Modelling local bending stiffness based on fibre orientation in sawn timber

Strength of structural timber depends to a high degree on the occurrence of knots and on the local fibre deviation around such defects. Knowledge of local fibre orientation, obtained by laser scanning, has been utilized in a previously developed machine strength grading method, but rather crude assumptions regarding the fibre orientation in the interior of boards and a mechanical model that does not capture the full compliance of knotty sections were adopted. The purpose of the present study was to suggest and verify a model with which local bending stiffness can be predicted with high accuracy. This study included development of a model of fibre orientation in the interior of boards, and application of a three-dimensional finite element model that is able to capture the compliance of the board. Verification included bending of boards in the laboratory and application of digital image correlation to obtain strain fields comparable to those obtained by finite element simulation. Results presented comprise strain fields of boards subjected to bending and calculated bending stiffness profiles along boards. Comparisons of results indicated that the model suggested here was sufficient to capture the variation of local bending stiffness along boards with very high accuracy.     

Prediction of global bending stiffness of timber beams by local sampling data and visual inspection

The results of visual inspection according to UNI 11119:2004 and bending tests made on 20 old chestnut (Castanea sativa Mill.) beams according to EN 408:2010 were statistically analyzed in order to provide a consistent and feasible procedure to predict their modulus of elasticity (MOE) in bending. Local data obtained from smaller size specimens was used for the prediction of the global mechanical properties of full structural size members and compared with the results of mechanical tests. The prediction models took into account the visual strength classes and the influence of defects on the determination of the MOE. Moreover, random sampling was considered in order to demonstrate the possibility of using smaller representative samples, thus avoiding excessive need of removal of on-site samples and allowing for a smaller number of mechanical tests. The models using random sampling predicted the behavior of full size scale elements accurately, with strong correlations to the experimental results (coefficient of determination r ² ranging from 0.70 to 0.79) and a percentage error lower than 20 %, thus allowing a reliable estimation of mechanical characteristics of existing timber members with a combination of visual inspection and local sampling.     

Influence of wood moisture content on bending and shear stiffness of cross laminated timber panels

The bending and shear stiffness of cross-laminated timber (CLT) made from softwood depend on the moisture content of the panel. In principle, the stiffness properties drop with increasing moisture content within the hygroscopic range. However, swelling provokes closing of open gaps of the middle layers and leads to an apparent increase of stiffness due to internal friction. This increase in stiffness is only relevant for low deflections and can provoke misinterpretation of measuring results when deriving stiffness properties of CLT by means of dynamic methods such as modal analysis. Drying, on the other hand causes cracking within CLT, which manifests itself in a distinct reduction of the bending stiffness perpendicular to the grain direction of the face layers. Calculating bending stiffness of CLT perpendicular to the grain direction of the face layers by means of the compound theory not taking into account the face layers therefore is a common and correct procedure.     

Prediction of strength and stiffness of concentrated compressive load applied to the narrow face of cross-laminated timber

European Journal of Wood and Wood Products - The behavior of the concentrated compressive load applied to the narrow face of cross-laminated timber (CLT) panels is an essential parameter in the...     

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.     

Determination of settings in combined strength, stiffness and density grading of timber

A new method is presented for the determination of settings in machine strength grading of timber. The method is compared with the European standard for “machine control” method. Numerical examples are shown based on large samples of spruce in bending from all over Europe. The proposed method gives similar values of grade determining properties and increased yields when a single indicating property is measured and timber is sorted to three grades. The method is based on strength, stiffness and density as grade determining parameters. Prediction limits on 10?% probability level are determined for all grade determining properties, and all three thresholds are met. The use of two indicating properties, one with good correlation with density, results in economically better grading results and more reliable quality of timber.     

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.     

Relationship between local and global modulus of elasticity in bending and its consequence on structural timber grading

The study analyses the relationship between local and global modulus of elasticity and develops and evaluates different models to predict local from global modulus measurements. The mechanical tests were performed on four species commonly used in Italy for structural purposes: fir, Douglas-fir, Corsican pine and chestnut. Two or three cross-sections and two provenances were sampled for each species. A theoretical analysis showed that the local–global modulus relationship was of polynomial form with only one coefficient. The effect of the species on the relationship was significant as well as the cross-section but only for softwoods. The effect of the cross-section was explained by the presence and the size of defects in the mid span. The different models were applied and then compared by means of the optimum grading: only slight differences among models emerged. Although optimum grading was strongly dependent on the sampling and on the grade combination, for softwoods the model for species and section showed very similar results to the grading with the true local modulus; inclusion of the knot values in the model led to only slight improvements. For chestnut all models were found to be comparable.     

Modeling the bending vibration of cross-laminated timber beams

Cross-laminated timber (CLT) is an economical construction material combining good structural properties and environmental advantages. Due to the layered geometry and the associated internal stress boundary conditions, modelling its dynamic behavior is complex. However, accurate vibration level estimations are necessary to estimate the sound insulation performance of CLT constructions. This paper outlines a method to determine the relevant elastic constants and the appropriate equations of motion to predict the vibration of beams cut from a three-ply CLT plate. Modal analysis performed on strips cut along the two principal directions provides insight in the material’s dynamics. The bending mode frequencies are determined experimentally by analyzing the sweep response spectrum using the linear prediction method. In a second step, analytical beam vibration models of increasing complexity are fitted to the measurements using a genetic optimization algorithm. Thin beam theory—the simplest model—does not predict the bending modes with sufficient accuracy. Depending on the strip direction, the bending vibrations can be modeled using Timoshenko’s theory for thick beams, or by a model for three layer sandwich beams respectively. The results show that taking the beam geometry into account is as important as estimating the orthotropic material constants to model the bending modes properly.     

The influence of compression failure on the bending, impact bending and tensile strength of spruce wood and the evaluation of non-destructive methods for early detection

Bending strength (MOR) and bending Young’s modulus (MOE) according to DIN 52186 and MOE calculated on the basis of eigenfrequency and sound velocity were tested on small clear wood specimens of Norway spruce wood with and without compression failure. One group of specimens was climatised in a normal climate of 20°C and 65% relative humidity, while the other group was stored for one month under water before testing. The MOR of specimens with compression failure decreased about 20% on average (normal climate and wet) compared with the specimens without compression failure. The MOE of the specimens with compression failure was reduced only minimally compared with the specimens without compression failure stored in a normal climate, but very distinct differences (more then 30%) were found under wet conditions. The MOE of the specimens with compression failure calculated on the basis of eigenfrequency and sound velocity were not reduced or only minimally compared with the specimens without compression failure. It is therefore not possible to detect compression failure and to determine reduction in MOR using eigenfrequency or sound velocity. In addition, impact bending (DIN 52189), tensile strength and tensile MOE (DIN 52188) were tested on small clear wood specimens of Norway spruce wood with and without compression failure. The specimens with compression failure revealed an average reduction in impact strength of about 40% and an average reduction in tensile strength of about 20% compared with the specimens without compression failure, whereas tensile MOE of the specimens with compression failure was not reduced compared with the specimens without compression failure. The detection of compression failure by computer tomography (CT) was tested on Norway spruce wood boards 10 cm in thickness, and detection by optical scanner was tested on planed Norway spruce wood boards. CT recognised large compression failures easily, whereas the scanner was not able to detect them.     

基于桥联模型预报三维五向编织复合材料的刚度和强度

基于三维五向编织复合材料细观单胞模型,将材料内部、表面、棱角区域内具有相同走向的纤维束简化为单向复合材料,利用桥联模型,确定了单向复合材料的柔度矩阵,将具有不同材料主向的单向复合材料的刚度通过体积进行平均,得到了三维五向编织复合材料的总体刚度矩阵,进一步计算得到材料的工程弹性常数;以单向复合材料为基础,宏观上基于等应变假设,结合桥联模型,确定出材料内各组分(纤维束和基体)的细观应力分布,对纤维束采用Hoffman失效准则,基体采用Mises失效准则,预报了三维五向编织复合材料的拉伸强度。结果表明,三维五向编织复合材料的刚度和强度的预测值与实验值吻合较好,验证了本文分析模型的有效性。     

Shear strength parallel to grain with distinct ray orientation on four Brazilian wood species

Ray orientation (perpendicular, parallel and diagonal) was compared with respect to shear strength parallel to grain and specific gravity in four Brazilian wood species. Higher absolute strengths occurred with ray oriented perpendicular to the shear plane, and lower ones for parallel orientation for almost all species. In particular, specimens with parallel ray orientation to shear plane did not differ statistically from shear values when compared to specimens having perpendicular and diagonal orientation to shear plane.     

缝缝对纺织面料抗弯硬挺度的影响

对于服装产品模型化,抗弯硬挺度是一个主要的原料性能参数.由于根据面料的硬挺度及局部面密度的变化而确定的缝合工艺,通过成型,服装展现出与纺织面料不同的外观.本研究开发了一种测定缝缝硬挺度对缝合面料影响的新方法.适用于本研究的新型抗弯硬挺度测试仪ACPM 200是由德累斯顿工业大学纺织服装学院开发的.     

Effect of knots on the flatwise bending stiffness of lumber members

In machine stress-rating of lumber where flatwise bending stiffness is used as a predictor of strength, it is customary to assume that the effect of knots is accounted for by stiffness. However, only few data in the published literature can be used to substantiate this claim. The present study was undertaken to evaluate the relationship between stiffness and knot size for lumber members loaded in bending with a test geometry similar to that used by grading machines. Experiments were carried out with spruce lumber specimens containing a single centerline knot. A theory-of- elasticity based model was derived for analysis purposes. Theory and experiments agreed in showing that the effect of knots on flatwise bending stiffness is very small. This low sensitivity may explain why correlations between strength and machine measured stiffness are rather poor for commercial lumber. Implications of this finding on the practice of machine stress-rating of lumber are discussed.     

Some simple theoretical considerations on the bending stiffness of human hair

Factors affecting the cantilever bending of human hair have been investigated by theoretical structural mechanics in which the hair was treated as if it was of isotropic structure and internal mechanical property. Despite the simplicity of the model it is a valuable one for assessing the major geometric factors affecting a hair's bending behaviour in relation to perceptions of coarseness and fineness. Simple equations have been used for predicting relative bending stiffness between hairs of different racial types using published fibre cross-sectional information and these accord with existing perceptions of behaviour. Strikingly it is the hair's minor elliptic diameter which has a dominant effect upon bending resistance and it is shown that the hair's cuticle could be making a very high proportional contribution. It is shown that triangular hairs of the uncombable hair syndrome will be stiffer than normal elliptical hairs of the same cross-sectional area and that this goes some way to explaining the condition.     

微米木纤维细胞视频变换及重构方法分析

用视频技术在微米木纤维细胞的描述方法的理论基础上,分析了微米级的木纤维在细胞学基础上的图像采集方法,分析利用显微镜扫描得到微米级木纤维的处理工艺方法,开创出一种新的微米木纤维视频描述的科学方法,设计微米级木纤维的视频描述的系统。对细胞尺寸的几何变换和投影变换进行了研究方法的分析,通过微米木纤维的计算机视频识别技术,将细胞尺寸投影变换,描述了计算机模拟三维重建的方法研究,分析了纤维与细胞边缘检测理论意义和实现方法的前景。     

木纤维压缩包装机的研制

介绍了木纤维压缩包装机的总体设计思想、主要结构及工作过程,主要技术参数和技术特点,以及在工厂化制备绿化基材生产的重要性与绿化喷播中的应用.     

Creep of wood in bending and non-symmetrical moistening

The effect of one sided moistening zones under both tension and compression, as well as the effect of simultaneous moistening both of these zones of bent wood on the course of creep was investigated. The increase in deflection of the bent samples was registered and the changes in moisture content distribution were observed in control samples. Moistening of the compressed zone has the maximum effect on creep in the bent wood samples. It was found that a bending stress of not more than 17% of the ultimate strength when moistening the compressed zone, decreases the deflection and even a reversion in its direction can be expected. The warping forces are probably responsible for this behaviour. The increase in deflection in the bent and moistened wood may sufficiently be described by a parabolic function of creep compliance.