弦向角对应力波在原木横截面传播速度的影响

为了研究应力波在原木横截面上传播时应力波波速的变化规律及其影响因素,选择柞木、红松、臭冷杉和大青杨4个树种的原木为试验材料,在原木的横截面上,测试不同弦向角所对应的应力波波速,并用MATLAB软件对弦向角和应力波波速进行曲线拟合。结果表明:1)随着弦向角的增加,应力波在柞木、红松、臭冷杉和大青杨原木横截面上的传播速度均呈3次曲线趋势增加;2)当弦向角大于36°时,应力波在大青杨原木横截面上的传播速度最快,在柞木原木横截面上的传播速度最慢;3)应力波在柞木、红松、臭冷杉和大青杨原木横截面上的径向最大传播速度分别为1109.0,1220.4,1245.8和1618.3m·s-1。     

应力波在原木中传播理论的研究

对原木中微小单元进行动力学分析,确定应力波沿原木轴向传播的波动方程,应用分离变量法,得出应力波沿原木轴向传播的位移、速度、应力和应变方程。根据波动学理论中的反射和透射原理,从传播方向与界面垂直和不垂直2个方面得出界面两侧位移、速度、应力、应变的关系式。在此基础上,建立传播方向与界面垂直情况下的应力波沿原木径向或弦向传播的位移、速度、应力和应变方程。从这些方程可以得出:微小单元的位移、速度、应力和应变不仅与原木试件的弹性模量、密度有关,而且与小锤的敲击力、小锤与原木试件作用时间、敲击方向有关。     

小兴安岭天然林中冷杉立木含水率对应力波传播速度的影响

对小兴安岭天然林中30棵冷杉立木的含水率和应力波传播速度的检测结果表明:常温下,应力波纵向传播速度远大于径向传播速度；光照条件对于纵向传播速度的影响不显著；应力波纵向传播速度与含水率存在不显著的负相关性.     

应力波在树木径切面内的传播速度模型

【目的】研究应力波在不同树木径切面内传播速度的变化情况,建立传播速度模型,以期进一步认识应力波在树木径切面内的传播规律及其影响因素,为树木内部缺陷的三维成像技术提供理论依据。【方法】首先通过理论分析,建立应力波在树木径切面内的传播速度模型;然后以浙江农林大学植物园内8类有代表性的树种(香樟、枫香、乐昌含笑、鹅掌楸、响叶杨、悬铃木、松树、白杨)为试验材料,在样本径切面上,采用Arbotom应力波木材无损检测仪测量与径向成不同角度方向的应力波传播时间,计算不同角度方向上的应力波传播速度,并对健康样本径切面内沿方向角θ的应力波速度vθ和径向应力波速度v0的比值vθ/v0与方向角θ之间的关系进行回归分析。【结果】应力波在健康香樟样本径切面内的传播速度随方向角的增大而增大,径向传播速度最小,其原因是当应力波在树木内部沿径向传播时,传播方向与树木纤维方向垂直,受到细胞壁边界的阻碍较多,传播速度较慢;而随着方向角的增大,应力波传播方向与树木纤维方向逐渐平行,受到细胞壁边界的阻碍变少,传播速度逐渐增加。枫香、乐昌含笑、鹅掌楸、响叶杨4种树种的健康样本在相同方向角上的应力波传播速度大小不同,但其变化规律与香樟活力木相同。对健康样本试验数据的拟合结果为vθ/v0≈kθ2+1(0≤k≤1),k值取决于被测树木的物理力学参数。在所建立的回归模型中,决定系数R2均大于0.92,表明模型具有较高的拟合优度。在有缺陷的悬铃木样本试验中,径切面上方向角为-20°~-50°的应力波传播路径经过缺陷区域,其余传播路径均位于健康区域内。当应力波传播路径位于径切面的健康区域时,传播速度随方向角的变化趋势满足一元二次函数模型;但当应力波经过径切面的缺陷区域时,传播速度明显降低,不再符合正常情况下的传播速度模型。针对松树和白杨原木试样,人工设计了空洞缺陷,并对产生空洞前后的原木径切面的应力波传播速度进行了对比。最后,基于建立的径切面内应力波传播速度模型,设计了一种树木内部缺陷四向交叉检测方法,该方法能够较准确地检测木材内部缺陷的位置。【结论】在健康树木的径切面上,方向角θ与vθ/v0之间满足一元二次函数关系vθ/v0≈kθ2+1(0≤k≤1),对不同树种的检测结果均表明了该模型的有效性;进一步的试验证明了该速度模型对于树木内部缺陷检测具有很好的指导作用。     

古建筑木材中应力波传播速度的影响因素

对应力波在古建筑木材中传播速度的影响因素及其影响规律进行了检测和分析,目的是找出各种因素变化与应力波扫描图像之间的关系,从而更好地判定木材内部缺陷的位置和面积。研究表明:含水率对应力波传播速度影响显著;在相同含水率下,应力波径向传播速度大于弦向,其传播方向偏离髓心越远,传播速度越小;树种和年代对应力波传播速度也有影响。     

樟子松活立木与原木及其板材应力波传播速度的相关性研究

对樟子松活立木、原木及其板材的应力波传播速度进行检测,得出活立木与原木的应力波传播速度非常接近;樟子松原木段与其板材的应力波传播速度具有较大的相关性,相关系数为R2=0.6,因此可以通过活立木的应力波传播速度来预测原木制成板材后的应力波传播速度。     

交趾黄檀心边材径向弯曲蠕变性能

采用三点加载方式对交趾黄檀心边材试件进行短时间(5 h)内径向弯曲蠕变测试,获得交趾黄檀心边材的弯曲蠕变特性曲线,分别用五单元模型、六单元模型模拟交趾黄檀径向弯曲蠕变过程,对比分析交趾黄檀心材与边材蠕变性能的差异。结果表明:五单元模型可准确地模拟交趾黄檀的径向弯曲蠕变特性;相同应力水平下交趾黄檀心材的蠕变小于边材的蠕变,随应力水平提高,心材的蠕变增大不明显,而边材蠕变明显增大。     

国产麻栎和栓皮栎木材的主要物理性能

按照国家标准要求,对国产麻栎和栓皮栎木材的主要物理性能进行测试。结果表明:麻栎与栓皮栎木材的密度等级为大,气干密度径向变异均表现为心材密度比边材密度大;麻栎和栓皮栎木材的径向尺寸稳定性优于弦向,边材优于心材。栓皮栎尺寸稳定性稍优于麻栎。     

应力波和超声波在立木无缺陷断面的传播速度

为进行立木无损健康检测,采用Arbotom应力波成像系统、Resistograph针式阻抗仪和RSM-SY5超声仪对4个树种共计120株树分断面进行测试,研究超声波和应力波在立木108个无缺陷断面传播速度变化规律,并对比分析二者的差异和相关性。结果表明:1)采用8个测点对立木测试时,超声波和应力波在立木无缺陷断面具有类似的传播规律,即沿Path A(相邻两传感器)传播时速度最低,沿Path D(径向传播)传播时速度最高,从Path A到Path D传播速度呈递增的变化趋势;2)对于小叶杨、榆树、旱柳、水曲柳4种立木,Arbotom测得的无缺陷断面应力波径向传播速度平均值分别为788.46,1 025.45,940.62和1 146.06 m.s-1;RSM-SY5测得的超声波径向传播速度平均值分别为1 159.57,1 537.5,1 323.15和1 558.6 m.s-1;3)尽管Arbotom和RSM-SY5测得的应力波和超声波传播速度差值较大,但二者的径向传播速度具有显著正相关性,决定系数R2均在0.77以上,因此采用这2种仪器对立木进行无损检测都是可行的。     

工艺条件对杉木微波处理后渗透性能影响的研究

采用单因素试验,对不同工艺因素条件下微波处理后杉木的阻燃渗透性进行了分析。试验结果表明：微波处理后,杉木的渗透性得到明显改善;微波功率、处理时间、心边材和含水率对载药量、注入深度和氧指数有不同程度的影响,但均没有呈现出明显的规律性;心边材对结果影响很小,可忽略这一因素的影响;径向、弦向和纵向三个方向之间的注入深度差异显著,总体表现为纵向大于弦向大于径向。     

Early prediction of basic density, shrinking, presence of growth stress, and dynamic elastic modulus based on the morphological tree parameters of Tectona grandis

Significant efforts have been made to improve teak; however, evaluation in juvenile step is limited. The objective of this study was to conduct an early assessment of the wood properties of 4-year-old Tectona grandis. Samples of 36 clones were collected to determine their morphological tree parameters [diameter a breast height (DBH), diameter of the second log, tree height, and log quality]. Presence of growth stress, heartwood percentage, shrinkage (radial, tangential, and volumetric), basic density, and ultrasound velocity (USV) were determined for standing trees, logs, green lumber, and dry lumber. The results indicate that DBH and USV in standing trees can be used to predict elastic module (ED), mainly ED of the standing tree and dry lumber, as well as the possible presence of growth stress. Additionally, growth stress can be predicted by UVS in standing tree. Tangential and volumetric shrinkage were not predicted by tree morphology, but radial shrinkage was predicted by diameter and UVS was not affected by any shrinkage. Basic density was predicted by DBH and UVS measured in log.     

A comparative study on the velocities of stress wave propagation in standing Fraxinus mandshurica trees in frozen and non-frozen states

In order to improve the accuracy and reliability of identifying wood defects and to realize the maximum wood utilization of trees, we employed an experimental method to test the stress wave propagation velocity in standing Fraxinus mandshurica trees selected from the Harbin Forest Experimental Station in winter. Thirty standing trees in good conditions were taken as test specimens and stress wave propagation velocities were measured using a FAKOPP Microsecond Timer in trees in both fall and winter. The test data were processed with the aid of Excel and SPSS software. The results show that 1) the velocities in longitudinal and radial stress wave propagation in frozen F. mandshurica trees were much higher than those in the non-frozen trees; 2) there was a highly positive correlation between longitudinal stress wave propagation velocity in frozen and non-frozen states, with a correlation coefficient of 0.82, as well as a positive correlation between radial stress wave propagation velocity in frozen and non-frozen states with a correlation coefficient of 0.87; 3) in the frozen state, the longitudinal stress wave propagation velocity was significantly affected by the moisture content (MC) of standing tree, while it was not obvious in the non-frozen state and 4) the radial stress wave propagation velocity was not significantly affected by MC in either frozen or non-frozen state.     

Investigation on transversal variation of poplar tension wood quality using ultrasound wave parameters

Transversal variations of some ultrasound wave parameters (phase velocity, group velocity, attenuation coefficient and acoustic radiation) were measured from the pith to the bark of poplar tension wood discs in different orthotropic directions. Over 80 cubic specimens of 2 × 2 × 10 cm³ (radial, tangential and longitudinal, respectively) were prepared and tested acoustically. Samples containing tension woods were separated from normal ones using anatomical experiments. Results showed acoustical behaviour of normal and tension wood improved in longitudinal direction while the distance between pith and bark increased; also, wave parameter variations were less important in radial and tangential directions. In addition, phase velocity and acoustic radiation – which significantly varied – were the best parameters for quality assessment of poplar wood compared with group velocity and attenuation coefficients. Since samples near the bark were acoustically better than counterparts near the pith, they could be used in products requiring more strength, like lumber. Finally, acoustic radiation and phase velocity were correlated more strongly with density than group velocity and attenuation.     

Stress wave sorting of red maple logs for structural quality

Existing log grading procedures in the United States make only visual assessments of log quality. These procedures do not incorporate estimates of the modulus of elasticity (MOE) of logs. It is questionable whether the visual grading procedures currently used for logs adequately assess the potential quality of structural products manufactured from them, especially those for which MOE is of primary concern. The purpose of this study was to investigate the use of stress wave nondestructive evaluation techniques to sort red maple logs for the potential quality of lumber obtained from them. Ninety-five red maple logs were nondestructively evaluated using longitudinal stress wave techniques and sorted into four stress wave grades. The logs were then sawn into cants and lumber. The same procedure was used to obtain stress wave times in the cants and lumber. The lumber specimens were then dried and graded using a transverse vibration technique. The results of this study showed that good relationships existed between stress wave times measured in logs, cants, and the lumber produced from the logs. It was found that log stress wave grades have positive relationships with the lumber grades. Logs with high stress wave grades produced high-grade lumber. These findings indicate that the longitudinal stress wave technique has potential in sorting logs and cants for the production of high MOE products.The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright.     

Acoustoelastic effect of wood II: Effect of compressive stress on the velocity of ultrasonic longitudinal waves parallel to the transverse direction of the wood

The changes in the velocity of ultrasonic waves propagating in wood parallel to the direction of applied stress are discussed. The ultrasonic mode was longitudinal waves traveling along the direction of applied stress with the compressive load applied parallel to the transverse direction of the wood. The ultrasonic velocities were measured by the sing-around method. The experimental results indicated the existence of an acoustoelastic phenomenon in the transverse direction of the wood. The percent change in the ultrasonic velocity was given as a function of the applied stress. The change in the velocity depended on the species and structural direction of the wood. That is, in the radial direction of hardwood, the ultrasonic velocity increased with increases in compressive stress at the initial stress level of less than 2MPa; it then gradually decreased with increases in stress. A change in velocity from an increase to a decrease was considered a unique phenomenon for wood. In contrast, in the radial direction of softwood and the tangential direction of hardwood, the ultrasonic velocity decreased with increases in stress from the beginning of loading. This phenomenon is also generally observed in metallic materials. The relations between velocity and stress at the initial stress level and between velocity and strain in the range of large deformation are represented by essentially straight lines. The acoustoelastic constants of wood were obtained from these relations at the initial stress level. The absolute values of the constants in the transverse direction of wood were larger than those for metals and were larger than those for the longitudinal direction of wood reported in our previous paper.This research was presented at the 1st Meeting of the Research Society of the Acoustoelastic Measurements in the Japan Society of Non-Destructive Inspection at Osaka, October 1996 and at the 47th Annual Meeting of Japan Wood Research Society at Kochi, April 1997     

Acoustoelastic birefringence effect in wood II: influence of texture anisotropy on the polarization direction of shear wave in wood

Ultrasonic shear waves were propagated through the breadth direction of a wood beam which was subjected to a bending load such that it was in a plane-stress state. The oscillation direction of the shear waves with respect to the wood beam axis was varied by rotating an ultrasonic sensor, and the relationship between the shear wave velocity and the oscillation direction was examined. The results indicate that when the oscillation direction of the shear wave corresponds to the tangential direction of the wood beam, the shear wave velocity decreases sharply and the relationship between shear wave velocity and rotation angle tends to become discontinuous. When the oscillation of the shear waves occurs in the anisotropic direction of the wood beam instead of in the direction of principal stress, the shear wave velocity exhibits a peak value. In addition, the polarization direction was found to correspond to the direction of anisotropy of the wood beam according to the theory of acoustoelastic birefringence with respect to plane stress. This indicates that when the acoustoelastic birefringence method is applied to stress measurement of wood, it is appropriate to align the oscillation direction of the shear wave with the principal axial direction of anisotropy in order to carry out ultrasonic measurement.     

Estimation of shrinkage coefficients in radial and tangential directions from CT images

The aim of the present work was to use the displacement information generated from the spatial alignment in order to compute wood shrinkage in the radial and tangential directions in computed tomography (CT) images, and to compare the results with those obtained with computer-aided design software on the same images. To estimate the shrinkage coefficients from tomography images, wood specimens in the green state, equilibrium moisture content 15% and 8% state and oven dry condition were scanned. Specimens were taken from Norway spruce and Scots pine logs. The root-mean-square-error calculations showed acceptable small differences between the two measuring methods, which means that the algorithm is a useful tool for estimating the shrinkage coefficients in radial and tangential direction from CT images. This provides an image processing tool to monitor the dimensional changes during the drying and heat treatment process.     

Torsional-shear stress of wood at various temperatures

Summary The maximum torsional-shear stress of 5 softwoods and 4 hardwoods were tested in the radial and tangential planes. These tests were carried out in glycerin which was preheated to different temperatures between 22°C and 150°C, inclusively. The dense hardwoods possess 2.4 times higher shear stress than softwoods at 22°C. However, at elevated temperatures, the same degree of shear stress reduction (77 %) is obtained for hardwoods and softwoods in both radial and tangential failure. Thus, the reduction in shear stress is independent of physical and structural wood variables.