Optimization Design Of WPC For Tree Support Frame Based On Fluid-structure Coupling Analysis

With the continuous expansion of urban landscaping area,more and more trees are transplanted into the city to be used as street trees or landscape trees.However,the newly transplanted trees may distort,broken branches,lodging and other damages under strong winds,resulting in economic losses and even casualties.Therefore,it is important to study the wind resistance of trees and methods to improve the wind resistance of trees.This article takes camphor and spruce as the research objects,compares their wind resistance,and analyzes the causes of tree damage under strong winds.It is of great theoretical significance and practical value to study the influence of different support heights on the wind resistance of trees when using wood-plastic composite（WPC）as support rot material,optimize the use method of tree support frame,and provide theoretical basis for the use of tree support frame.The main research contents and conclusions of this article include:（1）Taking the representative tree species in north and south of China,camphor and spruce,as the research objects,the structural characteristics of trees and their supporting methods were investigated,and the finite element modeling of trees before and after the support frame was added was carried out.The natural frequencies of camphor model and spruce model in modal analysis（Modal）are compared with the natural frequencies calculated by empirical formula.The results are close,indicating that the model has certain reliability.（2）Compared with the wind-induced response of camphor and spruce without support,with the increase of wind speed,the treetop displacement,shear stress and bending moment of camphor and spruce increase,so the risk of branch breaking and lodging of camphor and spruce increases.Under the condition of 20 m/s wind speed,the maximum transverse shear stress（3.48 MPa）of camphor is less than 85%of its transverse shear strength（3.91 MPa）,and the possibility of transverse shear failure of camphor is small.The maximum transverse shear stress（2.68 MPa）of the spruce is greater than 85%of its transverse shear strength（2.38 MPa）,and the spruce may undergo transverse shear failure,which indicates that the wind resistance of camphor is stronger than that of spruce.Through the post-processing of the simulation,it was found that the position of the maximum absolute value of the transverse shear stress of camphor and spruce（damage dangerous point）was located at the junction of the primary branch and the trunk below the tree.Therefore,when the tree has no support protection,the branches under the tree should be trimmed or properly reinforced to reduce the risk of strong wind damage to the branches under the tree.（3）Combined with the use scenario of tree support frame,WPC was prepared with poplar powder and high density polyethylene（HDPE）as raw materials for tree support rod materials.When the content of poplar powder was 30%,WPC had good physical and mechanical properties.The flexural strength,elastic modulus,density and poisson’s ratio of WPC were 53.1MPa,2.9 GPa,1.050 g·cm^-3 and 0.35,respectively.After six months of outdoor natural aging,WPC still had excellent physical and mechanical properties.The retention rates of flexural strength and elastic modulus were91.64%and 86.76%,respectively.Poisson’s ratio and density remained basically unchanged after aging.WPC has similar bending properties to Chinese fir support rod materials,and WPC also has excellent water resistance,aging resistance and environmental protection properties.The properties of WPC are compatible with the properties of tree support rods,so it is speculated that WPC is suitable for use as tree support rod materials.（4）The physical and mechanical properties of WPC after aging were given to the tree support rod.Compared with Chinese fir support rod,WPC support rod and Chinese fir support rod have almost the same support effect on trees under the same support conditions.Under the condition of 20 m/s wind speed,the wind-induced shear stress of camphor and spruce was reduced after the WPC support frame was set up at the height of 1/8th,2/8th and 3/8th of camphor and spruce respectively.When the support height of WPC support frame is located at 3/8th height of camphor tree and2/8th height of spruce tree,respectively,the wind-induced shear stress of camphor and spruce is relatively small,which is 1.17 MPa and 1.26 MPa respectively,which is 66%and 53%lower than that without support.The risk of camphor branches and spruce branches being broken by instantaneous strong wind is greatly reduced.The wind-induced bending moment of camphor and spruce was greatly reduced,only about 1/10 of that without support,and the probability of lodging of camphor and spruce was greatly reduced.The swing frequency of camphor and spruce above the support frame is higher than that without support,and the higher the support height,the greater the swing frequency.（5）Under the condition of 20 m/s wind speed,when the diameter of WPC support rod is 6 cm,the maximum stress of WPC support rod with different support heights is less than the allowable stress of WPC,and the strength of WPC support rod is sufficient and reliable.Considering the factors of cost and safety,when the diameter of WPC support rod is reduced to 4 cm,the maximum stress of WPC support rod is still less than the allowable stress of WPC,and the strength of WPC support rod is still surplus.In order to further reduce the cost and make full use of the properties of WPC materials,hollow or reinforced ribs can be designed inside the WPC support rod.