The influence of juvenility on the mechanical properties of douglas fir wood

Navadna ameriška duglazija (Pseudotsuga menziesii L.) je drevesna vrsta, ki bi lahko v prihodnje v Sloveniji nadomeščala morebitno pomanjkanje smrekovine, saj ima primerljive fizikalne in mehanske lastnosti ter boljšo naravno odpornost. Za optimalno izkoriščanje lastnosti te drevesne vrste smo na preizkušancih z rastišč Ravbarkomanda in Pečovnik ugotavljali, kakšen je vpliv juvenilnosti na izbrane fizikalne in mehanske lastnosti masivnega lesa ter njihovo variabilnost. Z merjenjem širin branik in deležem kasnega lesa smo potrdili trend zmanjševanja širin branik z oddaljenostjo od stržena. Delež kasnega lesa se je z oddaljenostjo od stržena večal, in značilno vplival na naraščanje gostote lesa. Gostota lesa se je najbolj povečevala med 20 in 30 let starosti kambija, pri višji starosti pa manj, tudi ob zmanjšani variabilnosti. Upogibna trdnost in togost lesa duglazije, določena s standardnim 3-točkovnim statičnim mehanskim testom, se je tako kot gostota lesa z oddaljenostjo od stržena povečevala. Krhke in enostavne lome smo potrdili pri juvenilnem lesu, dolgo-vlaknate in žilave lome pa večinoma pri preizkušancih adultnega lesa. Z oddaljenostjo od stržena so se vzdolž lesnih vlaken še značilno povečevale tlačna in strižna trdnost ter trdota lesa. Pri testiranjih prečno na vlakna so bile razlike manjše, značilne pa le med najzgodnejšim juvenilnim obdobjem (< 10 let) ter ostalimi starostnimi kategorijami. Vse proučevane mehanske lastnosti so bile značilno linearno korelirane z gostoto lesa.Douglas-fir (Pseudotsuga menziesii L.) is a tree species that could compensate for a possible shortage of spruce in Slovenia in the future, as it has comparable physical and mechanical properties and better natural resistance. In order to make the best use of the properties of this tree species, we determined the influence of juvenile period on selected physical and mechanical properties of solid wood and their variability on samples from the Ravbarkomanda and Pečovnik sites. By measuring annual ring widths and the proportion of latewood, we confirmed the trend that ring widths decrease with distance from the pith. The proportion of latewood increased with distance from the pith and significantly influenced the increase in wood density. Wood density increased most between the 20th and 30th growth rings, i.e., at the age of the cambium, and less at older ages, and variability was also lower. The bending strength and stiffness of Douglas-fir wood, as determined by a standardized static 3-point mechanical test, increased with distance from the pith, as did wood density. Fragile and simple bending fractures were confirmed in juvenile wood and long-fibre fractures mostly in adult wood samples with higher cambium age. With distance from the pith, the compressive and shear strength and Brinell hardness of the wood increased significantly along the grain. When testing properties transverse to the grain, the differences were smaller and characteristic only between the earliest juvenile period