2024-06-15T14:35:10Z
https://soar-ir.repo.nii.ac.jp/oai
oai:soar-ir.repo.nii.ac.jp:00011195
2022-12-14T03:59:10Z
1016:1018:1103:1106
信州産カラマツ実大材の強度評価の寸法効果と機械等級区分
Size Effects on Strength Evaluation and Mechanical Grading in Japanese Larch Lumber from Shinshu
武田, 孝志
The report was reconstructed from a series of the individual investigations on mechanical properties of Japanese larch (Larix kaempferi, Carriere) lumber, and it consists of the following five sections. 1. Introduction Japanese larch is one of the most important species in Shinshu (Nagano prefecture) as stock of the planted forests, and it has been increasingly expected for practical use as structural wood products. Thus, it is necessary to evaluate more precisely strength properties of Japanese larch lumber now. The principles of reliability-based design have been introduced to alter the code for timber structures in the developed countries. In the case, reliability calculations depend on the lower tail of the strength distribution, and it means that there will be small potentiality of the lumber without knowing the lower tail in the future. It is important to implement the true structural behavior of timber, and size effect is one of the chief concern of many researchers after 1990's. Then I focussed on the dependency of strength distribution on Young's modulus in Japanese larch lumber. Experimental works were done, and some suggestions on strength evaluation were presented. 2. The Fifth Percentile Estimate of Bending, Longitudinal Compressive, and Tensile Strengths of Square Sawn Larch Timbers from Shinshu. To investigate the mechanical properties of Japanese Larch square sawn timbers, bending, longitudinal compressive, and tensile tests were conducted. Sawn timbers for specimens were processed from 72 trees from seven stands in Shinshu. Specimen dimensions of cross-sections were nominal 12 cm × 12 cm. First, normal distribution, log-normal distribution and 2P-Weibull fit to bending, compressive, and tensile strength data in order to estimate the fifth percentile values. In the case of the 2P-Weibull, it fits to all data and to the lower tail 15% data. The results indicated that the values obtained from each distribution were almost equal to non-parametric estimates (NPE) in compression, but the values from log-normal and 2P-Weibull fits to the lower 15% data were near the NPE in bending and tension. Then, the distributions of the strengths of specimens classified by modulus of elasticity (MOE) were different in bending, compression, and tension. In the case of tension, there were small differences between the distributions of strengths of the low-grade specimens and the medium-grade specimens. From the distributions of the averages of MOE classified by strengths, it was shown that we may be analyzing experimental data as one group of lower 15% data in bending and the lower 50% data in tension. [Takeda T, Tokumoto M, Nakano T, Hashizume T, Nagao H (1998) Mokuzai Gakkaishi 44 : 170-177] 3. Effect of Number of Laminae on Mechanical Properties of Glued Laminated Timber Composed of Homogeneous-Grade Lumbers of Japanese Larch 3.1 Effect of Number of Laminae on Dynamic MOE of Glued Laminated Timber Composed of Homogeneous-Grade MSR Lumbers of Japanese Larch The tapping method to measure the Young's modulus of wood is useful for nondestructive evaluation of sawn lumbers. The dynamic MOE values are calculated from the resonance frequency of the tap tone with a FFT spectrum analyzer. However, if a glulam is used for a bending member such as a beam, it is not certain that the static MOE value of the glulam is equal to the dynamic MOE value. In case of glulams composed of sawn lumbers from juvenile wood, some adjustment to the dynamic MOE values may be necessary for use. Then the relationship between static MOE (E_s) and dynamic MOE(E_d) was investigated, since it was expected that the variation of MOE in the cross section of homogeneous-grade glulams would be relatively small. Two types of specimens were used: glulams laminated horizontally (H-type) and vertically (V-type). Dynamic MOE was measured by longitudinal vibration(E_l) and flexural vibration, whose directions of which were horizontal(E_h) and vertical(E_v) in the direction of the adhesive face of the glulam. The results indicated that E_h/E_s in H-type and E_v/E_s in V-type were not influenced by the number of laminae. As it was supposed that E_l/E_h should be influenced by the number of laminae, the relationship between E_l and Eh was estimated by simple simulation for different numbers of laminae. The estimated ratios of E_l, E_h and E_v to E_s were almost identical with the experimental data. [Takeda T, Hashizume T (1997) J Soc Mater Sci 46 : 839-844] 3.2 Properties of Compressive Strength Parallel to the Grain of Structural Glued Laminated Timber Composed of Homogeneous-Grade Lumbers of Japanese Larch It is known that bending strengths of glued laminated beams (glulams) may be dominated by tensile strengths of outer layers of glulams. Since compressive strengths as well as tensile strengths of laminae should effect the stress distribution through the depth of the beam, the compressive strength may be important to estimate the bending strength of the beam. The effects of number of laminae on compressive strengths of glulams composed of homogeneous-grade lumbers of Japanese larch were investigated. The compression tests parallel to the grains with various number of laminae and different grades of glulams were conducted. The test results showed that the effect of lamina grades on the mean compressive strength (CS) were clear and the correlation between the mean CS and specific gravity (SG) were considerably high. Calculation was done for the ratio of compressive strength to specific gravity, so called specific strengths(SCS). The effects of lamination on mean CS and mean SCS were not observed for both high and low grade glulams, then the size effects on CS and SCS were very small. These test results were compared to bending strength (MOR) in the literature. [Kadowaki T, Takeda T, Hashizume T, Tokumoto M (1998) J Soc Mater Sci 47: 631-636] 4. Effect of Longitudinal Quality Variation on Mechanical Properties of Japanese Larch Lumber 4.1 Variation of Localized Young's Modulus within Japanese Larch Lumber for Glued Laminated Timbers. Structural lumber of Japanese Larch used for glued laminated timbers often is graded mechanically with a continuous mechanical grading machine. This lumber, however, has great variations in properties within a piece, and the variations of localized Young's modulus (E) are especially large. The variation of E within a piece of lumber was reported by Hashizume and others to the effect that the average of difference between the means and the lowest values of E within a piece of lumber measured with the grading machine was 0.92 GPa. There should be one reasonable grading method for selecting lumber according to the lowest E value. So we measured the apparent localized E of Japanese larch lumber with the grading machine by bending to investigate the variation of E within the lumber. The measuring points within a piece of lumber were 41 and the distance between adjacent points were 5.6 cm. The E data for each piece of lumber were used for calculating the standard deviations (ESD), auto-correlation coefficient (R) of E, and the increasing trends of E in the longitudinal direction of the lumber (SL) by the linear regression method. The results indicated that the cycles of E variations estimated from R data were about 60 cm, and ESD was caused mainly by SL. In most lumber, the differences of the mean and the lowest values of individual piece-within E were smaller than the average of the differences because the distribution of SL was distorted positively by calculating the skewness coefficient. Then we supposed that the grading method according to the smallest E might be adequate for calculating the allowable strength but was too conservative for calculating the stiffness for some of the lumber. [Takeda T, Hashizume T (1999) Mokuzai Gakkaishi 45 : 1-8] 4.2 Effect of Length on Tensile Strength of Japanese Larch Lumber An experimental study was conducted to evaluate the effect of length has on the parallel-to-grain tensile strength of Japanese larch lumber. Six hundred pieces of mechanically graded lumber were tested at gauge lengths of 60, 100, and 180 cm. The lumber was sorted into matched groups according to the dynamic Young's modulus measured by the longitudinal vibration method before the lumber was cut to the particular length. The averages of the dynamic Young's modulus of high-grade (H) and low-grade (L) specimens were 12.8 and 7.5 GPa, respectively. Using non-parametric estimates, the estimated length effect parameters of H and L were 0.268, 0.304 for 50th percentile and 0.121 and 0.256 for the 5th percentile, respectively. We then concluded that the different length effect factors between H and L could be used when using the lumber for practical purposes. The parameters of L were larger than those for H, and the parameters for 5th percentiles were smaller than the parameters for 50th percentiles. When two-parameter Weibull distribution functions were fitted to the strength data, the estimated shape parameters of the Weibull distribution by the parametric method were almost identified to the inverse of non-parametric parameters except 5th percentiles in H. The influence of defects such as knots on the lower tail of the strength distribution in H may be different compared with L. [Takeda T, Hashizume T (1999) J Wood Sci 45: 200-206] 4.3 Effect of Knots on Tensile Strength Distribution in Japanese Larch Lumber The tensile strength (TS) test results of Japanese larch lumbers of varying have shown that the length effects on TS were different between high-grade (H) and lowgrade (L) lumber. In this section, we examined the effect of knots on the TS distribution by measuring number of knots and the knot area ratio of each specimen. There were more knots in L than in H ; and the knot area ratio in L distinctly increased as the length increased compared to that in H. The correlation coefficients between physical properties and TS indicated that knots were the most influential factor for TS among several physical properties ; annual ring width, distance from pith, density, dynamic Young's modulus, and knots. We attempted to estimate the length effect parameters by introducing the concept of assumed knot strength. We thought that the length effect parameters for 50th percentiles of TS could be estimated well with fitted 3P-Weibull, and that the parameters for 5th percentiles could be estimated well with 2P-Weibull fitted to lower-tail 10% data by the likelihood method. The differences of length effect on TS between H and L should be governed by the presence of knots. The independent model based on the concept of assumed knot strength may express the TS of structural lumbers of various lengths. [Takeda T, Hashizume T (1999) J Wood Sci 45 : 207-212] 4.4 Effect of Knot Restriction on Tensile Strength Distribution in Japanese Larch Lumber It is well known that the presence of knots in structural lumbers is one of the most important strength-reducing factors. In practical purpose, visual grading including knot restriction is an effective method for non-destructive evaluation of their strength, and the edge knot restriction for not only visually grade lumbers but also mechanically grade lumbers is specified in Japanese agricultural standard for glued laminated lumber. We had conducted experimental studies on differences of tensile strength distributions between mechanically high grade and low grade Japanese larch lumbers daily used for manufacturing glued laminated timbers in Nagano. Then we examined the additional visual grading of mechanically grade lumbers for non-destructive evaluation. We graded visually the prepared mechanically grade lumbers by focusing on knot's area ratio of grouped knots. Then we also confirmed that the higher in visual grade related the stronger tensile strength as similar to our present knowledge. But the effects of knot restriction became very small when the length of lumbers was increasing in view of nonparametric 5th percentiles of tensile strength. The differences of strength/elasticity ratio between mechanically high-grade and low-grade lumbers were negligible. It was clear that the length effect on the ratio in visually high grade was smaller than visually low grade. It may be judged that the knot restriction should have little effects on tensile strength of mechanically grade lumbers. 5. Conclusion The above results showed that the function of mechanical grading according to Young's modulus related to the strength-reducing factors such as knots and specific gravity. It cleared that the strength distribution varied respect to Young's modulus, and size effects were different between 50th percentile and 5th percentile. I expect that these results will be applied to other species.
Article
信州大学農学部演習林報告 36: 21-81(2000)
信州大学農学部附属演習林
2000-03-01
jpn
departmental bulletin paper
VoR
http://hdl.handle.net/10091/14067
https://soar-ir.repo.nii.ac.jp/records/11195
0559-8613
AN00121330
信州大学農学部演習林報告
36
21
81
https://soar-ir.repo.nii.ac.jp/record/11195/files/Agri_Forests-36-03.pdf
application/pdf
4.7 MB
2015-09-25