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。     

人工林尾巨桉原木内部轴向残余应变径向分布

采用"中心板锯割法"结合"应变片法"对人工林尾巨桉原木内部轴向残余应变径向分布进行测试和研究.结果表明:原木内部轴向残余应变径向上分布沿髓心的对称性较好,在靠近树皮处为拉应变,向髓心方向拉应变逐渐减小,到某个树干半径后减为O,随即转变为压应变,向髓心方向压应变逐渐增大,径向分布的回归方程为:y=-21.075+4.529 7x+890.75(r=0.87),在0.01水平相关性极显著,拉应变向压应变转变的树干半径为0.681R;在不同原木高度上内部轴向残余应变的径向分布模式相似,回归曲线相关性均在0.01水平极显著,但随着原木高度的增加相关性下降;原木直径影响内部轴向残余应变径向分布曲线的变化梯度,直径越小,曲线越陡峭.本研究提出原木直径对内部轴向残余应变径向变化梯度的影响具有一个临界值的观点,原木直径小于这个临界值时,随着直径的减小,内部轴向残余应变径向分布的变化梯度显著增大,对于本研究中的尾巨桉,原木直径的临界值为19.9 cm.     

原木内部腐朽应力波二维图像的获取与分析

应用Arbotom应力波检测系统,对东北林区3个针叶树种(云杉、冷杉、落叶松)和9个阔叶树种(白桦、色木、椴木、杨木、水曲柳、核桃楸、榆木、铁木和柞木)的原木内部腐朽进行检测,并对原木内部腐朽面积的实际估算值和检测估算值进行比较分析.结果表明:Arbotom应力波无损检测系统可以获取原木内部腐朽的二维图像,但检测准确率较低,建议采用多点测量和改进应力波测试仪器,以提高检测准确率.     

杨树与落叶松原木中应力波的不同传播速度

通过应力波测试仪对健康的杨树和落叶松原木试件进行检测试验,研究应力波在这2种原木试件中径向、弦向和纵向(原木心材和边材)传播速度之间的关系以及径向传播速度和直径、弦向传播速度和检测角之间的相关关系.结果表明:在这2种树种中,应力波径向传播速度和原木直径呈正相关,原木纵向边材传播速度比相应心材中的要大,弦向传播速度与检测角呈现二次函数关系.通过试验分析得出应力波在原木纵向心材和边材传播速度与径向传播速度比的域值.     

X射线在原木无损检测中的应用

提出了一种采用X射线为检测手段的原木无损检测方法,分析了X射线用于原木无损检测的可行性、原木木材质量吸收系数μm的变化规律和X射线无损检测的特点.     

基于计算机断层扫描技术的木材密度检测研究

使用计算机断层扫描技术进行原木无损检测,通过对原木树种断层 CT 值的统计,对 CT 设备在检测过程中的窗宽与窗位进行了设定,同时拟合出了木材 CT 值一密度值的线性方程,为原木缺陷的无损检测及木材密度的测定提供一种新方法。     

计算机断层扫描技术检测木材缺陷的研究

木材无损检测技术是一门新兴的、综合性的木材非破坏性检测技术，可以应用计算机断层扫描技术（Cr）对原木进行无损检测。计算机断层扫描技术是X射线无损检测的一个分支，是对X射线的应用。本论文通过对Cr的利用，通过对Cr图像的提取、分析和处理，找出了一套方便于Cr图像的重建和去除噪声的算法，从而提出利用计算机断层技术（Cr）对木材进行缺陷检测，该方法是可行的。     

基于应力波的活立木力学特性无损检测研究进展

活立木力学特性无损检测是科学评价森林质量的基础,为合理制定采伐作业时间、原木分级和木材高效利用提供有效依据.介绍应力波无损检测的基本原理以及基于应力波的活立木力学特性无损检测的研究进展,分析影响应力波在活立木中传播的因素,在此基础上阐述木材力学特性变化的原理以及基于应力波技术的木材力学特性季节动态变化检测的现状和意义,提出应力波检测活立木性质存在的问题,针对其存在的问题提出几点建议.     

计算机断层扫描技术(CT)在原木无损检测中的应用

介绍了计算机断层扫描技术(CT)对原木缺陷进行检测的原理,针对第三代CT对原木检测的特点,总结了计算机断层扫描技术对原木缺陷进行检测的优点,阐述了当今国内外的计算机断层扫描技术对原木检测的研究现状,指出其在原木无损检测中应用的发展趋势。     

用超声波探测仪检查胶合板“鼓泡”缺陷

用超声波探测仪检查胶合板“鼓泡”缺陷王执煜（黑龙江省木材采运研究所）刘颖波（哈尔滨国营松江胶合板厂）李娜（黑龙江省林业厅）１检测概况世界发达国家已在各类工程机械设备、化工设备上广泛采用超声波无损检测技术，特别是近年来，微型计算机技术与超声波检测技术的...     

Evaluation of non-destructive methods of measuring growth stress in Eucalyptus globulus: relationships between strain, wood properties and stress

High levels of growth stress are implicated in causing end splitting of logs, deflection during sawing and deformation of boards as stresses are released during sawing operations. Level of stress is a function of strain and the elastic modulus of the wood (MOE). Levels of peripheral strain can be measured on standing trees and, if the MOE is known, stresses can be estimated. The validity of using peripheral strain measurements relies on underlying theoretical models that relate strain to expected patterns of stress distribution and levels of board deflection. This study evaluates these theoretical relationships by determining relationships of stress and strain with board deflection, end splitting and a range of wood properties.

Peripheral strain levels were extremely variable within the bottom log and little evidence was found for consistent patterns of variation, although measurements generally increased with increasing height above ground. Sampling on two sides of the standing tree at breast height appeared to be a suitable strategy, with the mean for these strain readings having a correlation (r) of 0.86 with the average strain in the bottom log.

Growth strain was not a reliable predictor of board deflection and cannot be recommended as a non-destructive sampling method. Overall there was a poor relationship between growth strain and board deflection. No consistent relationships were found between a range of wood properties and growth strain or board deflection across both sites. Stress levels were calculated for each tree as the product of growth strain and modulus of elasticity and the relationship between calculated stress and mean board deflection determined. No relationship was found at either site with correlations being very close to zero.

The underlying theoretical relationships between stress and strain were examined and several questions raised about the validity of such models.     

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。     

竹质工程材弹性模量检测方法

对竹质工程材弹性摸量无损检测方法初步探索,采用超声波法和冲击回波法检测竹质工程材动弹性模量,对比静态荷栽法所测静弹性模量,分析动、静弹性模量的相关性和配对试件的差值。实验发现：所测动弹性模量均高于静弹性模量,且具有较显著的线性相关性;超声波法相比冲击回波法,其动弹性模量与静弹性模量有较好的线性相关性,且差均值较小。研究表明：超声波法较冲击回波法更能有效检测竹质工程材弹性模量。     

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。     

Nondestructive evaluation of veneer quality using acoustic wave measurements

Two nondestructive evaluation methods, impact-induced stress wave techniques and ultrasonics, were investigated to detect lathe checks and knots in veneer, which were identified as key veneer quality properties for some engineered applications. Measurements included wave velocity and attenuation in the directions parallel and perpendicular to the grain. The results showed that both techniques were sensitive to lathe checks when using wave propagation perpendicular to grain. For wave transmission parallel to grain, signals showed some sensitivity to knots. There was no significant difference in wave velocity measurements between stress wave and ultrasonic techniques. Regression models based on stress wave velocities in these two orthogonal directions were developed to estimate the veneer quality index giving a coefficient of determination ranging between 0.39 and 0.50. Received: 20 December 1998     

Moisture content and temperature effect on ultrasound timber grading

Summary Because wood is a natural material, the variability of its properties is very large. In order to use wood efficiently in building, it has to be stress graded. The ultrasonic stress grading was developped as an improved alternative to visual grading. This non destructive evaluative technique allows reliable higher strength values while working with new products. Corrected models were studied to reference the ultrasonic propagation speed at constant moisture content and temperature. Using a referential ultrasonic wave velocity, stress grading can be carried out on trees or logs before cutting, or on fresh beams. The correlation between conditioned beams and test specimens is very high, especially for the strongest material.The author wishes to thank the Swiss Federal Institute of Technology Lausanne for acceptance of his doctoral thesis work, June 1990     

Shrinkage stresses in wood logs considered as layered,cylindrically orthotropic materials

Summary The deformation and stresses in a circular wood log resulting from an arbitrary radial moisture distribution are examined. In this paper the log is modeled as a layered cylinder, with each layer assumed to be linearly elastic, cylindrically orthotropic, and homogeneous. The general solution to the equations of elasticity for a representative layer is given; constants of integration in the solution are determined through application of appropriate continuity conditions at the layer interfaces. Numerical examples are presented for logs of Scots pine which illustrate the effect of nonuniform moisture content upon the displacement and stress distributions.     

Acoustoelastic effect of wood III: effect of applied stresses on the velocity of ultrasonic waves propagating normal to the direction of the applied stress

Changes in the velocity of ultrasonic waves propagating in wood normal to the direction of applied stresses are discussed. The ultrasonic modes considered here are longitudinal waves and shear waves with particle motion along the direction of the applied stress. The ultrasonic velocities in wood were measured by the sing-around method. From the results of the acoustoelastic experiments in wood, changes in the ultrasonic velocities were expressed as a function of the applied stress. For the shear waves, the ultrasonic velocities decreased with an increase in compressive stress from the initial stress level. On the other hand, the ultrasonic velocities under tensile stress increased with an increase in stress at low stress levels and then gradually decreased with further a increase in the stress. In contrast, the longitudinal wave velocities increased with an increase in compressive stress at low stress levels and then decreased with additional increase in the stress. The wave velocities under a tensile stress decreased with an increase in the stress. The proportional relations between velocities and stresses at low stress levels are confirmed, and acoustoelastic constants were obtained from these relations. Their absolute values were smaller than those reported in previous studies but larger than those of metals. The acoustoelastic effect seemed to be almost equivalent on the sensitivity for stress measurement as the strain-gauge method.Part of this research was presented at the 48th annual meeting of the Japan Wood Research Society, Shizuoka, April, 1998     

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.