Investigation On Elastic Behavior Of Palm Sheath Fiber Based On Short Fiber Reinforced Composite Theory

Palm sheath fiber is a kind of natural material widely distributed in nature,and also a biological composite material with obvious multi-scale structure.Because of the environmental protection and sustainability,its mechanical properties have attracted the attention of many researchers.In this paper,analytical models and finite element models were established to study the elastic mechanical characteristics of palm sheath fiber.Palm sheath fiber was regarded as a kind of composite material which takes the slender single fiber with cavity as the reinforcement phase and the intercellular material between the single fibers as the matrix.The mechanical characteristics of different scales of palm sheath fiber,including palm sheath fiber,single fiber and single fiber cell wall,were measured.The stress-strain elastic behavior characteristics of palm sheath fiber and bamboo vascular bundle under elastic strain were simulated by using analytical model and finite element model,and the role of each component and its contribution to the performance of composite were discussed.Because the vascular bundle of moso bamboo has similar structure with the sheath of palm fiber,and there are abundant research bases on its structure and mechanical properties.This paper quoted the performance parameters of moso bamboo vascular bundle from other researchers to establish the models to conducted a comparative study with the sheath of palm fiber.The main conclusions are as follows:(1)The tensile test of palm sheath fiber was carried out.Results show that its modulus is between 1.1-1.7GPa and the tensile strength is between 130-160 MPa.The crystallinity of palm sheath fiber is 34.13% by XRD.Combining 30% hydrogen peroxide + glacial acetic acid,mechanical separation and ultrasonic,palm single fiber was separated by different ways,and tensile tests were carried out.From the SEM graphs of single fiber,circular pits unevenly distribution on the cell wall.The average length of single fiber is 761.38?m,and the average diameter is 6.57?m.Comparing the influence of different separation methods on the mechanical properties of single fiber,the method of separation solution + mechanical separation has the smallest damage to the single fibers.The modulus of single fiber obtained by this method range from 970 MPa to 1900 MPa,and the average modulus is 1.43 GPa.The strength ranges in 170-510 MPa,and the average strength is 337 MPa.The average modulus of palm cell wall measured by AFM is 6058 MPa.(2)Finite element models were established to study the stress distribution of palm sheath fiber and bamboo vascular bundle under elastic deformation.The finite element representative volume element model was hexagonal prisms,which increased the upper limit of the volume content of single fiber.The prediction results of palm sheath fiber and bamboo vascular bundle solid model are close to the experimental test results.The prediction modulus are 1198.94 MPa and 23222.88 MPa,respectively,and the error of prediction value is only 2.4% and 4.4% compared with the test results.When the defect parameters of palm sheath fiber and bamboo vascular bundle are 0.571 and 0.904 respectively,the prediction modulus of hollow model is equal to the experimental results,which shows that the defect of single fiber has a certain influence on the modulus of composite material,and the influence on palm sheath fiber is greater than that of bamboo vascular bundle.Because the cell wall of moso bamboo single fiber is thick,it was easy to observe that the closer to the edge of the cell wall is,the greater the stress is.The modulus of single fiber cell wall has a certain influence on the stress distribution of the matrix.The larger the modulus is,the more concentrated the matrix stress is at the tip of the single fiber.However,the main part of the fiber is uniformly distributed along the longitudinal direction,and whether the stress concentration at the head end of the fibers is related to the overall modulus of the single fiber.The interface phase was introduced into the finite element model,and it is found that the influence of the interface on the material modulus still conforms to the rule of mixtures.(3)Combined with the Bowyer-Bader equation which based on the two-dimensional composite structure,an analytical model of composite material based on the three-dimensional structure was established to predict the elastic modulus of palm sheath fiber and bamboo vascular bundle.The results show that the two-dimensional model is not ideal for the prediction of the modulus of hollow fiber-reinforced composite material.The error of the improved three-dimensional solid model is 9.3% and 5.89% compared with the test results.When ? is 0.556 and 0.860 respectively,the predictive modulus of the three-dimensional hollow analytical model is equal to the experimental value,which shows that the single fiber defect has a certain impact on the modulus of the composite,and has a greater impact on the palm sheath fiber.(4)The effects of single fiber modulus,cell wall modulus,cell wall thickness and single fiber length on the predictive modulus of palm sheath fiber and bamboo vascular bundle were analyzed with finite element model and analytical model.The results shows that the predictive modulus of solid and hollow model increase with the modulus of single fiber and the modulus of single fiber cell wall respectively;the prediction modulus of three-dimensional hollow model increase with the thickness of cell wall.With thickness of cell wall increase,the proportion of cavity decrease.So the prediction modulus increased,and the growth rate decreased gradually.The relationship between the predicted modulus and the volume content of single fiber is closely related to the overall modulus of single fiber.When the overall modulus of single fiber is larger than that of matrix,the predicted modulus increases with the increase of the volume content of single fiber.The analytical model analysis results show that when the length diameter ratio of single fiber is larger than 20,its influence of the predictive modulus is very small.