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Localized modulus of elasticity in timber and its significance for the accuracy of machine strength grading
SP Technical Research Institute of Sweden.ORCID iD: 0000-0002-8513-0394
Linnaeus University, Faculty of Technology, Department of Building Technology. (Träbyggteknik)
Linnaeus University, Faculty of Technology, Department of Building Technology. (Träbyggteknik)
2014 (English)In: Wood and Fiber Science, ISSN 0735-6161, Vol. 46, no 4, 489-501 p.Article in journal (Refereed) Published
Abstract [en]

From previous research, it is well known that a localized modulus of elasticity (MOE) is a better indicating property (IP) of strength than an MOE averaged across a longer span. In this study, it was investigated to what extent the relationship, in terms of coefficient of determination (R2), between strength and localized MOE was dependent on the length across which the MOE was determined. Localized MOE was calculated with MOE profiles based on dot laser scanning of fiber directions, axial dynamic excitation, and a scheme of integration across a board's cross-section. Two board samples were investigated. Maximum R2 values, which were as high as 0.68 and 0.77, respectively, were obtained for localized MOE determined across lengths corresponding to about half the depth of the investigated boards. Consequently, application of a highly localized bending MOE as an IP will result in very competitive grading.

Place, publisher, year, edition, pages
Society of Wood Science and Technology , 2014. Vol. 46, no 4, 489-501 p.
Keyword [en]
fibre angle, indicating property, laser scanning, machine strength grading, modulus of elasticity, strain fields
National Category
Building Technologies
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
URN: urn:nbn:se:lnu:diva-40316ISI: 000343355600007Scopus ID: 2-s2.0-84946014778OAI: oai:DiVA.org:lnu-40316DiVA: diva2:790022
Funder
VINNOVA
Available from: 2015-02-23 Created: 2015-02-23 Last updated: 2016-12-16Bibliographically approved
In thesis
1. Strength grading of structural timber and EWP laminations of Norway spruce - Development potentials and industrial applications
Open this publication in new window or tab >>Strength grading of structural timber and EWP laminations of Norway spruce - Development potentials and industrial applications
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Machine strength grading of structural timber is a sawmill process by which considerable value is added to sawn products. The principle of such grading is that the strength of a timber member is predicted on the basis of a so called indicating property (IP) which, in general, represents an averaged value of the modulus of elasticity (MOE) measured over a board length of about one meter or more.

A limitation of today’s grading methods is that the accuracy of strength predictions is often rather poor, which results in a low degree of utilization as regards structural potential of sawn timber. However, it has for many years been well known to researchers that much better strength predictions can be made by using localized MOE values, determined over a very short length, as IP. Still, the determination of such values in a sawmill production environment has been technically very difficult to achieve.

In the research presented in this thesis, dot laser scanning with high resolution was utilized for detection of local fibre orientation on the surfaces of timber members. Since wood is an orthotropic material with superior structural performance in the longitudinal fibre direction, information about fibre orientation was, in combination with beam theory and measured wood material properties, used to determine the bending MOE variation along boards. By application of an IP defined as the lowest MOE found along a board, more accurate strength predictions than what is obtained by common commercial grading techniques was attained.

The thesis also involves flatwise wet gluing of Norway spruce side boards into laminated beams. As side boards, being cut from the outer parts of a log, have excellent structural properties it was not surprising to find that the beams had high strength and stiffness, even when laminations of sawfalling quality were used. The possibility of grading boards in a wet state by means of axial dynamic excitation was investigated with a positive result and application of simple grading rules resulted in considerable improvement of beam bending strength. Finally, bending MOE variation determined on the basis of laser scanned fibre directions was used for identification of weak sections in laminations. Elimination of such sections by means of finger jointing showed that average lamination strength of a board sample could be improved by more than 35 percent.

Abstract [sv]

Hållfasthetssortering av konstruktionsvirke är en sågverksprocess som innebär att värdet av det sågade virket ökar väsentligt. Principen för denna typ av sortering är att styrkan hos ett virkesstycke predikteras med utgångspunkt från en så kallad indikerande egenskap (IP) som oftast representeras av medelvärdet av elasticitetsmodulen (E-modulen) mätt över en sträcka av minst en meter utmed virkesstyckets längd.

De sorteringsmetoder som används idag ger ofta prediktioner med relativt låg noggrannhet, vilket innebär att endast en begränsad del av det sågade virkets konstruktiva potential kan utnyttjas. Det är dock väl känt att avsevärt bättre prediktioner kan erhållas genom att använda en lokal E-modul, uppmätt över en mycket kort sträcka, som IP. I dagsläget saknas dock teknik för att kunna bestämma ett sådant lokalt värde vid produktionshastighet i ett sågverk.

I den forskning som presenteras i denna avhandling har punktlaserskanning med hög upplösning använts för att bestämma fiberriktningens variation på ytorna av virkesstycken. Eftersom trä är ett ortotropt material med högst styvhet och styrka i longitudinell fiberriktning ger skanningsresultaten värdefull information om hur dessa egenskaper varierar längs en planka. Genom att kombinera informationen om fiberriktning med uppmätta virkesegenskaper och klassisk balkteori, kan böjstyvhetens variation utmed en planka beräknas med hög upplösning och därefter omräknas till en E-modul i böjning. Med en IP definierad som det lägsta värdet på nämnda E-modul utmed en planka kan en högre noggrannhet i prediktionen av hållfasthet uppnås, jämfört med vad som kan erhållas med dagens sorteringsmetoder.

Avhandlingen omfattar också limträbalkar tillverkade av sidobräder av gran limmade i rått tillstånd. Eftersom sidobräder sågas från de yttre delarna av en stock har de vanligtvis utmärkta konstruktiva egenskaper. Det var därför inte förvånande att balkarna uppvisade hög styrka och styvhet, även i de fall lamellerna var av sågfallande kvalitet. Möjligheten att med hjälp av axiell dynamisk excitering sortera sidobräder i rått tillstånd undersöktes med positivt resultat och genom att använda sorterade lameller kunde balkarnas styrka förbättras avsevärt. Den ovan beskrivna metoden att med utgångspunkt från bl.a. skannade fibervinklar bestämma styvhetens variation längs virkesstycken utnyttjades sedan för att identifiera svaga snitt i lameller av sidobräder. Genom att eliminera sådana snitt med hjälp av fingerskarvning kunde medelhållfastheten för ett stickprov sidobrädor höjas med mer än 35 %.

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2014. 155 p.
Series
Linnaeus University Dissertations, 170/2014
Keyword
fibre angle, grain angle, knots, laser scanning, machine strength grading, modulus of elasticity, side boards, strain, structural timber, wet gluing, wood
National Category
Building Technologies
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-40319 (URN)
Public defence
Södrasalen (M1083), Hus M, Växjö, Lückligs plats 1, Växjö (English)
Opponent
Supervisors
Available from: 2015-02-23 Created: 2015-02-23 Last updated: 2016-12-15Bibliographically approved

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