell dynamisk excitering och vägning. Det visade sig att sådan sortering gav lika bra resultat som då samma metod användes efter torkning.

Sambandet mellan den nya indikerande egenskapen och hållfasthet visade sig också vara beroende av på vilken lokal nivå som egenskapen beräknades. Optimum uppnåddes då den bestämdes som ett glidande medelvärde beräknat över en längd motsvarande ungefär halva virkesstyckets höjd.

Implementering av den nya sorteringsmetoden kommer att resultera i sortering som är noggrannare än vad som kan erhållas med det stora flertalet av de sorteringsmetoder som finns idag. Den nya indikerande egenskapen kommer sannolikt att bli särskilt gynnsam att använda för utveckling av ingenjörsmässiga träprodukter bestående av smala lameller.

Place, publisher, year, edition, pages
Växjö: School of Engineering, Linnaeus University , 2012. , 72 p.
Series
Rapporter: Institutionen för teknik, Linnéuniversitetet, 15
Keyword [en]
bending strength, fibre angle, grain angle, knots, laser scanning, machine strength grading, modulus of elasticity, strain measurement, structural timber, tensile strength, wood
Keyword [sv]
böjhållfasthet, draghållfasthet, elasticitetsmodul, fibervinkel, konstruktionsvirke, kvistar, laserskanning, maskinell hållfasthetssortering, trä, töjningsmätning
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
URN: urn:nbn:se:lnu:diva-23757Libris ID: 13873346ISBN: 978-91-86983-92-5 (print)OAI: oai:DiVA.org:lnu-23757DiVA: diva2:600743
Presentation
2012-11-16, M1053, Linnéuniversitetet, 351 95 Växjö, Växjö, 10:00 (Swedish)
Opponent
Supervisors
Funder
Swedish Research Council FormasKnowledge Foundation
Available from: 2013-01-29 Created: 2013-01-25 Last updated: 2017-09-05Bibliographically approved
List of papers
1. Strain fields around knots in Norway spruce specimens exposed to tensile forces
Open this publication in new window or tab >>Strain fields around knots in Norway spruce specimens exposed to tensile forces
2012 (English)In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 46, no 4, 593-610 p.Article in journal (Refereed) Published
Abstract [en]

Two-dimensional strain fields around knots in two Norway spruce specimens subjected to tension loading were detected using a contact-free measuring technique based on white-light digital image correlation. The first specimen included a traversing edge knot and the second one a centric knot. The development of strain fields as function of load level were measured by consecutive cyclic load tests in which one side of a specimen was studied during each test. The objectives were to examine to what extent the strain fields could be detected, to investigate the correlation between strain fields measured on different sides of a specimen, and to analyse the strain distributions around the knots. The results show that the applied technique is very useful for catching both overall and detailed information about the behaviour of knots in wood members exposed to loading. Clear wood defects that could not have been detected by neither visual inspection nor scanning were observed and conclusions could be drawn regarding release of internal stresses. The correlations between strain fields on different sides of the specimens were excellent and the correspondence between measurement results and comparative finite element calculations was surprisingly good, considering the fact that the employed FE models were fairly simple.

Place, publisher, year, edition, pages
München: Springer, 2012
Keyword
knots, contact-free, strain measurement, strain distribution, wood
National Category
Building Technologies
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-10185 (URN)10.1007/s00226-011-0429-8 (DOI)2-s2.0-84863727713 (Scopus ID)
Note

online first 27/5-2011

Available from: 2011-01-18 Created: 2011-01-18 Last updated: 2017-12-11Bibliographically approved
2. Strength grading of narrow dimension Norway spruce side boards in the wet state using first axial resonance frequency
Open this publication in new window or tab >>Strength grading of narrow dimension Norway spruce side boards in the wet state using first axial resonance frequency
Show others...
2011 (English)In: International Wood Products Journal, ISSN 2042-6445, Vol. 2, no 2, 108-114 p.Article in journal (Refereed) Published
Abstract [en]

Strength grading of Norway spruce (Picea abies (L.) Karst.) side boards in the wet state was investigated. For a sample of 58 boards, density and dynamic modulus of elasticity in the axial direction, MOEdyn, were determined in the wet state. The boards were then split into two parts and the procedure of determining MOEdyn was repeated both before and after the boards were dried to a target moisture content of 12 %. Finally, tensile strength of the split boards was measured and its relationship to MOEdyn for both wet and dried split boards determined. The investigation also included an evaluation of a so called reversed lamination effect on the stiffness caused by the splitting of boards into two parts. The results show that strength grading of split boards in the wet state can give just as good results as grading performed after drying. The reversed lamination effect on the stiffness of split boards was found to be of lower order.

Place, publisher, year, edition, pages
Leeds/London United Kingdom: Maney Publishing, 2011
Keyword
strength-grading, axial stiffness, side boards, wet state, green state
National Category
Building Technologies
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-17871 (URN)10.1179/2042645311Y.0000000015 (DOI)
Note

Published online first 1/11-2011

Available from: 2012-03-06 Created: 2012-03-02 Last updated: 2016-12-15Bibliographically approved
3. Determination of tensile strain fields in narrow Norway spruce side boards as a basis for verification of new machine strength grading methods
Open this publication in new window or tab >>Determination of tensile strain fields in narrow Norway spruce side boards as a basis for verification of new machine strength grading methods
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Today’s methods for machine strength grading of structural timber result, in general, in strength predictions with a rather low accuracy. A need for development of more precise methods has been identified. Application of stiffness in terms of locally determined MOE as indicating property is an evident starting point for such a process. Development of new grading procedures and models require laboratory verification, and this research investigates the possibility of using contact-free deformation measurement technique based on white-light digital image correlation (DIC) for this purpose. A sample of nine Norway spruce (Picea abies) side boards of narrow dimensions was tested in tension according to the European Standard EN 408. Simultaneously, deformations along the entire length of one of the flatwise surfaces of each board were measured using two master-slave connected DIC systems. Strain fields were subsequently calculated. To evaluate the accuracy of the measurement technique, local MOE determined traditionally, i.e. on the basis of elongations measured in accordance with EN 408, was compared with corresponding MOE values calculated on the basis of DIC deformation measurements. Acceptable agreement between compared MOEs were achieved and the accuracy of MOE values determined on the basis of the DIC technique was on the same level as requirements laid down in EN 408. However, the resolution of the information supplied by the DIC technique can, in contrast to elongations measured traditionally, be used to gain detailed knowledge regarding local MOE in evaluated boards. Therefore, based upon achieved results, in combination with certain identified potentials for measurement improvements, it is concluded that DIC technique can be used as a tool for development and laboratory verification of new strength grading methods.

Keyword
contact-free, modulus of elasticity, side boards, strain distribution, strength grading, wood
National Category
Forest Science
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-23841 (URN)
Funder
Swedish Research Council Formas
Available from: 2013-01-29 Created: 2013-01-29 Last updated: 2016-12-15Bibliographically approved
4. Prediction of timber bending strength and in-member cross-sectional stiffness vartiation on basis of local wood fibre orientation
Open this publication in new window or tab >>Prediction of timber bending strength and in-member cross-sectional stiffness vartiation on basis of local wood fibre orientation
Show others...
2013 (English)In: European Journal of Wood and Wood Products, ISSN 0018-3768, E-ISSN 1436-736X, Vol. 71, no 3, 319-333 p.Article in journal (Refereed) Published
Abstract [en]

Machine strength grading of structural timber is based upon relationships between so called indicating properties (IPs) and bending strength. However, such relationships applied on the market today are rather poor. In this paper, new IPs and a new grading method resulting in more precise strength predictions are presented. The local fibre orientation on face and edge surfaces of wooden boards was identified using high resolution laser scanning. In combination with knowledge regarding basic wood material properties for each investigated board, the grain angle information enabled a calculation of the variation of the local MOE in the longitudinal direction of the boards. By integration over cross-sections along the board, an edgewise bending stiffness profile and a longitudinal stiffness profile, respectively, were calculated. A new IP was defined as the lowest bending stiffness determined along the board. For a sample of 105 boards of Norway spruce of dimension 45 × 145 × 3600 mm, a coefficient of determination as high as 0.68-0.71 was achieved between this new IP and bending strength. For the same sample, the coefficient of determination between global MOE, based on the first longitudinal resonance frequency and the board density, and strength was only 0.59. Furthermore, it is shown that improved accuracy when determining the stiffness profiles of boards will lead to even better predictions of bending strength. The results thus motivate both an industrial implementation of the suggested method and further research aiming at more accurately determined board stiffness profiles.

Place, publisher, year, edition, pages
Springer, 2013
Keyword
machine strength grading, bending stiffness, laser scanning, fibre angle, grain angle, wood, structural timber, lumber, dynamic stiffness, MOE
National Category
Materials Engineering
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-23846 (URN)10.1007/s00107-013-0684-5 (DOI)000317977800004 ()2-s2.0-84882925541 (Scopus ID)
Available from: 2013-01-29 Created: 2013-01-29 Last updated: 2017-12-06Bibliographically approved

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