Measurement and mechanical analysis of the strains–stresses induced by tree-pulling experiments in tree stems

Abstract

Our manuscript presents an approach to measure the maximum torsional shear and bending strains/stres ses of tree trunk. The measured strains are not affected by the direction of pulling force. Stem breakage is a common failure mode of standing trees. The stem of the tree with a lopsided crown is always subjected to bending and torsional combined deformation state under wind load. At a certain degree of torsional moment, the torsional stress exceeds the value of shearing strength of green timber resulting in longitudinal splitting. There is no valid experimental approach to measure shear strain of a trunk scientifically so far. In this paper, we propose an approach to measure the maximum torsional shear and bending strains of a stem which is under combined deformation state. It is that four strain meters are arranged on tree stem in a certain way. The measured strains are not affected by the direction of pulling force. We explain the principle of the measurement approach. To validate the measurement approach, tree-pulling tests are performed to model wind forces. The trunk strains are also converted to trunk stresses using Hooke’s Law. Analyses of the results demonstrate that the experimentally measured values are in good agreement with the theoretical values.