Effects Of Micro-structure On Wood Mechanical Properties Under Continuous Compression

Mechanical properties of wood and wood products are highly depended upon the interior structural features. Based on this theory, this paper introduced research methods and tendency of testing on wood mechanical properties, then introduced the applying of in-situ inspection on wood tissue mechanical level and analyzed the existing and unsolved problems of the present in-situ inspection system. A self-developed in-situ inspection platform had been developed, and then the structures of the platform and its implement methods were descried in detail. The mechanism of auto-focus photo-shooting system was given in the paper. By means of wood mechanical tests, detailed descriptions of the platform on how to achieve the variable technical characteristics had been obtained, and technical advantages of this platform were comprehensive analyzed.By virtue of the self-developed in-situ inspection system and earlywood simples in each growth ring of Chinese fir, wood mechanical behavior variations under continuous compressive loading and auto-focused in-situ inspection on microstructure characteristics of wood deformation had been systematically analyzed. Meanwhile, through the change of outer environment conditions, i.e. loading rate, temperature and moisture conditions, variations on mechanical properties in these situations were obtained, and quantitative analysis on microstructure variances were investigated with the help of the measuring and calculating software. The main conclusions of the paper are as follows:(1) The in-situ inspection system can be divided into two parts, one part is the loading system, and the other part is the picture-collection system. The two parts can be combined by the parameter of time. The picture collection system can automatically take photos of materials'surface deformation during the loading process. Meanwhile, by means of the quantitative calculation and measuring on the photos, the system can directly and precisely achieve the simulations of wood mechanical properties. (2) By using early wood samples in each growth ring, the result that yield strength and modulus of elasticity under radial compression are decreasing from heartwood to sapwood. Significant variations were observed among mechanical properties of different parts, i.e. heartwood, transition wood and sapwood. The in-situ inspection on wood micro-structural deformation and quantitative analysis results showed that the first yield positions of both heartwood and transition wood occurred at 15-20 cell layer of earlywood (643μm and 689μm from the surface of loading compression, respectively), while the first yield position of sapwood occurred at 5-10 cell layer of earlywood (153μm from the surface of loading compression).(3) The yield strength and modulus of elasticity at different loading rates are relevant to the first yield position. Loading rates of 1mm/min, 10mm/min and 50mm/min was adopted respectively for mechanical testing of typical softwood of Chinese fir (Cunninghamia lanceolata). The results indicated that the variations of wood mechanical performance at different loading rates were generated from the different position of the first yield point (689μm, 166μm and 23μm, respectively); the in-situ inspecting system could accurately characterize the variations of microstructure deformation. Furthermore, the microstructure characteristics could be used to explain mechanical behavior of wood suffered from different loading rates.(4) Mechanical properties, i.e. yield strength and modulus of elasticity, have the same tendency with the variation of temperature conditions. Mechanical properties at negative temperature are higher than that of at positive temperatures. Generally speaking, yield strength and modulus of elasticity are decreasing with the increase of temperature. The fracture behaviors are different at relatively low (≤60OC)and high temperature(s≥80OC).At relatively low temperatures (including -20OC,20OC and 60OC), the fracture behavior is tend to in a rigid way; While at relatively high temperatures (including 80OC and 120OC), the fracture behavior is tend to in a flexible way. At different temperatures(-20OC,20OC and 60OC), the first yield position is similar, which occurred at 15-20 cell layer of the earlywood (667μm,689μm and 643μm from the surface of loading compression, respectively), which inferred that the temperature had not affect the most vulnerable area in the sample.(5) The yield strength and modulus of elasticity are variable at different moisture conditions. Generally, the yield strength and modulus of elasticity are decreasing from fully dry to air-dry and then to fiber saturate condition. The first yield positions were similar at air-dry and full-dry moisture conditions, which occurred at 15-20 cell layer of the earlywood (664μm and 689μm from the surface of loading compression, respectively); while at firber saturate moisture condition, the free water in cell walls or cavities was compressed out which made it impossible to see the fracture surface.The fracture behaviors are different between the relatively low (fully-dry and air-dry)and high moisture conditions(fiber saturate).At relatively low moisture conditions, the fracture behavior is tend to be in a rigid way; While at relatively high moisture condition, the fracture behavior is tend to be in a flexible way.Above all, the self-developed inspecting method can help us understand and master the microstructure characteristics of wood deformation under continuous compressive loading, and can explain the variations of mechanical properties during continuous compression. The system can realize the effective anticipations of wood mechanical properties suffered from various compression conditions, and can quantitatively analyze the variance of wood microstructure characteristics.