Strain transfer from xylem to bark surface analyzed by digital image correlation

The goal of the study was to investigate the influence of bark of Turkish hazel (Corylus colurna L.) on strain transfer from xylem to bark surface. The study addresses issues of tree biomechanics that calls for a high-quality deformation data measured on trees’ surfaces in a noninvasive way. For these purposes, the optical technique employing 3D digital image correlation (3D-DIC) was used and tested against extensometers commonly used for such applications. The measured tree was subjected to bending by two pulling tests to induce the strain in tree stem, during which the analyzed area of interest (AOI) on the tree was studied in both variants with and without bark. The DIC technique successfully provided strain fields on a tree with bark and without despite its highly complicated structure. The absolute values of vertical strain measured by both techniques agree with each other in order of magnitude. However, 3D-DIC returns generally lower values than extensometers. Statistical tests proved (at p < 0.05) that the vertical strain measured on a bark was lower than that measured on wood, which confirms the hypothesis that bark layers reduce the strain transfer from wood to tree surface. The extent of strain reduction is highly dependent on force and was statistically detectable when the force reached ca. 5 kN. This is an important fact especially for practical use of optical measurements that are to replace standard extensometers. Measured displacements agreed with each other for both variants within a whole range of loads.