Morphology And Micromechanics Of Masson Pine Tracheids

The remarkable mechanical properties of multicellular biomaterials reside in their complex hierarchical architecture and in specific molecular mechanistic phenomena. Mechanical behaviors of wood are defined by the morphology and mechanical properties of wood tracheids and their bonding medium. Exploring the mechanical behavior underlying the single tracheid, and building the model of multi-tracheids, make them the most important influence at the tracheid level and molecular level. It is urgent to make further investigation of the morphology and mechanical properties of wood tracheids.Softwoods are usually treat as the model material for theirs simple microstructure. In this thesis, in order to explain the relationships between structure and tracheid mechanical properties, X-ray diffraction, Zero-span tensile test and nanoindentor were used to investigate the microfibril angle, longitudinal tensile strength, cell wall modulus of elasity and cell wall hardness, respectively. The main research results were sumerized as following:(1) From pith to 8th growth ring, the radial variation patterns of tracheid length are increasing rapidly. Between 8th and 14th growth ring, tracheids length changed annomally and reach the max values. After 14th growth ring, tracheids length decrease slightly. In sum, the distribution of tracheids length of Masson Pine is nomal. And 60% tracheids length of juvenilewood distribute between 2500μm and 4000μm, 70% tracheids length of maturewood distribute between 4000μm and 5500μm. From pith outward, the tracheid width is increasing gradually, at 7th growth ring reach constant value. The average value of maturewood tracheid width at 3 meter height is 45μm, 50μm at 6 meter height, 55μm at 9 meter height.From pith to 8th growth ring, the ratio of tracheid length to tracheid width is increasing rapidly, the min value emerge at corewood, which is 55. After 14th growth ring, tracheids length change slightly, and the mean value is 80-100. The tangential diameter almost at constant intr-ring, the value is between 40μm and 45μm. Radial diameter of earlywood is 60μm, while 20μm of latewood. Thickness of double cell wall is 5μm of earlywood, while 15μm of latewood.(2) The method we have proposed using X-ray diffraction has been shown to be capable of yielding microfibril angle values. By the peak-fitting method we can determine the shape of the microfibril angle distribution. Results indicate that the microfibril angle is 20°-35°in heartwood at any height. From pith to 8th growth ring, the microfibril angle is increasing linearly. Between 8th and 14th growth ring, microfibril angle changed annomally. After 14th growth ring, the tracheid length mainly maintains constant value. In longitudinal dirction, the microfibril angle reaches max value at 0 meter height, and averge value is 16°-20°in maturewood. At 3 meter height, the averge value is 12°-18°in maturewood. There is no significant diffence between 6 and 9 meters, the averge value is 10°-15°in maturewood. No matter of maturewood and juvenilewood, the microfibril angle in earlywood is bigger than latewood. The differences are about 10°in jucenilewood, while only 1°-3°in maturewood.(3) The zero-span tension technique was used to estimate rapidly the longitudinal tensile strength of tracheids of Masson pine plantation wood. It was found optimum thickness of samples and clamping pressure for zero-span tension was 80μm and 70 psi respectively. Statistical analyses indicated stable increase of tracheids tensile strength from bark to pith, while no significant variation was observed along tree height from 6 m to 9 m. Therefore, it was inferred that zero-span tension could be used as a novel and reliable method for assessing the logitudinal tensile strength of softwood tracheids.(4) In situ imaging nanoindentation was used to characterize the differences in longitudinal modulus of elasticity and hardness of different cell wall layers of softwood tracheids, It was found that the distribution of longitudinal modulus of elasticity and hardness along cell wall thickness was uneven with much lower values at the interface between S₃ layer and cell cavity as well as SK layer and compound middle lamella compared with that measured in S₂ layer. There also existed differences in modulus of elasticity and hardness between S₂ layers of adjacent tracheids. Both modulus of elasticity and hardness of mature wood trachedis were larger than that of juvenile wood. The sapwood of mature wood tracheids displayed a modulus of elasticity nearly 1.4 times of that of juvenile wood, but only 13% harder than juvenile wood. A positive linear correlation between modulus of elasticity and hardness was also found both for sapwood and compound middle lamella of tracheids.