Impact bending strength and structural properties of hardwood: branch versus stem

The branches of deciduous trees are optimised by nature to allow continuous adaptation and response to changing environmental conditions. As a result, the morphology and internal structure of the wood branches are often more variable than in the stem. Quantitatively, branches and stem tops represent 20–50% of the volume of the above-ground biomass of deciduous trees, which is currently under-utilized and mainly burned. To enable a higher-value application as a safe construction material, a comprehensive technological profile of branch wood is a prerequisite. Therefore, we performed single-blow impact pendulum tests in tangential wood direction on branch and stem wood samples of beech, oak, and poplar to investigate their relationship with wood density, macrostructural properties, fibre properties, and microfibril orientation of selected samples. Our results showed that the significant differences in mean impact bending strength between branch and stem for all species could not be explained by wood density. However, branch and stem wood with higher toughness are generally associated with longer fibres. Overall, branches showed higher MFA (microfibril angle) and lower fibre length compared to stems. We found that higher toughness in selected beech samples was associated with higher density and lower MFA. While oak also had a lower MFA, poplar had a higher MFA in high-toughness samples. Our empirical results provide insights into the species-specific structure-property relationships of hardwood branches, improving the understanding of their properties and variability, and potentially informing their use in structural applications.