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

摘要

Beating is the usual way to harvest Chinese hickory (Carya cathayensis Sarg.j nuts because of large size of trees and scattered distribution of nuts. Analyzing and comprehensively understanding the dynamic characteristics of Chinese hickory trees is important to design 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 a Chinese hickory tree sample. Its resonance frequencies, as well as the influence of the tree structure on transmission and attenuation of dynamic response, were analyzed based on the mechanical admittance analysis. The obtained primary resonance frequencies were detected on the main part of the sample tree. The variation of the dynamic response along the testing tree was greatly related to the Chinese hickory tree structure. The effect of the crotch and chain nodes on the acceleration response was quantified using the attenuation ratio of the acceleration admittance. Results obtained from this study revealed that the leader top crotch may amplify the acceleration admittance no matter what the crotch angle and the branch diameter was. Unlike the crotches, the branch chain nodes generally had depressed influence on the acceleration admittance along the branch chains which heavily depended on the branch chain configuration. However, the branch chains with large chain angle and uniform wood diameter could produce little influence on the vibration transmission which suggests that high mechanical harvest efficiency could be obtained on such branches.