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   

Utilization of digital image correlation in determining of both longitudinal shear moduli of wood at single torsion test

A sophisticated approach for the precise determination of both longitudinal shear moduli of wood at single test is introduced. The method is based on the combination of the torsion test inducing pure shear stresses in sample and an optical method providing the full-field strain data of such stress state. The proposed procedure of the longitudinal shear moduli determination consists of two main steps. In the first step, the apparent longitudinal shear modulus following the standardized procedure (EN 408+A1) was determined. Secondly, both longitudinal shear moduli were derived based on the apparent longitudinal shear modulus and the shear strain distribution on the radial and tangential sample surfaces. The wood of European beech (Fagus sylvatica L.) was used as material for the experiments. The exploratory analysis revealed the increasing difference between the longitudinal shear moduli determined in the longitudinal–radial plane and in the longitudinal–tangential plane as the total torsion angle increased as well as with the increase in the average torsion stiffness. Further, the longitudinal shear moduli and the torsional longitudinal shear strength did not correlate well. Therefore, they cannot be used in order to predict each other. Although such findings need more detailed studies, they should be taken into account when designing wood structures.     

Simultaneous determination of shear strength and shear modulus in glued-laminated timber using a full-scale shear block specimen

A newly developed full-scale shear block specimen was used to simultaneously determine the shear strength and shear modulus of glued-laminated timber. The shear modulus was calculated using the shear strain distribution measured by means of digital image correlation. To obtain the exact relationship between shear modulus and shear strength, the shear strain in the intended shear plane was measured. A relatively high correlation coefficient of 0.75 was found between the shear modulus and nominal shear strength in two types of glued-laminated timber tested: sugi (Cryptomeria japonica) and Douglas-fir (Pseudotsuga menziesii). Good correlation was also found between the density of middle lamina and shear strength. However, the relationships to density showed species dependence, which was not observed in the relationship between shear modulus and shear strength.     

An alternative solution for the determination of elastic parameters in free–free flexural vibration of a Timoshenko beam

An alternative inversion solution is presented to compute the elastic parameters based on the Timoshenko equation of motion in free flexural vibration. The interest of this solution was its relatively simple formulation and its validity domain, which was not restricted to the initial frequencies and a certain range of length-to-height ratio. The uniqueness of the solution was verified numerically, and the best optimization strategy was deduced. It was underlined that the shear modulus determination depended on the length-to-height ratio coupled with the number of resonance frequencies involved in the computation. The accuracy of the computed elastic modulus values was in the same scale as that for the measurement errors of frequency. However, the sensitivity of the shear modulus was found to be very high and depended on the number of frequencies taken into account. The theoretical model was found to be more accurate and to better estimate the frequency values than a classic solution of the Timoshenko equation. Furthermore, the inversion procedure gave equivalent elastic property values to those obtained with a classic solution (in the validity range of the classic solution) and was more robust when the number of frequencies taken into account was low.     

云杉属木材纵向和径向声振动特性参数关系的研究

采集7种我国具有代表性的云杉属木材及1种作对比用的美国产西加云杉试件,通过纵、横向木材试件在高次振动模式下的试验,测量木材各项声振动参数.研究发现:纵向材与径向材振动参数比之间存在着一定的相关性,据此可以选择各向异性适中,韧性好,纵向、径向振动受剪切影响小的木材制作乐器的音板.     

Shear modulus of several kinds of Japanese bamboo obtained by flexural vibration test

The vibrational properties of Japanese bamboo were examined. To obtain the Young’s modulus and shear modulus, a flexural vibration test and a longitudinal vibration test were conducted. The Young’s modulus with vibration in the R-direction was smaller than that measured in the longitudinal vibration test E _l . This was due to the shift of the neutral axis to the outer layer. On the other hand, the Young’s modulus with vibration in the T-direction was close to E _l . Hence, an adequate Young’s modulus should be used for each use of bamboo. The shear moduli of the LR and LT planes of bamboo were similar to those of beech. There were high correlations between shear moduli of the LR and LT planes and density.     

Application of the vibration method with additional mass to timber guardrail beams

This study examined whether or not weight, density, and Young’s modulus can be accurately measured by vibration test without weighing beams for timber guardrails. Bending vibration tests with and without the concentrated mass were performed on small clear round bars without the pith of spruce and cedar and on actual size round bars with the pith of cedar for the timber guardrail. The following results were obtained. The vibration method with additional mass could be applied to the round bar as well as the rectangular bar. It is possible that resonance frequency was decreased by the sawn split in the horizontal tapping. It is believed that the free ends condition is easier to realize than the fixed ends condition for cross beams for timber guardrails. The weight of the cross beam for the timber guardrail could be accurately estimated by the vibration method with additional mass under several testing conditions.     

Relationships between longitudinal and radial Picea genera sound vibration parameters

Longitudinal sawn wood are usually selected as samples in the study of sound properties of a musical instrument board. But in real production, radial sawn timber are cut and are also widely used as vibration component. Therefore, it is very important to evaluate the vibration properties of the board in the round, especially for the sound radiation characteristic of radial sawn timber and its relationship to longitudinal sawn timber. However, for the national and international experts, researches on radial sawn timber and its role and function in sound emission have not yet been developed. This paper describes a study of seven important spruces that grow up in the Sichuan and Heilongjiang provinces of China, and one Picea sitchensis specimen from North America. Under the high bending vibration mode, resonance frequency and other parameters of longitudinal and radial wood were tested. Analysis result disclosed the relationship between longitudinal and radial wood vibration property. An important conclusion of wood for musical instruments with proper anisotropy, fine toughness, and weak shear of longitudinal and radial vibration was inducted. __________ Translated from Scientia Silvae Sinicae, 2006, 42(3): 21–24 [译自: 林业科学, 2006, 42(3): 21–24]     

Determining Young’s modulus of timber on the basis of a strength database and stress wave propagation velocity I: an estimation method for Young’s modulus employing Monte Carlo simulation

In this article, we report on an estimation method for Young’s modulus that entails measuring only the stress wave propagation velocity of timber built into structures such as wooden buildings. Methods of estimating Young’s modulus that use the stress wave propagation velocity and characteristic frequency of timber in conjunction with timber density have long been used. In this article, we propose a method of easily and accurately estimating Young’s modulus from the stress wave propagation velocity without knowing the timber density. This method is based on a database of wood strength performance and density accumulated from a variety of research data and the method estimates Young’s modulus by a simulation method. We compared the Young’s moduli estimated by this method with those obtained by the bending test and by the measurement of the stress wave propagation velocity and density, and found similar results. This coincidence suggests that the method of estimating Young’s modulus presented in this article is valid. For example, the method is effective for convenient evaluation on site when determining whether a wooden building’s structural components should be reused or replaced when repairing or remodeling a building.     

Application of modal-based damage-detection method to locate and evaluate damage in timber beams

Modal-based damage-detection algorithms were used to identify the location of defects commonly found in timber and to estimate their severities. In this study, the authors propose modifications to an existing damage-detection algorithm for locating and evaluating damage by comparing the modal strain energy before and after damage using the first two flexural modes of vibration. Experimental verification was performed on pin-pin supported timber beams by employing the algorithms with extracted modal parameters using experimental modal analysis. Single and multiple cases of damage used to simulate pocket(s) of rot with various severities were inflicted by removing sections of timber beam specimens. The proposed damage indicator, computed from the first two flexural modes, was capable of detecting all damage locations. It was also able to estimate, with reasonable accuracy, the severity of damage in term of loss of sectional moment of inertia. The modified damage index method is generally reliable in detecting the location and estimating the severity of simulated defects in timber beams.     

Determining Young’s modulus of timber on the basis of a strength database and stress wave propagation velocity II: effect of the reference distribution database on the determination

A method of determining the Young’s modulus of timber using the stress wave propagation velocity without knowing the timber density was developed in our previous study. This method enables the estimation of Young’s modulus by Monte Carlo simulation using an existing database of the Young’s modulus versus density relationship as reference. Here, in Part II, we consider the effect of the reference distribution database on the accuracy of the estimated Young’s modulus by the developed method. Twelve different reference distribution databases were used in this study, containing Young’s modulus versus density data for more than 13 000 real-size timber specimens of ten different species. We obtained the following results: (1) the distribution of Young’s modulus estimated using an arbitrary stress wave propagation velocity depends on the reference distribution database employed, (2) the most important factor is not that the reference database has data on the same species as the timber in the test, but rather that the reference distribution database covers the foreseeable range of timber densities within the test, and (3) the estimation accuracy is higher than about 80% when the database covers many species and has wide ranges of densities and Young’s moduli. This estimation method was developed in order to measure the Young’s modulus of timber whose density cannot be measured. Considering that the quality of lumber has a large variation, such estimation accuracy will be useful for practical applications.     

Between- and within-site variation of density and bending properties of Picea abies structural timber from Norway

This study provides an analysis on the variability of structural timber of Norway spruce (Picea abies) grown in Norway. Density, modulus of elasticity (MOE) and bending strength were measured on 1188 boards from 205 trees, sampled from 14 sites throughout Southern Norway, Eastern Norway and Trøndelag. The area represents the procurement area for the majority of Norwegian sawmills. The variability of the timber properties was analysed in a linear mixed model where the random variance was divided into variance due to site, variance due to trees and within-tree variance. Models describing variance due to site based on site index, altitude and latitude were developed, and combined with data from the Norwegian National Forest Inventory to estimate mean values and variability of the timber properties. The results showed that major parts of the variance due to site are explained by altitude and site index, and for density also by latitude. Major parts of the variance due to site and the variance due to trees in bending strength and MOE were explained by density.     

Influence of moisture content on estimating Young’s modulus of full-scale timber using stress wave velocity

The effect of moisture content on the stress wave propagation velocity was investigated in order to estimate the Young’s modulus of full-scale timbers in an air-drying state using the measurement of stress wave propagation velocity above the fiber saturation point. Using Japanese cedar lumber, the velocity and the density under high-moisture condition and air-drying states were measured respectively; after measuring the modulus of elasticity in an air-drying state, the moisture content of each condition was measured. By performing numerical analysis on these data, the relationship between the moisture content and the rate of change of velocity of full-scale timbers was derived. This relationship was used to estimate the Young’s modulus of the timber in the air-drying state from the velocity in high-moisture condition. First, the velocity and the Young’s modulus in an air-drying state were estimated accurately from its density, moisture content and velocity under high-moisture condition. In cases where the density could not be measured, using the database of mechanical properties with the Monte Carlo simulation method, the Young’s modulus of the full-scale timber in an air-drying state might be estimated within 20% accuracy from its moisture content and velocity under high-moisture condition.     

Vibrational properties of Sitka spruce heat-treated in nitrogen gas

Sitka spruce (Picea sitchensis Carr.) wood was heated for 0.5–16.Oh at temperatures of 120°–200°C in nitrogen gas or air. The values for Young's modulus, shear modulus, and loss tangent were measured by free-free flexural vibration tests. X-ray diffractometry was carried out to estimate the crystallinity index and crystallite width. The results obtained are as follows: (1) Density decreased at higher temperatures and longer heating times. The specific Young's modulus, specific shear modulus, crystallinity index, and crystallite width increased during the initial stage and were constant after this stage at 120°C and 160°C, whereas they increased during the initial stage and decreased later when the temperature was high. Loss tangent in the longitudinal direction increased under all conditions, whereas that in the radial direction increased at 120°C and decreased at 160°C and 200°C. (2) From the relation between Young's modulus and moisture content, it can be safely said that Young's modulus is increased by the crystallization and the decrement in equilibrium moisture content, and that crystallization (rather than degradation) is predominant at the initial stage of the heat treatment, whereas the latter is predominant as the heating time increases. (3) It is implied that the specific Young's modulus, specific shear modulus, crystallinity index, and crystallite width decreased more in air than in nitrogen gas because of oxidation in air.This study was presented in part at the 43th Annual Meeting of Japan Wood Research Society at Morioka, August 1993, the 44th Annual Meeting of Japan Wood Research Society at Nara, April 1994, and the 45th Annual Meeting of Japan Wood Research Society at Tokyo, April 1995     

Measurement of the shear modulus of wood by asymmetric four-point bending tests

We conducted asymmetric four-point bending tests of wood and obtained the shear moduli on the basis of Timoshenko's theory of bending. Akamatsu (Japanese red pine,Pinus densiflora D. Don) and shioji (Japanese ash,Fraxinus spaethiana Lingelsh.) were used for the tests. Asymmetric four-point bending tests were undertaken by varying the depth/span ratios; and Young's modulus and the shear modulus were calculated by Timoshenko's bending theory. Independent of the asymmetric bending tests, we also conducted three-point bending tests, free-freeflexural vibration tests, and numerical calculations by the finite element method. Young's and shear moduli obtained by these methods were compared with those derived from the asymmetric bending tests. Based on these comparisons, we concluded that the shear modulus can be properly obtained by the asymmetric four-point bending tests when the span is 20 times larger than the depth.     

Capacity prediction of welded timber joints

Linear vibration welding of timber structural elements provides new opportunities to potentially achieve structural joints. This paper investigates to which extent welded joints can be considered for load-bearing structural joints. On the basis of a series of experimental and numerical investigations on a series of welded single-lap joints, failure modes were identified, and the associated failure criterion was quantified. A probabilistic method subsequently allowed accurately predicting the capacity of the tested wood welded joints exclusively based on objective input data, including an estimate of the scattering due to the material’s inherent variability.     

Evaluation of the fire endurance of mechanically graded timber in bending

AbstractThis study examined the performance of mechanically graded timber in bending when exposed to fire at various load ratios. The test specimens were 150 pieces, each with the dimensions of 60 × 120 × 3500mm. The modulus of elasticity (MOE) of 150 specimens was measured, and 60 among them were selected to formulate the prediction equation for MOE and modulus of rupture (MOR), which was used to predict the remaining 90 specimens. These were tested under fire exposure in bending using three-point loading at 11.1%, 16.7%, 33.3%, 66.7%, and 83.3% of the ultimate load. Using mechanically graded timber, which means acknowledging the actual strength of the bending member, permits fairly precise application to the targeted design load. This research confirmed that mechanically graded timber under fire exposure has the following tendencies: under the same load ratio, time to failure is independent of strength class, and, at any load ratio, the critical strength is dependent on the timber strength class. The obtained design bending strength under fire exposure using the reduced cross section method and the reduced strength method conformed to those calculated based on Eurocode 5. Following those findings, mechanically graded timber can be applied to obtain the design bending strength when taking into account the fire attack.     

玻璃纤维增强结构用单板层积材热压工艺研究

通过玻璃纤维增强速生杨木制备杨木单板层积材(LVL),可提高杨木的强度等级,使其达到结构集成材层板的使用要求。采用正交实验方法,研究温度、时间、压力、偶联剂浓度、涂胶量对杨木单板层积材弹性模量、静曲强度、剪切强度的影响,其中主要研究热压工艺对力学强度的影响,得出的最优工艺参数为:热压温度130℃、时间100s/mm、压力1.2MPa。     

Natural vibration analysis of clear wooden beams: a theoretical review

 In wood science, natural vibration analysis is being used to an increasing extent to characterise the longitudinal and the shear modulus of elacticity of various geometrical types of prismatic beams. A lateral or a axial percussion at one end of a beam set up on elastic support produces bending or longitudinal vibrations. Considering the hypothesis of the homogeneity of geometrical and mechanical properties of the beam, basic dynamics theorems can be applied to obtain the motion equations of longitudinal and transverse vibrations. The resolution of the differential equation for transverse motion leads to a search for solutions to the frequency equation. Because no exact analytical solution can be found, several approximate results are analysed. The application and validity range have to be defined to be able to achieve realistic results. The effects of the elastic support, the shear modulus and the height to length ratio are discussed. The conceptual bases help in understanding the analytical results and accuracy calculations can then be developed for practical applications. We present the most common theoretical models and define their validity range, application conditions, and accuracy levels with respect to measured values. Received 26 June 2000     

On mechanical behavior of traditional timber shear wall in Taiwan II: simplified calculation and experimental verification

In the previous report of this ongoing study, results of an extensive field survey were collated and a theoretical model was proposed to predict the mechanical behavior of timber shear walls of traditional design in Taiwan. The initial objective of the present report was to propose a simplified calculation method for estimating the initial stiffness and yield strength of traditional timber shear walls. Based on the results of the field survey, a total of 15 full-scale specimens were tested to verify the theoretical model and simplified calculation proposed previously. Good agreement was found from comparison of analytical and experimental results. The results of this study show that the friction behavior between board units and beams plays the major role in resisting the lateral force applied on the timber shear wall, followed by the resistance supplied by embedment. The resistance provided by bamboo nails is minor due to the small section. Another trend found was that for set dimensions of a timber shear wall, the board width can be increased to obtain higher stiffness and strength of the shear wall.