Assessment of local stiffness redistribution around knots in round timber under axial compression using digital image correlation
Giorgio Pagella (TU Delft - Bio-based Structures & Materials)
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Abstract
The local stiffness of a knot in wood can be up to 30 times lower than the longitudinal stiffness of clear wood, as reported in literature and codes. However, it remains difficult to determine the local effect of knots on the stiffness of round timber and to quantify the extent of the surrounding area influenced by this reduction in stiffness. The objective of this study was to experimentally investigate the local redistribution of stiffness around knots in round timber under axial compression using digital image correlation (DIC). By providing full-field strain measurements, DIC enables a detailed characterisation of strain localisation and offers a novel approach to quantify the influence zone of knots. Two spruce piles were tested in compression, and the deformations were obtained on three distinct zones: within the knot, a transition zone, and a clear wood zone. The stiffness in the knot was approximately 1/30 of the clear wood longitudinal stiffness, while the transition zone had a reduction of about 1/15. The study supports the validity of existing design assumptions and of DIC for capturing local stiffness variations in round timber.
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