The aim of this thesis was to evaluate the use of dynamic excitation as a method for stiffness and strength grading of wet side boards of narrow dimensions. The need for such an investigation has previously been identified in an ongoing research project in which the possibility to use side boards as lamellae in wet glued glulam beams is investigated.      The assessment of the dynamic excitation method was carried out by means of experimental work and measurements.. The approach was quantitative in the sense that the data was collected through experiments performed on a rather large population and that the results were analyzed using statistical methods.      To investigate the effect of moisture content on stiffness of narrow dimension, side boards, the stiffness was measured in three states: - in wet state, before splitting the boards, - in wet state, after splitting the boards, and - in dry state (splitted boards).      The conclusion, after calculations and analysis of all the results, was that the natural frequency and stiffness of wet boards could, with a high degree of reliability, be predicted by use of the dynamic excitation method. There was a strong correlation in stiffness between wet state split boards and dry state split boards, with a coefficient of determination of 0.93.

<p> The aim of this thesis was to evaluate the use of dynamic excitation as a method for stiffness and strength grading of wet side boards of narrow dimensions. The need for such an investigation has previously been identified in an ongoing research project in which the possibility to use side boards as lamellae in wet glued glulam beams is investigated.</p><p>     The assessment of the dynamic excitation method was carried out by means of experimental work and measurements.. The approach was quantitative in the sense that the data was collected through experiments performed on a rather large population and that the results were analyzed using statistical methods.</p><p>     To investigate the effect of moisture content on stiffness of narrow dimension, side boards, the stiffness was measured in three states:</p><ul><li>- in wet state, before splitting the boards,</li><li>- in wet state, after splitting the boards, and </li><li>- in dry state (splitted boards).</li></ul><p>     The conclusion, after calculations and analysis of all the results, was that the natural frequency and stiffness of wet boards could, with a high degree of reliability, be predicted by use of the dynamic excitation method. There was a strong correlation in stiffness between wet state split boards and dry state split boards, with a coefficient of determination of 0.93.</p>

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. / 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 %.