The impact of earlywood and latewood on the compressive strength of Douglas fir

Abstract

AbstractMechanical performance of wood is determined by its structural characteristics, among others earlywood and latewood in softwoods. Given that thermal modification also affects the mechanical strength, it is key to understand how the modification affects earlywood and latewood and how this on its turn impacts compressive strength. In this study, Douglas fir blocks measuring 30×50×150mm3were modified at 180°C (TM-180°C) and 210°C (TM-210°C). The compressive strength of pure earlywood (EW), pure latewood (LW) and a combination of earlywood and latewood (ELW) specimens was measured. The specimens were compressed in terms of 30% of their original thickness, and during the compression test the strain distribution of ELW was recorded. In addition, the microstructure before and after compression was investigated complemented with using SEM to understand the structural changes taking place. The results show that compressive strength of TM-180°C specimens was high, most probably because thermal modification increased stiffness of cell walls and homogenized strain distribution in ELW specimens. Control specimens had a higher compression set recovery than thermally modified specimens. Tracheid cell walls in EW and LW specimens were flattened and buckled respectively due to compression. While in thermally modified material, cell wall fissures and wood ray fractures in EW and LW specimens respectively were observed. For ELW specimens, structural changes in latewood were trivial, and structural changes in the earlywood part were less significant than in full EW specimens. Compared to EW specimens, earlywood in ELW showed higher compression set recovery. It seems that structural failure in earlywood is limited when in combination with latewood, resulting from homogenized strain distribution in earlywood.