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     

木质材料力学性能无损检测技术研究现状与发展趋势

木质材料在包装、建筑、家具、铁路等领域正发挥着巨大的作用,而木质材料力学性能的检测则是这类材料使用过程中的重要依据。介绍了木质材料力学性能无损检测的主要方法和原理;阐述了几种主要的检测技术(如机械应力法、振动检测法、应力波检测、超声波检测等)的研究现状及存在的问题;并在此基础上,提出木质材料力学性能检测技术的发展趋势。     

Grading of construction timeber by ultrasound

Summary The resistance of spruce beams was characterized with the aid of nondestructive ultrasonic measurements. The modulus of elasticity and the modulus of rupture in flexure for beams of commercial sized section can be estimated by observing the speed of propagation of a longitudinal wave applied to the longitudinal axis of the beam. The use of simple ultrasonic equipment and two experimental transducers (in direct and dry contact) permit measurements to be performed in industry. The technique constitutes the basis of an objective concept for classification of sawn timber guaranteeing a better reliability and profitability with regard to the visual grading rules used in Western Europe. This research was supported by The National Research Fund     

木质材料力学性能无损检测方法的研究现状与趋势

木质材料用途广泛,在建筑、家具和包装等领域发挥着重要作用。目前,木材无损检测技术发展很快,应用到木材性质检测的无损检测方法已达几十种,然而对木质材料力学性能的检测还处于相对初级阶段,具有较大的挑战性。本文对木质材料力学性能的无损检测方法和原理进行归纳与总结,阐述几种主要检测技术（如应力渡检测、超声渡检测、振动法检测）的国内外研究现状、检测特点及存在问题。在此基础上,提出木质材料力学性能无损检测技术的发展趋势。     

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     

Modeling wood pole failure

Summary In part 1 of this series, a three-dimensional, structural analysis, finite element program has been developed to predict the stress distribution in wood poles with and without spiral grain and variable material properties. This program serves as a basis for a model to predict the strength and failure location in full-size wood poles. Fundamental to this model is the ability to quantify the effects of key material and geometric properties of the pole. This paper deals with the enhancement of the program to quantify the effect of knots and their associated cross grain on the stress distribution of wood poles. The technique is based on the theoretical behavior of laminar fluid flow around an elliptical obstruction. The flow-grain analogy was employed to develop empirical relationships between knot diameter and pertinent variables (grain deviation angle near the knot and area of influence of the knot). Prior to the development of the empirical relationships, a study was conducted to determine the size and distribution of knots in Douglas-fir and western redcedar poles.The validity of the technique to describe knot behavior is reflected in the ability of the finite element model to predict the strength and failure location of wood poles. The results suggested that the flow-grain analogy is a rational mechanism to quantify the fiber orientation near a knot. Furthermore, this technique could have meaningful implication in improving visual grading methods for wood poles.The authors would like to recognize the contributions of Engineering Data Management, Inc. of Ft. Collins, Colorado for their contribution of test materials and facilities for this study.     

The analytical influence of all types of knots on bending

The influence of different types of knots and fiber deviations on the bending of wood, as managed using visual grading standards, was analytically discussed in this research by means of the finite element method. The effect of each geometrical variable on the strength and stress distribution has been studied individually. This was accomplished after validating a previously presented finite element model that accounted for the three-dimensional shape of the knots and fiber deviations. The simulated sizes and positions were those commonly managed by the standards for strength grading. The research presents the effect of: (1) the size and position of cylindrical face knots; (2) size and position of truncated conical face knots; (3) size and position of shallow conical face knots; (4) size and position of conical edge knots; (5) inclination of face knots. In each case, the analytical predictions of the modulus of rupture and stress distribution are detailed. Seven characteristic positions with distinct mechanical behavior were found, which should be accounted for in order to improve the wood grading efficiency.     

溶胶-凝胶法功能性改良木材研究进展

基于木材天然的双重毛细管系统结构,金属醇盐可在木材"微/纳米容器"内进行溶胶-凝胶反应最终制备木材-无机复合材料。在溶胶-凝胶改良木材过程中,以金属醇盐为主要组分的前驱体溶液可通过涂覆、浸渍/真空加压浸渍、微波/超声波辅助等方式对木材进行改性,且木材密度、基材初始含水率、溶胶反应体系、溶胶粒径、溶胶pH值等对木材溶胶-凝胶改性效果都会有一定的影响。文中主要从制备方法和影响因素2个方面对溶胶-凝胶法功能性改良木材的研究现状进行归纳总结,并提出溶胶-凝胶功能性改良木材的应用前景和发展方向。     

含LT型裂纹木梁起裂载荷确定方法的试验研究

木材裂纹萌生的准确判定对木材损伤断裂的评估具有重要的意义,起裂载荷是标定裂纹萌生的关键参数。本试验以杉木为研究对象,利用声发射技术(AE)、数字图像相关法(DIC)和电测法(EM),对含LT型裂纹木梁的损伤断裂特性进行了试验研究。通过研究木梁在加载过程中声发射参数变化规律以及裂尖区域的表面应变信息的演变,分析木梁裂纹萌生规律并确定起裂载荷Pini。结果表明:声发射累计振铃计数、幅度可有效反映木梁内部损伤的产生和演化,利用声发射参数的变化规律能准确确定含LT型裂纹木梁的起裂载荷Pini;数字图像相关法、电测法可以实时监测木梁表面裂缝尖端区域的应变变化,根据应变演变特征可以有效监测木梁表面裂纹的萌生和扩展。声发射技术、数字图像相关法、电测法在确定木梁起裂载荷Pini方面有较好的适用性,所确定的起裂载荷大小为:电测法>数字图像相关法>声发射。试验结果为研究监测含LT型裂纹木材裂纹萌生的试验方法提供了依据,应用时可结合实际工况选择合适的测量方法。     

Acoustic and bending properties in Pinus elliottii beams obtained from trees of different ages

In contrast to wood from native forests, the lumber harvested from planted forests is primarily composed of boards from younger trees. Given the possibility of using wave propagation methods to classify wood for structural purposes, it is important to evaluate if the ultrasonic parameters tend to vary with the age of the tree in a similar manner as static bending parameters. Experiments were conducted using structural beams taken from Pinus elliottii trees that were 8, 9, 13, 15, 22 and 23 years old. The beams were tested by ultrasound in the longitudinal direction and by static bending. The results showed that age of the tree influences the wave propagation velocity, stiffness constant and modulus of elasticity. The results were sorted into two groups based on the variation of these parameters: wood pieces from trees up to 15 years old and wood pieces from trees older than 15 years of age. Statistically, the flexural strength did not change with age due to the significant influence of knots on this parameter, which overcomes the influence of age. The data obtained by ultrasound follow the same trends with age as the elastic modulus in static bending.     

Failure analysis of laminated timber beams reinforced with glass fibre composites

Summary A qualitative analysis is presented of failure, perpendicular to the grain, in laminated timber reinforced with a glass fibre composite. The study is focused on beams with holes of different shape. The stress by corners, infinitesimal cracks and finite cracks are investigated. An initial crack model is suggested that brings about some of the phenomena observed in earlier performed experiments. A crack appears to propagate in the wood but is retarded in the reinforced beams. Eventually, the composite will fracture and failure of the beam follows. Finite element computations suggest that the reinforcement decreases the stress intensity at cracks in the wood and acts as a crack stopper. The reinforcing effect increases with the crack length. A point stress criterion is used to predict failure in the fibre composite.     

Stiffness gradients in radiata pine trees

A mill study of 62 trees, in which boards were reassembled into their original logs, permitted the construction of wood quality maps. In this instance stiffness profiles were obtained from butt to upper-top logs, based on machine stress grading of all boards and then averaging values from the 62 trees. Traditionally the butt log has been perceived to be the most valuable log in a tree, because it is bigger and gives a higher recovery of lumber. However, it is shown to contain a wide cone of very low stiffness wood that is confined to the first 2.4–2.7 m above ground level. Above this point stiffness gradients become cylindrical with no noticeable decrease in stiffness up the tree stem. Stiffness in all logs increased radially from pith to cambium with the greatest change being associated with the wood nearest the pith. The low stiffness at the base of the tree suggests that an alternative log bucking strategy should be considered, namely cutting a short 2.4–2.7 m butt log for plywood/LVL or for bolter sawing and only cutting standard length logs above this point.The least stiff logs (lowest 20%) yielded lumber that had an average stiffness that was over 1 GPa less than the average for the population. A case can be made for separating these logs and processing them differently.     

A stress transformation approach to predicting the failure mode of wood

Summary A simple model, based on the use of transformations of second-order tensors, is presented in this paper to predict the failure mode of wood members stressed in various degrees of parallel-and perpendicular-to-grain tension and parallel-to-grain shear. This type of loading is indicative of structural wood members with cross grain or grain deviations in the vicinity of knots subjected to bending or tension. The model is based on the assumptions that failure is dictated by the presence of any of the aforementioned stresses that exceed the clear wood strength in that mode and that failure does not result from stress interactions. The magnitudes of the applied stresses are normalized relative to the wood strength in that mode. The ratio of applied stress to material strength that is greatest at any particular angle of load to grain is presumed to be the failure mode at that angle. To verify model predictions, optical and microscopic analyses of surfaces of failed specimens loaded in uniaxial tension at angles between 0° and 90° to grain were compared to previously obtained, or otherwise known, surfaces of specimens tested in tension and shear. Specimens tested at various angles to grain demonstrated failed surfaces very much like those associated with specimens loaded in the modes predicted by the model.     

Application of modal analysis by transfer function to nondestructive testing of wood I: determination of localized defects in wood by the shape of the flexural vibration wave

This study was intended to detect nondestructively some defects such as knots and grain deviations in wood using modal analysis. The shapes of flexural vibration waves at the first mode generated by the tapping of wooden beams were determined using the transfer function. The wave shapes obtained were compared with the theoretical wave shape for a uniform material; and the possibility of detecting defects in wood was examined. The results are summarized as follows: (1) The shapes of flexural vibration waves at the first mode of wooden beams free of defects coincided almost completely with the theoretical wave shape. (2) The shapes of flexural vibration waves of wooden beams containing defects such as knots clearly differed from the theoretical wave shape, especially near the defect. (3) Based on these results, it should be possible to detect the presence of defects and to determine their location in wood.Part of this report was presented at the 49th Annual Meeting of the Japan Wood Research Society, Tokyo, April 1999. This article is translated from the Japanese edition published inMokuzai Gakkaishi Vol. 47, No. 4, 1988     

北美规格材机械分等综述

规格材的机械分等方法是北美一种很重要的分等体系, 即采用机械应力测定设备对木材进行非破坏性试验, 按测定的木材弯曲强度和弹性模量确定木材的材质等级。用于机械分等规格材的分选参数可以因不同工厂机械的具体情况而进行调整, 较目测分等更能适应现代化木材工业的发展要求。文中从分等方法的诞生、机械分等与目测分等相互关系、机械分等方法和标准、分等机构设置等方面介绍了北美规格材机械分等, 在此基础上分析了中国规格材分等的现状并对中国规格材分等提出了参考建议。     

Interaction between glycerin and wood at various temperatures from stress relaxation approach

In order to understand the reason why glycerin pre-treatment can accelerate the deformation fixation of compressed wood, the interaction between glycerin and wood at various temperatures was investigated in this study from stress relaxation approach. The compression stress relaxation curves of poplar (Populus cathayana Rehd.) samples impregnated with glycerin were measured at temperatures ranging from 25 to 180°C, together with the curves of oven-dry wood at temperatures between 100 and 180°C for comparison. The activation energy was calculated according to the Eyring’s absolute rate reaction theory. The results showed that temperature had very obvious effect on stress relaxation for both glycerin-treated wood (GTW) and oven-dry wood. The stress released very fast at higher temperatures. Glycerin showed an accelerating effect on stress relaxation. At temperatures exceeding 120°C, a complete relaxation of the stress could be expected. While for untreated wood, it cannot be reached until 160°C. By calculating the apparent activation energy (ΔE) of GTW at different temperatures, it is clear that two mechanisms are responsible for different temperature ranges. From 40 to 100°C, ΔE is only 8.24 kJ/mol, which corresponds to the hydrogen bonds formed between wood and glycerin molecules; from 120 to 180°C, ΔE reached 81.38 kJ/mol, which corresponds to the degradation of hemicelluloses or lignin, and during this process, new cross-linking would happen.     

Use of Leicester's “rheological model for mechano-sorptive deflections of beams”

Summary The model for mechano-sorptive deformation proposed by Leicester is used to predict stress relaxation of wood drying under constraint from apparent creep of drying wood under constant load. The predictions are compared with measurements on four pairs of beams, one beam in each pair being tested under constant load, the other at constant deflection. Agreement was sufficiently close to demonstrate the value of the model.     

EMC response along the tensile face of small wood beams

Small wood beams were loaded in a controlled environment to evaluate the effect of axial stress and strain on the equilibrium moisture content (EMC) of the tensile face. It was determined that the bending stress and the associated strain caused by the loading scheme increased the EMC of the wood along the extreme tensile face at the point of maximum bending moment.     

Basic study on nondestructive evaluation of artificial deterioration of a wooden rafter by ultrasonic measurement

Most old buildings in Korea are wood framed and, with age, deterioration is found in all wood components of antique buildings. Insects and rapid changes in humidity are among the main causes of deterioration. To preserve antique wooden buildings, nondestructive testing (NDT) methods are required. Various methods of nondestructive evaluation (NDE) such as X-ray, stress wave, drilling resistance test, and ultrasound are used to inspect the safety of wooden antique buildings. The ultrasonic method is relatively simple, inexpensive, and accurate. The rafters are one of the main components of antique buildings and are seriously affected by deterioration. This study aimed to develop a nondestructive ultrasonic technique for evaluation of wooden rafter deterioration. Regression models describing the relationship between the artificial deterioration of the specimen and ultrasonic parameters were proposed. The method was found to be reliable for evaluating wooden rafter deterioration.     

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.