The Longitudinal Tensile Elasticity And Creep Properties Of Masson Pine (Pinus Massoniana Lamb.) Wood From Plantation

The excellent longitudinal mechanical properties of wood not only directly relate to theproperties of the three main constituent materials in the wood cell wall, but also are affected bythe highly optimized hierarchical structure of wood. There are different mechanical propertiesbetween woods at macro-and micro-scale. To study the longitudinal mechanical properties ofwood at the millimetre level can build the connection between macro-and micro-mechanicalproperties of wood, which can provide scientific basis for understanding the transformationmechanisms of physical and mechanical properties of wood between different scales.Studies were conducted on the effects of moisture content (MC) and microfibril angle(MFA) on the longitudinal tensile elastic and viscoelastic properties of clear thin wood usingaxial micro-tensile test method. The test specimens (1.5mm×10mm×80mm in R×T×Ldirections) were cut from19discs at the breast height (BHT) of Masson pine (Pinusmassoniana Lamb.). It was the first time to experimentally study the longitudinal tensilecreepand the longitudinal tensile mechano-sorptive (MS) creep of wood in China.In this paper, the density, MFA, tensile modulus of elasticity parallel to grain (EL),short-term longitudinal tensile creep under the constant climatic conditions and short-termlongitudinal tensile MS creep under the cyclic variation of40%-90%relative humidity (RH) ofMasson pine wood were experimentally determined at room temperature. Then the radialvariations of density and MFA, and the effects of MFA and MC on the ELwere analyzed. Inaddition, the relationships between longitudinal tensile creep, longitudinal tensile MS creepand MFA, MC were revealed respectively. The main research results are as follows:1. The basic density of Masson pine plantation wood at the breast height average0.428g/cm3. The average earlywood density and latewood density are0.296g/cm3and0.460g/cm3respectively. The lower density of latewood is one of the main reasons why Masson pine wood from plantation has lower density compared to its natural wood. The wood density increasesfrom the pith to the bark.2. The MFA of Masson pine plantation wood at the breast height ranges from9.8°to45.6°with an average of17.5°. The MFA decreases as the growth ring goes outward from thepith until after the20th growth ring remains constant.3. The ELof Masson pine plantation wood averages10948MPa with7812MPa forjuvenile wood and11950MPa for mature wood. The average ELof early-and latewood are5455MPa and8964MPa.4. The wood density is significantly positively correlated with ELat0.01level and50%ofthe variation of ELis caused by the wood density. Contrary to density, the relationship betweenMFA and ELis nonlinear. In general, the wood ELdecreases as MFA increases. But when MFAis larger than15°, it can linearly relate the ELto MFA and the determination coefficient R2isaround0.6.5. The ELof Masson pine plantation wood increases as MC decreases below fibersaturation point (FSP), and reach a maximum value about5.5%MC and then decreases withfurther drying. The effect of MFA on the MC dependency of the ELis obvious. In general, thelarger the MFA of wood, the more its ELdecreases as MC increases. This demonstrates that theMC dependency of the ELis according with the traditional theory, which considers that thedecrease of wood ELis mainly caused by the MC dependency of matrix oflignin-hemicellulose.6. The longitudinal tensile creep deformation of Masson pine wood under steady moisturecondition tends to increase with the elapsed time. The longitudinal tensile creep property ofwood is highly dependent on the MFA. The larger the MFA of wood, the larger the initialsubstantial creep compliance becomes. Both the initial substantial creep compliance incrementand the time to reach the steady-state creep increase with the MFA increases. This can beexplained by the composite structures of the wood cell wall. The longitudinal tensile creep ofwood with small MFA is dominated mainly by the cellulose microfibril (CMF) in the S2layer. Because the viscosity of the CMF is much larger than that of the matrix substance. By contrast,at a large MFA, the matrix substance in the cell wall affects the longitudinal tensile creepbehavior, because the mechanical contribution of the CMF decreases. Because the matrixsubstance is less viscous than the CMF, the creep compliance becomes several times largerthan at a small MFA. The simplified three components model can explain the longitudinaltensile creep of1.5mm thick Masson pine wood.7. The longitudinal tensile creep property of Masson pine wood is sensitive to MC. Theinstantaneous substantial creep compliance, the substantial creep compliance increment and thetime to achieve the steady-state creep all increase with the MC increases.8. MS is a complex phenomenon which is ascribed to transient hydrogen bonding,changing in molecular mobility, and internal stress gradients due to the presence of moisturegradients. However, the underlying mechanisms remain poorly understood. But with regard toMasson pine wood, the MFA affects the sensitivity of the longitudinal tensile MS creep. Thelarger the MFA, the larger the longitudinal tensile MS creep deformation is. The longitudinaltensile MS creep of wood is related to its initial MC. The greater the initial MC, the larger thelongitudinal tensile MS creep deformation is. The longitudinal tensile MS creep of wood tendsto be a constant creep limit under a cyclic change of RH at room temperature.This paper carried out the experiments about the effects of density, microfibril angle andmoisture content on the tensile modulus of elasticity parallel to grain of wood, longitudinaltensile creep and longitudinal tensile mechano-sorptive creep, and obtained abundantexperimental data of longitudinal tensile elastic and creep properties of Masson pine woodfrom plantation. It also analysed and revealed the longitudinal mechanical behavior mechanismof wood under millimetre scale. These results may provide a reference for the efficient use ofMasson pine wood from plantation, as well as enrich the content of the wood rheology. So ithas important academic values and instruction meaning for practical application.