Partial modal analysis for health assessment of living trees

Abstract:
Rot in living trees cause substantial losses for the forestry industry. The common practice when evaluating forest stands for, e.g., purchase, is assessment based on visual signs. In this paper a new non-destructive assessment method based on the impact excitation method is proposed. The trunk of a living tree is excited by the impact of a hammer, and the vibrations are measured by accelerometers. Resonance frequencies, circumferential mode shapes and propagation velocity of a surface wave are analysed. A function describing the expected frequency for a sound tree is derived, and used in a detector whose performance is evaluated for 93 trees of species Norway spruce. The partial mode shape is used to ensure that the corresponding resonance frequencies are compared to each other. It is found that the detector is successful and outperforms assessments by skilled experts in forestry.