Analysis of dynamic mechanical properties of wood during convectional kiln drying

Raziskali smo kinetiko sušenja žaganega lesa invazivnih lesnih vrst, robinije, amerikanskega javorja in navadnega divjega kostanja. Preizkušance debelin 22 mm, 28 mm in 46 mm smo sušili v laboratorijskem sušilnem kanalu, pri temperaturi 40 °C in 60 °C. Dinamične mehanske lastnosti smo med sušenjem analizirali s frekvenčnim odzivom pri vzdolžnem in prečnem nihanju 490 mm dolgih preizkušancev. Intervalno smo gravimetrično določali še povprečno vlažnost in vlažnostni gradient, ter prečno krčenje lesa. Sušenje je potekalo hitreje pri višji temperaturi, debelina preizkušancev pa je na hitrost sušenja vplivala negativno. Najnižji modul elastičnosti, vzdolžni in upogibni, smo pri vseh lesnih vrstah in pogojih sušenja potrdili v svežem stanju. Pri sušenju, se je s padanjem vlažnosti lesa najprej začel povečevati vzdolžni modul elastičnosti. Spremembe so se pojavile že pred krčenjem lesa, pri povprečni vlažnosti nad točko nasičenja lesnih vlaken. Odziv je bil večji pri debelejših sortimentih, kjer je bil prisoten večji vlažnostni gradient. Upogibni modul elastičnosti je bil vselej nižji kot vzdolžni modul elastičnosti. Upogibni modul elastičnosti se pri vlažnosti lesa nad točko nasičenosti značilno ne spreminja, prične pa naraščati s padanjem vlažnosti lesa v higroskopskem območju.We investigated the drying kinetics of sawn wood of invasive wood species, black locust, American maple, and European horse chestnut. Specimens of 22 mm, 28 mm and 46 mm thick were dried in a laboratory convective kiln drier at 40 °C and 60 °C. Dynamic mechanical properties were analysed during drying by the frequency response in the longitudinal and flexural vibrations of 490 mm long specimens. We also determined gravimetrically in short periods the average moisture content (MC) and moisture content gradient, as well as the transverse shrinkage of wood. Drying was faster at higher temperatures, where the thickness of the specimens had a negative effect on the drying rate. The lowest longitudinal (EL) and bending modulus of elasticity (EB), were confirmed in the green state for all wood species and drying conditions. During drying, the reduction of MC of wood first caused an increase of the longitudinal modulus of elasticity. The changes appeared even at an average moisture content above the fibre saturation point (FSP), before shrinking of wood. The change was greater in thicker boards where a higher MC-gradient was present. The EB was always lower than the EL. The bending modulus of elasticity did not change significantly above FSP, but began to increase with the reduction of MC of wood in the hygroscopic range