Longitudinal Viscoelasticity And Its Main Influencing Factors Of Tracheids Of Pinus Massoniana Lamb

Wood is a kind of natural macromolecule composite material mainly composed of cellulose,hemicellulose and lignin.Its viscoelastic behavior is closely related to the micro-structural factors and chemical components of wood.The macro viscoelastic behavior of wood has been deeply studied,and its micro viscoelastic behavior needs further breakthroughs.As an important means to study wood micromechanics,nano-indentation technology has been accepted by many scientific researchers,which provides a new means to further study wood microviscoelastic behavior.The purpose of this paper is to elucidate the mechanism of micro-structure,chemical compositionand water on wood viscoelastic properties,and to understand the essence of wood viscoelasticity.Based on the project of Natural Science Foundation and previous research,the cell wall of coniferous wood with relatively simple cell structure was selected as the research object;The results of this study provide basic data and theoretical basis for the later study of the changes of cell wall viscoelasticity after wood modification.Pinus massoniana,which is most widely planted in the south of China,has obvious distinction between early and late timber.Firstly,the effects of loading function parameters on the modulus of elasticity,hardness and creep of wood cell wall during nanoindentation were studied,so as to determine the parameters standards that should be used in later experiments.Then,the changes of wood cell wall viscoelasticity and its influencing factors were explored;Respectively,in terms of wood micro-structure,wood chemical composition and water content,and combined with microfibril angle and crystallinity characteristic parameters.Finally,combined with various micro-mechanical models of wood cell wall,the specific influencing factors in this paper are analyzed and verified concretely,so as to verify the reliability and validity of the methods and experimental data used in the study.The main conclusions of this paper are as follows:(1)With the increase of loading load,the modulus of elasticity increases gradually.When the load reaches 200?N,it reaches its maximum and then tends to be stable.When the load is 400?N,the elasticity modulus begins to decrease;The cell wall hardness tends to increase gradually,and the maximum increase is about 10%;The maximum difference of modulus of elasticity is 16.85%.It can be seen that the change of loading load has an effect on elasticity modulus and hardness.When the effect of loading time on elasticity modulus and hardness was studied,the change of loading time had a significant effect on the elasticity modulus of wood cell wall,and the hardness increased with the increase of loading time.When the effect of holding time on elasticity modulus and hardness was studied,the elasticity modulus and hardness decreased gradually with the increase of holding time,and both of them decreased by more than 20%.Creep ratio increases with the increase of loading load,decreases with the increase of loading time and increases with the increase of holding time.According to the experimental data and the power law principle,it can be seen that the linear viscoelastic behavior can be approximated when the loading load is 150-300?N,It can be concluded that the appropriate loading range is200-300?N for nanoindentation experiments on conifers such as Pinus massoniana.(2)According to the above research results,the variation of longitudinal viscoelastic property of masson pine cell wall was studied by loading 250?N in the nano indentation experiment.To investigate the viscoelasticity of cell walls at different heights,rings and early and late wood,and to analyze the relationship between the viscoelasticity and microfibril Angle and chemical composition of wood.First,the elastic modulus of cell wall changed significantly at height 1 m,3 m,6 m and 9 m,and increased with the increase of height;And its hardness value changed slightly;In the radial direction,the elastic modulus and hardness of wood increased gradually with the increase of annual rings,and the elastic modulus and hardness of juvenile wood were significantly lower than that of mature wood due to the change of microfibril angle and chemical composition;The creep rate decreases with the increase of height,and the creep compliance decreases;The creep rate of radial juvenile wood is greater than that of mature wood,and the creep compliance is greater.In the study of dynamic viscoelasticity,the storage modulus of cell wall increases with the increase of height,while the loss modulus decreases gradually;The storage modulus increases and the loss modulus decreases with the increase of annual rings in the radial direction,The storage modulus of juvenile wood is obviously smaller than that of mature wood,but its loss modulus is larger.In this study,the height of the tree ranged from 1 to 9 m,which only accounted for less than half of the whole tree's height of 20.1 m.It was not found that there was a turning change in the mechanical properties of the cell wall of the wood in this study area.(3)In the third chapter,we can see that the viscoelasticity of wood cell wall is not only related to the angle of microfibril,but also to the chemical composition of wood.In this chapter,the effects of wood chemical composition on the viscoelastic properties of cell wall are studied.The hemicellulose and lignin are gradually removed by chemical treatment,and the viscoelastic properties of cell wall are studied and analyzed.The results show that:Wood extracts have limited effects on cell wall mechanics,while the removal of hemicellulose and lignin significantly reduces the elasticity modulus and hardness of cell wall,and improves creep flexibility.From the Burgers model,it can be seen that cellulose plays a leading role in the mechanical properties of wood cell wall,hemicellulose contributes more to the cell wall viscosity than elasticity,and lignin contributes more to the cell wall elasticity;Wood extract had limited effect on the dynamic viscoelasticity of cell wall,while the removal of hemicellulose and lignin significantly reduced the storage modulus of cell wall and increased the loss modulus.(4)In addition to the effect of wood microfibril angle and chemical composition on cell wall viscoelasticity,wood moisture content is also an important factor.This chapter will study the effect of water content on the viscoelastic properties of wood cell wall through four different water content ranges.The results show that there is a negative correlation between elastic modulus and hardness and moisture content;The creep compliance calculated by the experimental results is compared with the fitting curve obtained by Burgers model.The results show that with the increase of water content,the viscoelastic parameters E₁,E₂,?₁and?₂tend to decrease;Experiments show that the storage modulus of wood cell wall of Pinus massoniana decreases gradually with the increase of water content,while the loss modulus increases gradually.(5)In this paper,the effects of wood microfiber Angle,chemical composition and moisture content on wood cell wall viscoelasticity were studied;Furthermore,the micromechanical theory and the mechanical constitutive model of wood cell wall were used to analyze the results.The influence of fiber content,microfiber Angle and moisture content on the longitudinal viscoelastic properties of wood cell wall was predicted by the correlative mechanical model,and the accuracy of experimental data and the rationality of experimental method were evaluated.