Zgoščevanje lesa

The paper treats the processes involved in wood densification and provides a summary of the state-of-the-art, as presented in the literature, with regard to densification as achieved by compression, accompanied by some form of hydrothermal treatment. The viscoelastic nature of wood is discussed, togetherwith its thermal softening and typical stress-strain relationships. The properties of densified wood products depend, apart from processing parameters, on various anatomical features such as density, the percentage of late wood material, ray volume and the loading direction. The problems associated with wood stabilization after densification are also treated. Relevant examples of wood densification from fundamental research, and the results of applied studies significant for everyday practice, are presented. Aspecial focus is given on the process of viscoelastic thermal compression (VTC) of wood.Članek obravnava procese, ki potekajo med postopkom zgoščevanja lesa, in podaja pregled dosežkov relevantnih raziskav, v katerih je bilo proučevano zgoščevanje lesa s kombiniranim postopkom hidrotermične obdelave in stiskanja.Predstavljena je viskoelastična narava lesa, skupaj s toplotnim mehčanjem oziroma plastifikacijo. Prikazane so značilne zveze med napetostjo in deformacijo lesa pri zgoščevanju. Pojasnjen je vpliv tehnoloških parametrovzgoščevanja in inherentnih značilnosti lesa, kot so gostota, odstotek kasnega lesa, delež trakovnega tkiva in smer obremenjevanja na končnelastnosti zgoščenega proizvoda. Članek obravnava tudi problem stabilizacije lesa po postopku zgoščevanja. Predstavljeni so relevantni primeri zgoščevanja lesa temeljnih raziskav in rezultati študij, ki so pomembni za aplikacijo. Podan je podroben opis procesa viskoelastične toplotnezgostitve (VTC) lesa

Transverse Compression Behavior of Wood in Saturated Steam at 150-170°C

The transverse compression behavior of wood in high temperature (150, 160, and 170°C) and saturated steam conditions was studied. The effect of the temperature on the stress-strain response, nonlinear strain function, and relative density change was examined by a modified Hooke's law based on the load-compression behavior of flexible foams. The influence of environmental conditions during compression on the set recovery of the compression deformation was determined. It was found that temperature and moisture content affected the compression behavior of wood in saturated steam conditions. A small difference in moisture content of specimens compressed at 160 and 170°C caused approximately the same stress-strain and relative density curves with minimum temperature affect on the compression behavior. The compressive modulus of the wood and cell wall modulus were found to decrease with increasing temperature from 150 to 160°C with no change when increased to 170°C. The densification region was entered at notably lower stress levels at 160 and 170°C when compared with 150°C. The results established that temperature and moisture content did not affect the nonlinear strain function at strain levels lower than 0.63. Furthermore, it was found that the set recovery of compressive deformation decreased with increasing temperature of compression from 150 to 160°C. In addition, the results showed that compression at 160 and 170°C significantly lowered the equilibrium moisture content

Creep behaviour of densified wood

Due to the reproducibility, good workability, suitable mechanical properties, and attractive aesthetic appearance, timber is widely used in the building industry. Among those properties, mechanical properties are important for the useability of timber in construction applications. It is well known that there is a positive relationship between wood density and its mechanical properties. That means the thermo-hydro-mechanical (THM) densification, i.e. transverse compression of the wood cells only by using additional temperature, moisture and mechanical action to increase its density without structural fracturing is a practicable method to increase the performance of low-density species and thereby improve its mechanical properties. The previous studies on wood densification mainly focused on the influence of process parameters on wood physical and mechanical properties and how to use post-treatment to reduce the set recovery. This study is in the field of increasing the use of densified timber in construction applications and thereby strengthen the competitiveness of wood as a construction material. In construction, however, densified timber normally needs to be exposed to long-term loading which may lead to creep deformation and reduction of load-bearing capacity. There is an obvious risk of reduced serviceability and safety of constructions containing densified wood. Studies of creep characteristics of densified wood are rare, and therefore the purpose of this study was to fill the gap in knowledge if the field of densified under bending load. Scots pine specimens subjected to THM densification, THM densification with a post-heat treatment, and THM densification combined with phenol resin impregnation were loaded under 3-point bending under the 35% of maximum stress level at 20℃ and 65% RH. Results from these tests will be presented.Finansiär: Republic of Slovenia</p