先进生物质复合材料在风电叶片中的应用

竹材是生长速度快、性能好的生物质材料之一。由于竹材外侧性能远高于内侧, 采用分级加工方法可制备由外侧到内侧的A、B、C、D 级竹层积材。通过与其他风电叶片材料比较, A 级竹篾层积材的性能超过了目前风电叶片使用的木层积材, 与玻璃纤维增强塑料的性能接近。用这种先进生物质复合材料制作的新一代风电叶片, 具有可再生性、加工消耗能源少、无废弃物、成本低、废旧产品易于处理等一系列优点。      

以非结构用单板层积材为例的现代家具材料的创新与实践

针对非结构用单板层积材结构均匀、强度高、稳定性好等优良特性,在家具的造型设计和结构设计上进行有益的探索与实践,提出了适合其材性的家具造型与结构设计方法,为单板层积材家具的开发开辟了新的空间和方向。     

小径木白桦锯材干燥特性的研究

对小径木白桦锯材(1 000 mm ×45 mm×30 mm)进行了连续加热和间歇加热干燥试验,分析了小径木白桦锯材在不同加热方式下干燥曲线、含水率及干缩率的变化规律.结果表明:锯材的干燥速率在含水率15%左右明显降低;干燥过程中锯材厚度方向上的含水率梯度呈不对称分布,这种不对称分布在含水率降至20%以下才得到改善;锯材在较高的初含水率时就发生了干缩,干缩速率在含水率为15%左右减少;一元三次回归方程较好地反映了干缩率与含水率的相关关系,复相关系数R2在0.97以上;干缩率可以代替含水率作为控制小径木锯材干燥过程的参数.     

干燥过程中小径木表层应力的研究

采用连续加热和间歇加热(间歇2h和间歇6h)方式对小径木锯材进行常规干燥,研究了干燥过程中小径木的表层应力(全应力和残余应力)发生、发展变化规律.结果表明,随着试材的含水率降低,其表层全应力最初表现为拉应力,拉应力增大到最大值后,逐渐减小直至消失,然后转变为压应力,压应力达到最大值后,逐渐减小直到干燥结束;残余应力是先逐渐增大,达到最大后又减小;3种加热方式下,试材表层应力的变化趋势基本一致,但是试材的表层应力间歇加热的明显小于连续加热的,间歇6h试材的表层应力最小;同一含水率水平下,表层应力弦切板的比径切板大.     

锯材指接自动涂胶设备的研制与应用

针对锯材指接涂胶过程中存在的涂胶不均、效率低、涂胶辊易磨损等问题，提出一种锯材指接自动涂胶的方法。基于PLC(programmable logic controller，可编程逻辑控制器)，采用自动化检测技术，经过模块化设计，设计了一种新型自动涂胶设备，并进行了锯材指接自动涂胶试验。工厂化试验结果表明：自动涂料设备以200根/30 min的速度工作时，自动1次循环涂胶的合格率可达到97%及以上，且随时间逐渐提高，整个过程中设备无故障；自动2次循环涂胶时，在60—90 min、90—120 min内设备各出现1次故障，合格率均达95%及以上，无故障工作时间内自动2次循环涂胶的合格率可达到98%及以上，而人工涂胶合格率在90%以下。试验表明该自动涂胶设备能够实现快速自动涂胶，保证锯材指接涂胶质量，提高锯材指接涂胶的自动化程度，适合应用于锯材指接的工业化生产。     

Implementation of a mechanics-based system for estimating the strength of timber

The most accurate way of determining the strength of lumber requires destructive testing. An intelligent mechanics-based lumber-grading system was developed to provide a better estimation of the strength of a board nondestructively. This system processed X-ray-extracted geometric features (of 1080 boards that eventually underwent destructive strength testing) by using finite element methods to generate associated stress fields. The stress fields were then fed to a feature-extracting-processor, which produced 26 strength predicting features. The best strength predicting features were determined from the coefficient of determination (correlation r/sup 2/) between the features and actual strengths of the boards. The coefficients of determination of each feature (or combination of features), with the actual strength of the board, were calculated and compared. A coefficient of determination of 0.4158 was achieved by using a longitudinal (along the local grain angle) maximum stress concentration (MSC) feature to predict the estimated strength of lumber.     

Constrained displacement boundary condition in embedment testing of dowel‐type fasteners in LVL

The influence of the loading orientation with respect to the grain direction of wood and the influence of the lateral dowel displacement boundary condition on the embedment behaviour of steel dowels in laminated veneer lumber (with parallel‐laminated veneers) are investigated in this study. For limit states of the lateral boundary condition, the load‐displacement behaviour was experimentally studied by means of full‐hole embedment tests on screw‐reinforced laminated veneer lumber, for two dowel diameters and up to large dowel displacements. A novel biaxial test set‐up is proposed for embedment tests with constrained lateral dowel displacement boundary condition, in order to quantify laterally evoked reaction forces. Corresponding forces were found to change orientation with increasing dowel displacement and amounted to about 20% and 40% of the vertical reaction force for dowel displacements of 5 mm and twice the dowel diameter, respectively. The influence of the lateral displacement boundary condition was highlighted by comparison of the test data with a previously established data set for unconstrained embedment testing. Constrained loading showed a stiffer response and higher nominal embedment stresses, as well as a more pronounced displacement hardening, compared to unconstrained loading.     

铁纤维的气流分级效果研究

为了将不同直径分布范围的铁纤雏分离出来，根据平抛运动原理自行设计了气流分级装置，并在该装置上进行气流分级实验，得到不同直径分布范围的3个组分。借助扫描电镜和矢量网络分析仪对所得组分进行了分析．结果表明，分级后纤维的直径分布范围更加集中，电磁参数区别明显。其模拟反射率计算结果表明，直径较细的纤维吸波性能好。     

Theory and practice in measuring wood grain angle using microwaves

Knowledge of wood grain inclination is critical for achieving improved lumber strength grading. This paper describes an instrumentation system and theory for using microwaves to measure wood grain angle in real time. By using an electrically modulated scattering dipole with a homodyne microwave system, it is found both theoretically and experimentally that the amplitude reading of the modulated scattered signal provides an excellent means far measuring wood grain angle. In addition, the phase output of the system can be used to identify the sign of the grain angle. Two widely used wood species, Douglas Fir and Spruce, are measured at different moisture levels to verify the theoretical prediction experimentally. Preliminary data analysis shows that reliable measurement of grain angles up to about 30° can be made for wood within a wide range of specific gravity and moisture content. Through the experimental measurements, the potential of such a system for industrial use is demonstrated     

Effects of grading procedures on the scatter of characteristic values of European grown sawn timber

The natural scatter in mechanical properties of sawn timber must be reduced by grading the material either visually or mechanically. Depending on the grading procedure, the scatter of these properties varies. This study deals with their variation as influenced by the grading procedure. The effect of the grading principle is analyzed based on 4,893 sawn timber specimens from several European natural forests with widths up to 167 mm and depths up to 284 mm and using the method given in EN 14081-2:2010, CEN, Brussels (2010). Grading models for visual grading and machine grading are derived considering different source countries, strength classes and strength class combinations. Material safety factors for the graded material are then estimated in accordance with ISO 2394 (1998) to evaluate the grading outcomes. Analyzing and comparing the lower 5th-percentile to the requirements of EN 384: 2009, CEN, Brussels (2009), it is found that the actual strength for class C24 can be up to 20 % lower than required by the standard. This is true, regardless of whether the timber is graded visually or by an advanced grading machine using dynamic modulus of elasticity and knots. Low strength values can be expected especially in cases where a batch of timber is graded into a single strength class and reject only. High coefficients of variation of the graded material lead to the conclusion that high material safety factors are needed. On the contrary, if the material is graded by a machine and into more than two strength classes in one pass, it can be shown that the required material safety factors can be lower.     

Glulam beams reinforced with FRP externally-bonded: theoretical and experimental evaluation

The glued- laminated lumber (glulam) technique is an efficient process for the rational use of wood. Fiber-reinforced polymer (FRPs) associated with glulam beams provide significant improvements in strength and stiffness and alter the failure mode of these structural elements. In this context, this paper presents guidance for glulam beam production, an experimental analysis of glulam beams made of Pinus caribea var. hondurensis species without and with externally-bonded FRP and theoretical models to evaluate reinforced glulam beams (bending strength and stiffness). Concerning the bending strength of the beams, this paper aims only to analyze the limit state of ultimate strength in compression and tension. A specific disposal was used in order to avoid lateral buckling, once the tested beams have a higher ratio height-to-width. The results indicate the need of production control so as to guarantee a higher efficiency of the glulam beams. The FRP introduced in the tensile section of glulam beams resulted in improvements on their bending strength and stiffness due to the reinforcement thickness increase. During the beams testing, two failure stages were observed. The first was a tensile failure on the sheet positioned under the reinforcement layer, while the second occurred as a result of a preliminary compression yielding on the upper side of the lumber, followed by both a shear failure on the fiber-lumber interface and a tensile failure in wood. The model shows a good correlation between the experimental and estimated results.     

基于连续损伤的岩石渗流有限元分析

该文基于连续损伤力学,建立了含损伤的岩石渗流模型,研究岩石损伤破坏并分析其渗流特性。该文采用Weibull分布函数来模拟岩石弹性模量和强度的非均质特性,利用Biot本构关系建立含孔隙结构的弹性变形方程,结合Drucker-Prager强度准则、统计强度理论和连续损伤理论推导出岩石连续损伤演化控制方程及其定解条件,进而采用有限元法对其求解。该文给出的数值算例结果表明所提出的含损伤渗流模型适用于研究岩石的渗流特性,分析结果与实验数据吻合较好,且该求解方法可以对岩石体积压裂、岩层弱面进行可靠、有效的数值分析。     

Applying Real-Time Statistical Process Control to Manufacturing Processes Exhibiting Between and Within Part Size Variability in the Wood Products Industry

In this paper, we develop a control charting procedure to monitor average part size, as well as between and within part size variation for sawn lumber in an automated lumbermill. We present a review of the sawing process followed by a discussion of sampling methods for a real-time noncontact laser measuring device. A statistical model based on the components of variation analysis of variance model is proposed both for the analysis of the data and the construction of control charts that can be used to monitor the process. The details of the resulting statistical process control system are developed and an example from the lumber industry is provided and compared to other possible approaches. The resulting techniques may have applicability in many other industries where within and between variation in processes occurs.     

高频热压杨木单板层积材纵向弹性模量和泊松比的测定与分析

基于四点弯曲法,对一种基于高频热压技术的厚型杨木单板层积材的纵向弹性模量和纵横向泊松比进行了试验测定。其中纵向弹性模量的测定,分别采用了挠度法和应变片法。实验结果表明,2种测量方法所得纵向弹性模量数值比较接近,但又存在一定的差异,并对其差异的影响因素进行了探讨。基于高频热压技术的杨木单板层积材纵向弹性模量的测定结果表明,其纵向弹性模量接近甚至超过杨木单板,为该材料引入重型产品包装箱领域甚至结构用材料,从而替代原木,提供了一定的参考价值。     

基于磁流变液阻尼减振器的建筑结构半主动控制研究

基于相似理论，制作了某3层建筑结构的相似结构，建立了一个4自由度的振动系统；根据结构的振动特性对磁流变液阻尼减振器进行了优化设计，求得了磁流变液阻尼减振器的质量、阻尼系数及刚度；设计一套用于建筑结构的半主动振动控制测试系统，对磁流变液阻尼减振器的减振效果进行了试验研究，在同时考虑结构固有频率以及外界磁场强度的情况下，安装磁流变液阻尼减振器能使结构的振幅明显减小，整个振动过程较为稳定，验证了磁流变液阻尼减振器的减振效果。     

高速列车车间连接处车内噪声特性研究

参考ISO 3381-2005标准，对运营中速度等级为300 km/h的某型高速列车进行车间连接处车内噪声测试，给出了车间连接处车内噪声的频谱特性及其空间分布规律。进而，基于球谐函数声场分解和重构的球形阵列声源识别原理，采用球形阵列声源识别系统，对车间连接处车内噪声进行声源识别，明确了车间连接处车内噪声的源强和分布特性。最后，参考TB 3094-2004标准，对典型的车间连接风挡结构进行隔声特性测试。综合上述测试结果，对车间连接处噪声的产生机理进行了综合的分析。结果表明，现有高速列车风挡结构不单有隔声不足的问题，还存在较显著的结构振动声辐射，对风挡结构的优化设计需同时考虑上述两大因素。     

A stereological analysis of aggregate grading and size effect on concrete tensile strength

One of the most important topics in solid mechanics is the study of the so-called size effects, whose importance has been widely recognised during the last decades. Size effects are particularly strong in quasi-brittle (i.e., concrete-like) materials. In this paper we focus our attention on the tensile strength decrease associated with the size of concrete structures. An original explanation of this well-known size effect was proposed by the first Author based on the assumption of a fractal-like damage localisation at the mesostructural level. This hypothesis leads to a multifractal scaling law (MFSL) for concrete tensile strength. The present contribution provides a scaling law for concrete tensile strength based on its aggregate size distribution. Since the weakest link in normal strength concrete is represented by the interface between the cementitious matrix and the aggregates, it seems reasonable to look for a relationship between the aggregate grading and the material strength. Based on the hypothesis that the strength depends on the largest flaw, we compute the strength of a concrete specimen as a function of its size. Differently from other statistical approaches, we use a truncated distribution (namely the Füller distribution) in order to describe realistically the flaw population inside the specimen. Calculating the distribution of the largest flaw size by means of statistics of extremes, and relating it to the specimen size, we obtain a scaling law for concrete tensile strength whose trend strictly agrees with the MFSL. Finally, we pay particular attention to the computation of the power law exponent characterising the strength scaling at the smallest sizes and present a comparison with available experimental data.     

A stereological analysis of aggregate grading and size effect on concrete tensile strength

One of the most important topics in solid mechanics is the study of the so-called size effects, whose importance has been widely recognised during the last decades. Size effects are particularly strong in quasi-brittle (i.e., concrete-like) materials. In this paper we focus our attention on the tensile strength decrease associated with the size of concrete structures. An original explanation of this well-known size effect was proposed by the first Author based on the assumption of a fractal-like damage localisation at the mesostructural level. This hypothesis leads to a multifractal scaling law (MFSL) for concrete tensile strength. The present contribution provides a scaling law for concrete tensile strength based on its aggregate size distribution. Since the weakest link in normal strength concrete is represented by the interface between the cementitious matrix and the aggregates, it seems reasonable to look for a relationship between the aggregate grading and the material strength. Based on the hypothesis that the strength depends on the largest flaw, we compute the strength of a concrete specimen as a function of its size. Differently from other statistical approaches, we use a truncated distribution (namely the Füller distribution) in order to describe realistically the flaw population inside the specimen. Calculating the distribution of the largest flaw size by means of statistics of extremes, and relating it to the specimen size, we obtain a scaling law for concrete tensile strength whose trend strictly agrees with the MFSL. Finally, we pay particular attention to the computation of the power law exponent characterising the strength scaling at the smallest sizes and present a comparison with available experimental data.     

Neural network approach for estimating the residual tensile strength after drilling in uni-directional glass fiber reinforced plastic laminates

The drilling of fiber reinforced plastics (FRP) often results in damage around the drilled hole. The drilling induced damage often serves to impair the long-term performance of the composite products with drilled holes. The present research investigation focuses on developing a predictive model for the residual tensile strength of uni-directional glass fiber reinforced plastic (UD-GFRP) laminates with drilled hole which has not been developed worldwide till now. Artificial neural network (ANN) predictive approach has been used. The drill point geometry, the feed rate and the spindle speed have been used as the input variables and the residual tensile strength as the output. The results of the predictive model are in close agreement with the training and the testing data.     

Indirect determination of size effect on strength of asphalt mixtures at low temperatures

Low temperature cracking of asphalt pavements is a major distress in cold regions. Accurate assessment of strength of asphalt mixtures at low temperatures is of great importance for ensuring the structural integrity of asphalt pavements. It has been shown that asphalt mixtures behave in a quasibrittle manner at low temperatures and consequently its nominal strength strongly depends on the structure size. The size effect on the strength of asphalt mixtures can be directly measured by testing geometrically similar specimens with a sufficiently large size range. Recent studies have shown in theory that for quasibrittle structures, which fail at the macrocrack initiation from one representative volume element, the mean size effect curve can also be derived from the scaling of strength statistics based on the finite weakest link model. This paper presents a comprehensive experimental investigation on the strength statistics as well as the size effect on the mean strength of asphalt mixtures at ?24 °C. It is shown that the size effect on mean structural strength can be obtained by strength histogram testing on specimens of a single size. The present study also indicates that the three-parameter Weibull distribution is not applicable for asphalt mixtures.