Abstract
The paper reports on creep of ash wood (Fraxinus excelsior L.) thermally modified at 180 and 200 °C, and subsequently subjected to compression in tangential and radial directions and simultaneously wetted, from the moisture content (MC) of 6% to above the fibre saturation point (FSP). The compressing load made 0.00, 0.25, 0.50 and 0.75 of impact stress at the proportional limit (Rc). The compression stress needed to restrain the swelling of wood, the so-called swelling pressure, was indirectly determined from isochrones of mechano-sorptive creep. The most important finding was that thermal modification reduces the strain of ash wood subjected to compression perpendicular to the grain to a degree proportional to the mass loss. The compression stress needed to restrain the swelling of thermally modified wood is ca. 10 and 20% smaller in the tangential and radial directions, respectively. This effect leads to a reduction in the anisotropy of swelling pressure of thermally modified wood perpendicular to the grain. Moreover, although upon thermal modification the mass loss of wood takes place, at the MC of 6% it shows practically the same modulus of elasticity (MOE) and Rc as the unmodified wood. After wetting to MC higher than the FSP, the thermally modified wood at 200 °C shows significantly higher MOE and Rc than the wood modified at 180 °C and untreated wood. Reduction of wood hygroscopicity, an inevitable effect of thermal modification, also reduces the range of changes in mechanical properties of wood caused by the increase in its MC to the FSP.
Highlights
The effects of thermal modification of wood on its chemical structure and properties have been extensively studied
Thermal modification of wood at 180 or 200 °C for 3 h leads to a decrease in equilibrium moisture content (EMC) corresponding to the air relative humidity (RH) near saturation by 18 and 24%, respectively
Assuming the sorption model proposed by Guggenheim–Anderson–de Boer (GAB) (Timmermann 2003), the theoretical moisture content (MC) of wood corresponding to the fibre saturation point (FSP) was calculated: EMC
Summary
The effects of thermal modification of wood on its chemical structure and properties have been extensively studied. Thermal modification always leads to a reduction in wood density, sometimes an increase in its compression strength parallel and perpendicular to the grain or an increase in bending strength, modulus of elasticity (MOE) and hardness is observed. It depends on the wood species and parameters of Arnold (2010) studied the MC effect of thermally modified beech and spruce wood on the bending strength and revealed that the mechanical parameters of these two wood species after modification undergo smaller changes with. According to Moliński et al (2016), who studied the mechanical properties of thermally modified and unmodified ash wood subjected to compression in the radial direction at the same and constant MC of 4 or 12%, the decrease in the mechanical properties with increasing MC was more pronounced for the modified than for the unmodified wood
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