Grading Technology Research On Structural Larch Dimension Lumber

As an important commercial forest resource,Larix gmelinii is a kind of ideal material for wood structural timber, which owns fast growth, high strength, decay resistance characteristics and so forth. For structural dimension lumber, the grading technology plays a significant role in improving wood utilization efficiency.In this paper, Dahurian larch was studied, and the cross-section dimensions of samples were three types: 40mm×65mm, 40mm×90mm and 40mm×140mm, and the length was 4000mm. Through visual grading, we obtained the visual grades and the major grade-reducing defects of larch dimension lumber. Full-size bending test was carried out on the visual graded dimension lumbers to achieve the relationships between different mechanical properties and meanwhile provide the references for mechanical grading. Based on knots as the main factor, the theoretical models of bending properties were established. In addition, according to the strength properties of larch dimension lumber, the grade boundaries were set and then mechanical grades of larch dimension lumber were got. At the end, the suitable grading system for Chinese larch dimension lumber was obtained through a comprehensive comparison of different grading methods.The main results of this paper were summarized as follows:(1) For larch dimension lumber with the same thickness, the proportion ofâ… c grade obviously increased with the lumber width, but the ratios ofâ…£c and R grade decreased in different extents. The ratio ofâ… c grade of 40mm×140mm dimension lumber accounted for nearly 50%, and as to theâ…£c grade, the percentage was more than 80%. The major grade-reducing defects of larch dimension lumber were knot, shake/checks, wane and decay. The knot was the main grade-reducing defect ofâ…¡c grade and shake/checks had the most serious impact on low-grade lumber compared with other defects. In addition, the processing methods of dimension lumber were needed to optimized under the premise of satisfying the volume ratio, because those could affect the types and proportions of defects that influenced the strength of lumber.(2) Forâ… c grade of larch dimension lumber, the characteristic value of MOR was significantly higher than that ofâ…¡c andâ…£c grade, but the mean value of MOE was slighter higher than that of other grades. Visual grades and dimensions had significant effects on the bending properties. The results showed that there was a good relationship between MOE and MOR. Based on the basis of bending properties, the study of mechanical properties can provide technical support for assessing the mechanical properties of other domestic structural materials.(3) The best fitted effect for full-size bending strength was obtained using average density and the ratio of moment of inertia I_K/I_g for the tension side of knot which located within the maximum strength reducing defect (MSRD) zone to the whole cross-section . The theoretical model of full-size bending strength for larch dimension lumber was , and the coefficient of determination (R²) was f_full=- 65 .04I_k/I_g+51.56ρ^{1 .25}+15.120.798 and RMSE was 4.73. With average density and the ratio of moment of inertia IK/Ig for all the knots located within the test span to the entire cross-section as independent variables, in additon, the raito of moment of flexure MK/Mo for the location of knot to the center position of the testing specimen was also considered, the theoretical model of MOE for larch dimension lumber was E_full = -5.19∑[(I_K/I_g)·(M_K/M_o)] +8.30ρ^1.25+6.71 , R² was 0.0.681 and RMSE was 0.97. As for the full-size bending properties, clear specimen bending properties and knot information, the models among the three factors were f_full / f_clear=-0.70I_k/I_g+0.46 andE_full/ E_clear=-0.49∑(I_k/I_g)·(M_k/M₀ )+0.96 . The size of knot has very serious effcet on the bending strength of full-size dimension lumber.(4) The MOE data of larch dimension lumber obeyed normal distribution, but MOR data was biased normal distribution and the data was scattered. According to the data distribution, the mechanical grade boundaries was determined with the static bending test MOE as a reference index for larch dimension lumber, and then they were sorted into M14, M30 and M40 grade. Among mechanical grades, the proportion of M30 and M40 accounted for more than 80%. More higher grades for larch dimension lumber were achieved, such as grade M30, M40 and other grades. Mechanical grading is a good grading method for larch lumber in the construction. Besides, mechanical grading method can increase utilization value and provide a guarantee for the effective application of larch lumber in the construction.(5) Mechanical grading can enhance the grading efficiency and improve the strength characteristic values of high-grade dimension lumber greatly, for instance, the characteristic value of M40 grade is higher nearly 30% than that ofâ… c grade . Besides, mechanical grading can decrease the coefficient of variation of characteristic values for dimension lumber. That is to say, the strength properties of lumber were more stable. After a comprehensive comparison of visual grading and mechanical grading for larch dimensipon lumber, it can be concluded that mechanical grading is more suitable for Chinese larch dimension lumber Mechanical grading can fully develop domestic forest resources, play geographical advantages and reduce energy consumption. The ultimate goal of mechanical grading is to obtain the best combination of economic and environmental benefits.