裂纹对木梁承压与抗弯强度的影响

设计了裂纹木梁的承压与抗弯性能试验,研究了裂纹对木梁强度的影响.结果表明:裂纹对横纹承压强度的影响较小,对抗弯强度影响较大;在承压面为50mm×50mm时木材自身的变异性影响较小;试件长度≥150mm时,局部长度承压值趋于稳定;上下表层横向裂纹使抗弯强度降低近一半.     

The influence of spruce wood heat treatment on its thermal stability and burning process

This paper deals with the assessment of the influence of heat treatment of Norway spruce wood (Picea abies L.) on its thermal stability and burning process. Three types of samples were used for the research. The first group was comprised of untreated samples. The second group was comprised of samples heat treated according to the ThermoWood—Thermo-S thermal programme (maximum temperature during heat treatment is 190 °C), and the third group was modified according to the ThermoWood—Thermo-D thermal programme (maximum temperature during heat treatment is 212 °C). The influence of heat treatment on the burning process was assessed based on the heat release rate, effective heat of combustion and the yield of carbon monoxide. The influence of heat treatment on the thermal stability was assessed based on the resistant residue weight. Heat release rate, effective heat of combustion, yield of carbon monoxide and resistant residue weight were determined with a cone calorimeter for different heat flux densities. The results obtained prove that the spruce wood heat treatment has an important influence on the decrease of the maximum heat release rate and the increase of resistant residue weight. The results obtained further prove that the heat treatment has only a modest influence on the increase of the carbon monoxide yield, and its influence on the effective heat of combustion significantly depends on the heat flux density.     

Relation on microscopic compression creases and crack of wood under static cyclic bending load

The relationship between microscopic compression failure lines and crack-nucleation is reported. Static and reversed cyclic bending load was applied to the specimens made from wood of Agathis species (Agathis sp.). The maximum subjected load was ±450～650 kgf/cm². The observation of the microscopic compression failure line and cracks nucleated at the root of the specimen was made every half cycle of load. The microscopic compression creases nucleate at the tip of the notch on the one side face of the specimen in the compression cycle. The length of the microscopic compression creases shortened in the tension cycle. This is because the microscopic compression creases are stretched parallel to the cell axis. In the next compression cycle, the microscopic compression, creases appear again. After repeating these processes several times and when the shape of the microscopic compression creases has not yet recovered to its original shape a large part of the microscopic compression creases change into cracks. In other word, the cracks nucleate. Mostly, the tip of the microscopic compression creases preceeds the tip of the cracks. The microscopic compression creases and the cracks propagate by steps, and after repeating these processes several times failure of the specimen occurs. The cracks nucleate in the compression cycle and sometimes in the tension cycle too. The cracks follow microscopic compression creases. The microscopic compression creases and the cracks run at nearly right angles to the cell axis.     

On the influence of crack closure on strength estimates of wood

Three well-known duration of load models (Gerhard, Barrett/Foschi, DVM) are considered in this note with respect to their ability to predict lifetime of wood subjected to harmonically varying loads. The result obtained is that they practically predict the same lifetime—which for low frequency loading can be considered approximately true. For higher frequencies, however, this result can be far too overestimated. The reason is that the models considered do not take into account the effect of the crack closure phenomenon (which are the main mechanisms of energy dissipation causing fatigue failure in metals). It is suggested that any of the simple models can be used in practice when low frequency load variations are considered. The DVM model, however, should be preferred because of its ability to predict residual strength, and because of its ‘build in’ flexibility with respect to wood quality and ambient climatic conditions. For high frequency load histories more refined models are required. The extended DVM model, recently developed by the author, is suggested as such a model—especially because it has the potentials of being further developed to consider arbitrary load variations (such as earthquakes). Finally, the widely spread concept of estimating long-term strength by multiplying short time strength with a codified factor (so-called k _MOD factor) is discussed. It is concluded that the k _MOD -method can be justified in practice with low frequency load variations. When high frequency load histories or unexpected peak loads are considered, the k _MOD -method may cause considerably overestimated lifetimes.     

木材顺纹抗拉强度试件的数控加工技术

主要介绍了木材顺纹抗拉强度试件的数控铣床加工技术,指出数控加工技术在木材加工行业的广泛应用已成为必然的发展趋势,它将会大大提高木材加工机械的自动化水平和生产率,有利于实现木材制品的辅助设计和辅助制造(CAD/CAM),便于加工复杂的工件。     

Correspondences between manually estimated compression wood in Norway spruce and the warp of the sawn timber

Picea abies L.) and the warping of the sawn timber in terms of bow, spring and twist as well as further deformation after ripping of the dried products. The amount of compression wood was defined and measured on logs according to the methods of the Swedish Timber Measurement Council (Regulations for measuring of round wood) and on the sawn timber according to the Nordic Timber. The impact of two different drying schedules was also investigated. The study shows that visible compression wood in both the butt end of the log and within the sawn timber was a rather poor indicator of the warp of the dried sawn timber. In no comparison did the correlation coefficient, r, exceed 0.3. In contrast to this, the correlation between the amount of compression wood and the warp of secondary products was fair, r = 0.79. This means that it should be possible to identify sawn timber less suitable for secondary processing by the amount of compression wood. The corresponding correlation between compression wood in the butt end of the log and the warp of the secondary products was r = 0.46. No significant differences could be shown in the degree of warp, as related to compression wood, between sawn timber or secondary products, dried at a wet-bulb temperature of 55 °C/117 h, LT-schedule, and a dry-bulb temperature of 110 °C/24 h, HT-schedule, respectively.

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r  = 0,79). Es sollte also m?glich sein, weniger geeignete Schnittware anhand ihres Druckholzanteils auszusortieren. Die Korrelation zwischen Druckholzanteil am Stammende und der Verwerfung im Schnittholz betrug 0,46. Für das Ausma? der Verformungen wurde kein signifikanter Unterschied zwischen den beiden Tocknungsbedingungen (55 °C/117 h und 110 °C/24 h) gefunden.

     

碳纤维管状复合材料拉伸强度测试方法研究

针对目前高强度管状复合材料测试方法和手段不成熟的问题,提出了碳纤维管状复合材料拉伸强度的两种测试方法——取样拉伸法和整体拉伸法,它们分别用于大管径和小管径的测试.对试件的制作方法进行了研究和实验,并研制出专用夹具用于整体拉伸测试,通过实验验证了夹具的可行性和优越性,从而突破了高强度管状复合材料强度测试时夹持困难的瓶颈.     

空气湿度对新闻纸裂断长的影响及补正系数的探讨

按行业检测标准,纸张物理指标的检测应在恒温(23±1)℃,恒湿(50±2)%条件下进行,但是对于规模较小的公司,特别是在生产现场,许多企业并不具备标准条件,所以物理指标的检测数据值(检测值)与报告值(近实际值)存在不相符的情况,特别是对于不施胶纸的裂断长指标,受空气湿度的影响很大.     

管状复合材料拉伸性能测试及夹具原型系统

针对目前高强度管状复合材料测试方法和手段不成熟的问题,提出了碳纤维管状复合材料拉伸强度的两种测试方法——取样拉伸法和整体拉伸法,它们分别用于大管径和小管径的测试。对试件的制作方法进行了研究和实验,并研制出专用夹具用于整体拉伸测试,并通过实验验证了夹具的可行性和优越性,从而突破了高强度管状复合材料强度测试时夹持困难的瓶颈。     

Prediction of timber bending strength and in-member cross-sectional stiffness variation on the basis of local wood fibre orientation

Machine strength grading of structural timber is based upon relationships between so called indicating properties (IPs) and bending strength. However, such relationships applied on the market today are rather poor. In this paper, new IPs and a new grading method resulting in more precise strength predictions are presented. The local fibre orientation on face and edge surfaces of wooden boards was identified using high resolution laser scanning. In combination with knowledge regarding basic wood material properties for each investigated board, the grain angle information enabled a calculation of the variation of the local MOE in the longitudinal direction of the boards. By integration over cross-sections along the board, an edgewise bending stiffness profile and a longitudinal stiffness profile, respectively, were calculated. A new IP was defined as the lowest bending stiffness determined along the board. For a sample of 105 boards of Norway spruce of dimension 45 × 145 × 3,600 mm³, a coefficient of determination as high as 0.68–0.71 was achieved between this new IP and bending strength. For the same sample, the coefficient of determination between global MOE, based on the first longitudinal resonance frequency and the board density, and strength was only 0.59. Furthermore, it is shown that improved accuracy when determining the stiffness profiles of boards will lead to even better predictions of bending strength. The results thus motivate both an industrial implementation of the suggested method and further research aiming at more accurately determined board stiffness profiles.     

A new method for the measurement of the tensile strength of rice grains by using the diametral compression test

Breakage of rice grains during milling can be attributed to the presence of fissures, caused by stresses related to shrinkage during drying. These fissures are usually caused by a tensile failure in the center of the grain. Very few data of the tensile strength of rice grains have been published, which is related to experimental difficulties. An attractive method for the measurement of the tensile strength is the diametral compression test, in which a cylinder is pressed between two flat plates. In this paper, results are shown of the application of the diametral compression test on rice grains. At a temperature of 20°C, a moisture content of 5.7% (w.b.) and a deformation rate of 7·10⁻³ min⁻¹ the tensile strength of rice grains is 10 ± 2 MPa, the compressive strength is 73 ± 15 MPa and the Young's modulus is 1.1 ± 0.1 GPa. The Young’s moduli from the diametral compression tests are not significantly different from those from uniaxial compression tests, which agrees with the theory. The obtained tensile strength values are in accordance with those from the literature. The repeatability of the diametral compression tests is of the same order as the repeatability of uniaxial compression tests.     

Improvement of tensile shear strength and wood failure percentage of 1C PUR bonded wooden joints at wet stage by means of DMF priming

Tensile shear tests according to EN 302-1 for load-bearing timber structures were performed on European beech wood (Fagus sylvatica L.) and Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] bonded by means of a one-component polyurethane adhesive (1C PUR). Results reveal a substantial loss of tensile shear strength (TSS) and wood failure percentage (WFP) at the wet stage compared to the dry stage. As can be seen from microscopic images, this is accompanied by a loss of adhesion at the boundary layer. Therefore, the aim of this work was to find a priming fluid that improves the load transmission between adhesive and adherend at the wet stage without introducing formaldehyde into the gluing process. A substantial improvement of TSS and WFP was achieved by means of the hygroscopic organic solvent N,N-dimethylformamide (DMF). In addition, contact angle measurements were carried out, revealing that DMF heavily enhances the wettability of the joining surface. Furthermore, it was attempted to integrate the outcomes into the swelling strain model stated by Frihart in 2009. By way of comparison a hydroxymethylated resorcinol coupling agent, a mixture of diphenylmethane-4,4′-diisocyanate isomers and water were also tested as priming fluids. The data confirm that TSS and WFP of 1C PUR bonded wooden joints do not correlate, whilst WFP is mostly not normally (at wet stage often bimodally) distributed.     

针叶材管胞纵向零距抗拉强度的研究

研究了杉木、马尾松木材管胞零距纵向抗拉强度.结果表明,杉木早材管胞纵向抗拉强度介于310MPa～733MPa,平均499MPa;马尾松早材管胞纵向抗拉强度介于300MPa～590MPa,平均461MPa.两个树种的早材管胞纵向抗拉强度在树高方向(1.3m～7.2m)差异不显著,没有明显的变化规律;含水率对早材管胞纵向抗拉强度的影响不显著,但总体趋势是气干态的强度高于饱水态.     

不同组织结构织物的拉伸、撕裂和冲击强力效果

织物的组织结构通过组织因子和交织指数表达,这些因素之间的联系是r=0.98,R2=0.97。研究了这些参数对织物拉伸、撕裂和冲击强力的影响。织物的拉伸性能受其参数和结构的影响。因此,这项工作可以帮助理解织物相关强度的决定因素。试验研究织物中纱线相互交织或交错模式对拉伸、撕裂和冲击强力的影响。以12种具有代表性的不同交织方式织成的织物作为研究的对象。本试验工作表明:在这些不同的组织结构中,平纹的组织因子和交织指数是12种组织中最高的,花式组织最低,表明织造类型影响织物的强度性能。利用方差分析帮助进行结果分析。     

Model for the prediction of the tensile strength and tensile stiffness of knot clusters within structural timber

In the present paper, a model for the prediction of the local strength and stiffness properties is developed. Compared to existing models, here the local material properties are described according to their morphological characteristics; i.e. the timber boards are subdivided into sections containing knots (knot sections) and sections without knots (clear wood sections). The strains of the corresponding sections are measured during non-destructive tensile tests using an optical camera device. Based on these measurements the tensile stiffness of each particular section is estimated. For the estimation of the tensile strength, destructive tensile tests are performed. Herewith, the tensile strength of the entire timber board is measured. The strength of the other knot clusters are estimated using censored regression analysis. Taking into account the results of the experimental investigation, material models are developed to predict the tensile strength and the tensile stiffness of knot clusters.