Viscoelastic characterization of Camellia oleifera wood and dynamic analysis of whole tree

To precisely obtain the dynamic responses of fruits and flower buds under vibratory excitation and determine their detachment states through coupled modeling of wood viscoelasticity and tree dynamics, so as to provide reference for the design of vibratory harvesting parameters for Camellia oleifera fruits.

Creep data of C. oleifera wood samples were first obtained via compression creep tests, and the generalized Kelvin-Voigt model was employed to fit the creep data to determine the viscoelastic parameters of the wood. Subsequently, finite element simulations of compression creep tests were conducted to verify the predictive accuracy of the identified viscoelastic parameters. On this basis, a finite element model of the C. oleifera tree was further established based on the viscoelastic constitutive model, and both modal analysis and transient dynamic analysis were performed to obtain the modal shapes and natural frequencies of the tree. The inertial force responses of C. oleifera fruits and flower buds under vibratory excitation were then calculated, and finally the detachment rates of the fruits and buds were predicted based on the peak inertial forces.

1) Compared with conventional binary and ternary models, the generalized viscoelastic models can significantly enhance the goodness-of-fit for creep data within the testing duration, which enables accurate characterization of the short-term creep mechanical behavior of C. oleifera wood (achieving 99.92% predictive accuracy); 2) At specific excitation frequencies, branch amplitudes far exceed that of the trunk; this “branch-dominated resonance” pattern can serve as a basis for excitation frequency selection; 3) Under trunk excitation at the selected frequency of 9.428 Hz (with a 10 mm amplitude and a 400 mm excitation height), the fruit detachment rate reaches 93.94% and the bud detachment rate remains only 6.06%, meeting the requirements for high-efficiency and low-damage harvesting.

This study reveals the “branch-dominant resonance” phenomenon in the C. oleifera tree at specific frequencies, delineates the distinct inertial force responses between fruits and flower buds under vibratory excitation, and establishes detachment criteria based on peak inertial forces. These findings provide a quantifiable parametric design guideline for the efficient and low-damage vibratory harvesting of C. oleifera fruits.