Mechanical Admittance Measurement and Analysis of Dwarf Chinese Hickory Trees under Impact Excitations

Abstract

<abstract><title><italic>Abstract.</italic></title> Beating is the usual way to harvest Chinese hickory nuts (Carya cathayensis Sarg.) because of the large size of the trees and the scattered distribution of the nuts. Analyzing and comprehensively understanding the dynamic characteristics of Chinese hickory trees is important to design a high-efficiency mechanical harvester. An impact testing system consisting of a data acquisition and analysis system, an impact hammer, and an acceleration sensor was used to measure the mechanical admittance of three dwarf Chinese hickory trees selected at random from a plantation. Their resonant frequencies, as well as the influence of the tree structure on transmission of the dynamic response, were analyzed based on a mechanical admittance analysis. The obtained primary resonant frequencies were detected on the main parts of the sample trees, and the resonant frequencies of the offshoots without secondary crotches were very close to those of the leader limbs. The variation of the dynamic response in the tested trees was strongly related to the tree structure. The effect of crotches and chain nodes on the acceleration response of tree branches to impact excitation was quantified using the acceleration admittance attenuation rate. The results of this study revealed that the leader top crotch may amplify the acceleration admittance. As for the secondary crotch, significant vibration amplification was only observed on the response at 14 Hz. Contrary to the dynamic response variation at the crotch, the acceleration admittance was little affected by branch chains with uniform diameters and small angles. There was weak correlation between the acceleration admittance attenuation rate and the angle or diameter ratio based on correlation analysis and t-test results of all three structural features.