Three-dimensional elastic behaviour of common yew and Norway spruce

In view of its high density, yew wood has a remarkably low longitudinal Young’s modulus, which makes it unique among coniferous woods. However, the elastic response of yew related to other load directions is largely unknown. Therefore, our goal was to comprehensively characterise the three-dimensional elastic behaviour of yew wood. To achieve this, we performed tensile tests on dog-bone-shaped yew specimens and determined the three Young’s moduli and six Poisson’s ratios using a universal testing machine and a digital image correlation technique. All tests were also applied to spruce as reference species. After including the shear moduli determined in a prior study by our group, all elastic engineering parameters of yew and spruce were ascertained. Based on these values, the three-dimensional elastic behaviour was describable with deformation bodies and polar diagrams. Evaluating these illustrations revealed that yew had a lower stiffness only in the longitudinal direction. In all other three-dimensional directions, spruce was clearly more compliant than yew. Particularly, in the radial–tangential plane, both species varied largely in their degree of anisotropic elasticity. All mentioned differences between yew and spruce originate at the microstructural level.     

Bending and shear properties of compressed Sitka spruce

We examined the bending and shear properties of compressed wood using small and clear specimens of Sitka spruce (Picea sitchensis Carr.). For measuring the bending properties, three-point bending tests were conducted under the span/depth ratio of 14, which is standardized in the American Society for Testing and Materials [ASTM D143-94 (2005a)] and Japanese Industrial Standards [JIS Z2101-94 (1994)]. In the bending test, the load, deflection at the midspan, and strain at the bottom of the midspan were simultaneously measured, and Young’s modulus and bending strength were obtained by elementary beam theory. For obtaining the shear modulus and shear strength, asymmetric four-point bending tests were conducted using the specimens with rectangular and side-grooved cross sections, respectively, and the influence of the compression ratio on the shear properties was examined. The results are summarized as follows: (1) Young’s modulus increased with increasing compression ratio when it was determined by the load–strain relation. Nevertheless, this tendency was rather obscured when Young’s modulus was determined by the load–deflection relation. Hence, it is preferable that Young’s modulus is measured from the load–strain relation. (2) The shear modulus in the longitudinal–tangential plane was maximum at the compression ratio of 50%, whereas that in the longitudinal–radial plane was minimum at the compression ratio of 50%. (3) The influence of the compression on the bending and shear strength ratio was not significant.     

Applicability of the losipescu shear test on the measurement of the shear properties of wood

We examined the applicability of the Iosipescu shear test for measuring the shear properties of wood. Quarter-sawn board of sitka spruce (Picea sitchensis Carr.) and shioji (Japanese ash,Fraxinus spaethiana Lingelsh. were used for the specimens. Iosipescu shear tests were conducted with two types of specimen whose longitudinal and radial directions coincided with the loading direction. The shear modulus, yield shear stress, and shear strength were obtained and were compared with those obtained by the torsion tests of rectangular bars. The results are summarized as follows: (1) The Iosipescu shear test is effective in measuring the shear modulus and the yield shear stress. (2) To measure the shear strength properly by the Iosipescu shear test, the configuration of specimen and the supporting condition should be examined in more in detail.     

Estimation of shear moduli of wood by quasi-simple shear tests

A quasi-simple shear test, which is the most direct method for examining the shear properties of sheet metals, has been applied to measure the shear moduli of wood. Buna (Fagus crenata Blume) with variously sized shear regions was used for the test specimens. Strain gauges were mounted in the center of the shear regions to measure the shear strains. The shear tests were carried out to determine the shear moduli in the radial and tangential planes. Apparent shear moduli obtained from the experimental results were corrected by finite element method (FEM) simulation of the shear region, where both shearing and bending are produced. It was found that the corrected shear moduli are roughly independent of test conditions, and their values are in good agreement with the data obtained from bending-shear tests. This suggests that the method employed here can effectively estimate the shear moduli of wood.Part of this research was presented at the 50th Annual Meeting of the Society of Materials Science, Osaka, May 2001     

针叶树木材细胞力学及纵向弹性模量的计算--试件纵向弹性模量的预测

依据针叶树木材管胞和射线细胞的结构模型。使用计算机抽样模拟解剖结构参数。以及使用针叶树木材纵向弹性模量计算公式和方法,计算人工林杉木,马尾松幼龄材和成熟材试件纵向弹性模量,计算结果与常温条件下气干试件测定结果十分符合。在试件晚材率和管胞解剖结构参数改变的条件下。计算预测了人工林杉木,马尾松幼龄材和成熟材纵向弹性模量的变化。结果表明：试件纵向弹性模量随晚材率,管胞长度,管胞壁厚度的增加而增加,而试件纵向弹性模量随管胞直径增加而减小。本文提出的纵向弹性模量计算的预测方法,对于运用现代生物技术控制和改变针叶树木材的材质,材性有实际意义。     

Failure in timber part II: The angle of shear through the cell wall during longitudinal compression stressing

Summary The angle at which the slip plane traverses the wall of cells subjected to longitudinal compression has been studied in detail and appears to be a function of the angle of the microfibrils in the middle layer of the secondary wall together with the ratio of the modulus of elasticity in the longitudinal and radial planes. These parameters can adequately explain the observed variations in slip plane angle that occurred between species, between early and late wood, and at different temperatures. Significant differences were absent in the comparison of radial and tangential walls, normal and compression wood, and samples at different moisture content.     

Elastic parameters of poplar wood with end-cracks

? In this study, longitudinal specific modulus of elasticity along the grain (specific MOE_L = MOE_L/ρ) as well as radial and tangential shear moduli (G _LR and (G _LT ) of Populus Deltoides wood were examined in free flexural vibration in a free-free bar method, where end-cracks produced manually in LT plane along and parallel to annual rings in four different sizes.

? The effects of four different crack sizes (0, 6, 12, and 18 cm) on elastic parameters of the bars were examined for their vibration properties based on Timoshenko bar equations, in order to find a procedure to make a confident choice of a clear specimen among the cracked ones, considering three initial modes of vibration.

? Based on research findings, a significant correlation existed between radial and tangential shear moduli of the clear bars as G _LR was approximately 15 percents higher than (G _LT . After making the shortest crack sizes, however, this correlation entirely faded. Statistically for 6 cm crack, decreases in specific MOE _L for measurement on the tangential impact were not significant though they were for longer cracks.

? It was also revealed that in specimen under the study if longitudinal specific modulus of elasticity from both LR and LT flexural vibrations were almost equal and G _LR was slightly larger than (G _LT , the user could be confident enough to consider the specimen without severe longitudinal cracks.

     

定向刨花板剪切模量和弹性模量动态测试

【目的】研究定向刨花板(OSB)的各向异性,探讨OSB面内剪切模量动态和静态测试方法,以提供一种快速、简便、重复性好、精度高的动态测试方法测量和分析OSB弹性常数。【方法】应用ANSYS程序计算OSB自由板和悬臂板试件的振形系数,给出振形系数依赖于板长宽比和宽厚比的关系式,通过仿真计算、动态试验和方板静态扭转试验验证其正确性。动态试验测试OSB剪切模量试件从一块整张OSB上下料制作,分为3个方向,即沿整板纵向下料制作的试件(0°或x向)、横向方向下料制作的试件(90°或y向)和沿与纵向呈45°方向下料制作的试件;方板扭转试验测试OSB剪切模量试件沿整板纵向或横向下料制作;动态测试OSB纵向、横向和45°方向弹性模量以及面内剪切模量和45°方向剪切模量。【结果】OSB实测纵向弹性模量是横向弹性模量的2.89倍,45°方向剪切模量小于面内剪切模量。正交各向异性材料方板扭转试验测试剪切模量推算公式需用±45°方向应变测量值的差值进行推算,将其用于OSB,测得的静态剪切模量与动态测试的剪切模量相当吻合。【结论】OSB弹性模量具有方向性,纵向最大,横向最小,45°方向介于二者之间;自由板扭转振形法和悬臂板扭转模态法适用于动态测试OSB面内剪切模量,其正确性得到方板扭转试验验证;0°和90°OSB动态测得的剪切模量几乎相等,可作为OSB面内剪切模量Gxy的估计值;OSB不宜按单向复合材料处理,在理论分析时宜按正交各向异性处理,OSB45°方向的剪切模量G45°相似文献   

Derivation and application of an equation for calculating shear modulus of three-ply laminated material beam from shear moduli of individual laminae

In a detailed study of the relation between the deflection caused by shear force and the constitution of a laminated material beam, we derived an equation for calculating the shear modulus of a laminated material beam from the shear moduli of individual laminae. The validity of the derived equation was investigated using crosslaminated wood beams made with five species. The calculated shear moduli parallel to the grain of face laminae ranged from 48.3 MPa to 351 MPa, while those perpendicular to the grain of face laminae ranged from 58.0 MPa to 350 MPa. The calculated shear moduli increased markedly with increasing shear modulus in a cross section of perpendicular-direction lamina of a cross-laminated wood beam. The calculated apparent modulus of elasticity (MOE) of cross-laminated wood beams agreed fairly well with the measured apparent MOE values. This fact indicated that the apparent MOE of cross-laminated wood beam was able to be calculated from the true MOE values and shear moduli of individual laminae. The percentage of deflection caused by shear force obtained from the calculated apparent MOE (Y _sc) was close to that obtained from the measured apparent MOE (Y _s) and there was a high correlation between both values. From the above results, it was concluded that the derived equation had high validity in calculation of shear modulus of a cross-laminated wood beam.     

Annual ring orientation effect on bending strength of subfossil elm wood

The effect of annual rings’ orientation on bending strength was examined on subfossil elm wood. Elm is extremely rarely found as subfossil wood, and during the last 50–60 years it has almost disappeared from natural forest stands of south-eastern Europe, due to the Elm Dutch disease. The samples were cut from approximately 670 years old subfossil elm trunk retrieved from the bed of the river Sava in the area between the villages Grebnice and Domaljevac in north Bosnia. The wood was identified to the genus level based on optical microscopy analysis of three wood sections—transverse, tangential and radial. Bending strength was determined by the three-point bending test. The load was applied to the longitudinal–tangential surface (LT) and to the longitudinal–radial surface (LR). The bending strength values of subfossil elm do not differ from the values of recent elm, despite the 700-year resting in anoxic river conditions. Bending strength in LT direction was slightly higher than bending strength in LR direction. The coefficient of variation and the standard deviation of the arithmetical mean were higher for LT direction. It was found that the measured bending strength for both directions follows Weibull distribution. Coefficient of determination of Weibull functions was close to 1 for both directions. The Weibull shape parameter was higher for LR direction.     

Variability in physical properties of plantation-grown progenies of Melia composita and determination of a kiln-drying schedule

Plantation-grown progenies of Melia composita Willd.were studied for variability in several physical properties: density,radial and tangential shrinkage,longitudinal permeability of heartwood and sapwood.Furthermore,flat-sawn planks from each of the progeny were subjected to a quick-drying test for determination of kilndrying schedule.The mean density of the species was 0.39 g cm~(-3) and the wood may be categorized as light wood.Mean radial shrinkage(%) for the species was 2.8% with progeny-wise variation in radial shrinkage from 1.56 to 4.11%.Mean tangential shrinkage(%) for the species was 5.54% with progeny-wise variation in tangential shrinkage from 3.69 to 7.71%.The resultant tangentialradial shrinkage ratio was 1.98(less than two),which suggests that the wood is relatively stable with respect to drying behavior.Mean sapwood and heartwood longitudinal permeability of the species were 3.38 and 2.02 Darcy,respectively.Higher longitudinal permeability of sapwood and heartwood indicate better drying and preservative properties of the species.Terazawa quick-drying test method suggests that the species is less susceptible to drying defects.During the test,only moderate checks and cracks were observed.A tentative kiln-drying schedule was recommended based on these results.     

Stiffness of normal, opposite, and tension poplar wood determined using micro-samples in the three material directions

Mechanical tests on micro-samples were performed in the three material directions in normal, opposite, and tension wood collected from a poplar tree. Two custom micro-devices were designed and built in the laboratory to test samples under pure tension in the transverse direction and under 4-point bending conditions in the longitudinal direction. Both devices were designed to handle samples with a small transverse section (a few square mm), which allowed to select zones with homogenous anatomical features. The results indicate a very high longitudinal stiffness in tension wood (up to 35 GPa compared to an average of 18 GPa for normal wood). Considering wood density, the value represents a specific modulus that is nearly 70 % crystalline cellulose. However, tension wood is slightly less stiff in the tangential and radial directions (1,150 vs. 1,500 MPa for normal wood in the radial direction and 430 vs. 530 MPa in the tangential direction).     

Cell wall thickness and tangential Young's modulus in coniferous early wood

To investigate the effect of wall thickening around cell corners on the tangential Young's modulus of coniferous early wood, tapered beam cell models in which the variation of the cell wall thickness in the axial direction was taken into account were constructed for seven species. Their tangential Young's moduli were compared with the experimental results. The calculated Young's moduli of tapered beam cell models were larger than those of the models composed of the cell walls with uniform thickness, although both models showed almost the same density. For some species the calculated Young's moduli of the models in which the cell wall thickness increased curvilinearly in the axial direction were much closer to the experimental values. The reduction of the radial cell wall deflection due to the increase of the stiffness around cell corners was considered to increase the tangential Young's modulus of a wood cell.This report was presented at the 49th annual meeting of the Japan Wood Research Society, Tokyo, April 1999     

Determination of Young’s and shear moduli of common yew and Norway spruce by means of ultrasonic waves

Despite the exceptional position of yew among the gymnosperms concerning its elastomechanical properties, no reference values for its elastic constants apart from the longitudinal Young’s modulus have been available from literature so far. Hence, this study’s objective was to determine the Young’s moduli E _L, E _R and E _T and the shear moduli G _LR, G _LT and G _RT of yew wood. For that purpose, we measured the ultrasound velocities of longitudinal and transversal waves applied to small cubic specimens and derived the elastic constants from the results. The tests were carried out at varying wood moisture contents and were applied to spruce specimens as well in order to put the results into perspective. Results indicate that E _L is in the same order of magnitude for both species, which means that a high-density wood species like yew does not inevitably have to have a high longitudinal Young’s modulus. For the transverse Young’s moduli of yew, however, we obtained 1.5–2 times, for the shear moduli even 3–6 times higher values compared to spruce. The variation of moisture content primarily revealed differences between both species concerning the shear modulus of the RT plane. We concluded that anatomical features such as the microfibril angle, the high ray percentage and presumably the large amount of extractives must fulfil important functions for the extraordinary elastomechanical behaviour of yew wood which still has to be investigated in subsequent micromechanical studies.     

Acoustoelastic birefringence effect in wood III: ultrasonic stress determination of wood by acoustoelastic birefringence method

Stress conditions produced in wood were analyzed by means of the acoustoelastic birefringence method. Bending load was applied against a wood beam specimen. Under loading, ultrasonic shear waves were propagated through the breadth direction of the wood beam specimen. The velocities of shear waves polarized in the longitudinal or tangential direction of the wood beam specimen were measured with the sing-around method. Bending stresses were determined by dividing the difference between the acoustic anisotropy and the texture anisotropy by the acoustoelastic birefringence coefficient. Shear stresses were also determined. These stress distributions of the beam specimen were in good agreement with those obtained by the strain gauge method and mechanical calculation.     

Microscopic tensile tests in the transverse plane of earlywood and latewood parts of spruce

This paper presents a method which makes it possible to measure elastic properties of a small group of tracheids in the transverse plane. The method is based on tensile tests under microscope that are performed with the assistance of an image analysis system. The calculation of the strain field is based on a global comparison of the grey levels between each deformed image and the initial image. All tests were carried out within one annual ring of spruce: • radial and tangential Young's modulus and Poisson's ratio can be measured in earlywood with a good accuracy, • radial Young's modulus and Poisson's ratio of tracheids in latewood can be estimated with good confidence, • two tests of very thin samples allowed the evaluation of the tangential elastic modulus in latewood. The small size of the sample together with the local measurement of the strain field permitted us to perform several measurements along one single annual ring. Consequently, it was possible to reveal a good relationship between the mechanical properties and the local density determined by microdensitometry. Received 27 October 1997     

落叶松木材API胶粘剂弦径面胶接强度的差异

对落叶松木材水性高分子异氰酸酯 (API)胶粘剂弦径面胶接强度进行了研究。结果表明 :落叶松木材API胶粘剂弦径面胶接强度存在着差异 ,落叶松木材API胶粘剂径切板胶合强度试件的常态压缩剪切强度是弦切板的 1 4 1倍 ,而反复煮沸压缩剪切强度径切板的却比弦切板的低。落叶松木材本身弦径面顺纹抗剪强度和剪切强度率的试验结果表明 :造成落叶松胶合强度试件弦径面常态压缩剪切强度存在差异的根本原因在于落叶松木材本身弦径向的强度存在差异 ,木材径向的强度比弦向的大 ,文中对这造成这种差异的原因进行了分析     

Elastic modulus of lignin as related to moisture content

The Young's and shear moduli of two lignins have been measured at several moisture contents. Cylindrical test specimens moulded from periodate and Klason lignin powders were conditioned to the required moisture contents and tested in tension and torsion. The Young's modulus of periodate lignin increased linearly from 3.1x10⁹ to 6.7x10⁹ Pa, and the shear modulus from 1.2x10⁹ to 2.1x10⁹ Pa as the moisture content of the lignin decreased from 12 to 3.6%. Klason lignin showed similar behaviour but its moduli were always much lower. This was probably a consequence of the more drastic alteration undergone by the Klason lignin during its isolation from the wood cell wall.I am indebted to Messrs M. B. Forsyth and L. P. Lowe for assistance with the design and construction of the testing equipment used in this work.     

黄山马尾松顺纹剪切性能研究

参照ASTM D143标准对黄山马尾松进行顺纹剪切测试，结果表明，径切面剪切强度平均值为9.59MPa、弦切面剪切强度平均值为11.38MPa、45＆#176;纹理方向剪切强度平均值为11.15MPa。弦切面剪切强度略高于45＆#176;纹理方向剪切强度，径切面剪切强度最小。弦切面和径切面的剪切强度与密度均呈线性相关，密度越大，剪切强度越大。材料满足GB50005-2003《木结构设计规范》中对剪切强度的要求，可作为结构材使用。     

三倍体毛白杨无性系木材两个切面密度的可视化

This study uses two different mathematical models and gets two dimensional graphs of longitudinal and tangential section wood density at the same time by the radial wood density data measured by X-ray. The graph is direct and clear. It can be used to analyze the distribution and difference of wood density. It can also be done various kinds of transformation, including different angle, different colors, space and plane, part and entirety. This develops the visible analysis of wood density. Next the distribution of wood density of veneer will be simulated by new mathematical model and radial wood density data.