Tensile Mechanical Behavior Of Palm Leaf Sheath Fiber And Its Single Fiber Based On Fractal Theory

Palm leaf sheath fiber is a kind of natural fiber with multi-scale structure.Due to its characteristics of corrosion resistance and ultraviolet shielding property,it has been widely used in furniture building materials,environmental engineering and other fields,and its special mechanical properties have attracted the attention of many researchers.In this work,based on the fractal theory,the three-circle fractal model and the finite element model of the palm leaf sheath fiber and its single fiber were established respectively.The fractal dimension was introduced to study the tensile mechanical behavior of the palm leaf sheath fiber and its single fiber.This paper observed the morphology of palm leaf sheath fiber and its single fiber by SEM and TEM,then measured the palm leaf sheath fibers and single fibers performance characteristics of the two scales.A three-circle fractal model of palm sheath fiber was established,and each scale's fractal dimension of the palm sheath fiber was calculated by theoretical calculation and image analysis.Through stress-fractal dimension-diameter,fracture energy-fractal dimension-diameter,Young's modulus-fractal dimension-diameter relationship formula calculated the mechanical properties under different scales,and the contribution of all scales is discussed on the tensile properties of fiber.This paper built the finite element model to simulate the single fiber's tensile mechanics behavior.The ideal finite element model was a hollow cylinder,and the damage model was given spirals and grooves structure on this basis.Using the two of the models to study the stress distribution,mechanical curve and fracture surface morphology.The fractal dimension was introduced into the damage finite element model to quantitatively characterize the single fiber ‘s fracture surface morphology and explore the single fiber's fracture mechanism.The main conclusions were as follows:(1)The morphology of palm leaf sheath fiber and its single fiber was observed.Results showed that the porous structure of palm leaf sheath fiber cross-section had obvious fractal characteristics,the pore distribution was uniform and simple,the porosity was 57.69% and average pore diameter was 5.33 ?m,it was a macroporous material;After treated with 6 hours and 9 hours with delignification solution(30%hydrogen peroxide and glacial acetic acid 1:1 mix by volume),the surface topography has no obvious difference between two of palm single fibers,all smooth surface has a certain degree of shrinkage.While after 14 hours,single fibers surface topography showed unique Z-spiral structure,helix angle of about 37.75 ° and the gap between the spiral and the spiral was 4.94 ?m;Compared with palm leaf sheath fiber's chemical composition,the chemical composition of single fiber is cellulose and a small amount of hemicellulose.The crystalline form of cellulose remains unchanged,the crystallinity decreases to 20.78°,and the grain size decreases to 1.3 nm.(2)The tensile mechanical properties of palm sheath fibers and single fibers were studied.the result shows the palm leaf sheath fiber's tensile curve has obvious yield point while single fibers not.Moreover,single fiber's strength follows the Weibull distribution,and the fracture strength ranges from 150 MPa to 350 MPa;The fracture surface of single fiber was rough and uneven,showing transverse fracture with spiral crack,which belongs to ductile fracture.(3)Three-circle fractal model was established for different scales of palm leaf sheath fiber,and the contributions of different scales to the tensile properties were studied.Theoretical calculation and image analysis were used to calculate the fractal dimension of each fiber scales.The mechanical properties at different scales were calculated by the formulae of stress-fractal dimension-diameter,fracture energy-fractal dimension-diameter,and Young's modulus-fractal dimension-diameter.The results showed that the horizontal fractal dimensions of palm leaf sheath fibers,single fibers,microfibers and fibrils were 1.97,1.88,0.74 and 1.04,respectively.The stress and Young's modulus of the fibers are determined by the scale of fibril filaments-microfibril,while the fracture energy is determined by the microfibril-single fiber;Combining tensile tests with theoretical calculations,the microfibril determines the fiber's stiffness and the single fiber plays a key role in the fiber's toughness.(4)The ideal finite element model and damage finite element model of single fiber were established to study the tensile mechanical behavior and analyze the tensile fracture mechanism.The tensile curve analysis of the model showed that the curve trend of the two models is consistent with the experimental curve trend,and the error between the mechanical properties obtained by the damage model and the actual one was less than 2%;The simulation results the ideal single fiber model showed that the stress of cell wall peripheral part is greater than that of the part near the cell cavity,but the fracture surface is not consistent with the reality.The damage model showed that after the stress concentration occurs at the grooves,the cracks occurred along the spiral direction until the fracture,which is consistent with the actual situation;The stress and strain at different parts of the damage model's fracture surface were analyzed.The strain and fracture energy near the cell cavity were slightly larger than those outside the cell wall,while the stress and Young's modulus were not significantly different.The stress and strain at model's defect were all smaller than those at other intact parts and the strain at the helical tear was larger and the stress was slightly smaller;The fractal dimension of the single fiber's actual fracture surface was 1.94,and that of the model fracture surface is 1.93,which is close to the actual situation.(5)Different tensile rates were applied to the damage model,the results showed when the velocity was low,the fracture surface was more regular,the spiral tearing behavior was obvious and the fractal dimension was small.On the contrary,when the velocity was high,the fracture surface was more irregular,the spiral tearing behavior was not obvious,and the fractal dimension was large;The fractal dimension was about1.89 when the same rate was applied to different single fiber damage models,which has no obvious change rule with Young's modulus,breaking energy and elongation at break,but has a positive linear correlation with breaking strength;Combined with the experimental and model analysis of single fiber tensile fracture behavior,it is found that the spiral fracture of single fiber was caused by its own spiral structure,and the spiral tear was obviously related to the tensile rate.