弓形弯曲原木优化下锯建模与仿真

面对我国森林资源不足、质量不高的状况,建立和应用原木优化下锯理论,提高木材出材率已成为缓解木材供需矛盾的主要途径.本文通过提出弓形弯曲原木的数学模型,全面描述弯曲原木的解析形状,并对弯曲原木的主产出材率进行优化设计,借助计算机辅助设计实现原木优化下锯的三维再现,这对提高原木出材率和利用率、提高木材加工技术水平与现代化程度有着极为重要的实际意义和广泛的应用前景.     

“锯切用弯曲原木”出材率数学模型

影响“锯切用弯曲原木”出材率的相关因子有“弯曲率C2（弯曲拱高与弯曲内曲水平长的比值）”、“弯曲度C2（弯曲拱高与检尺径的比值）”2种。通过建立弯曲原木出材率数学模型的方法，证明了影响弯曲原木出材率的主要因子是弯曲度C2，并得出将弯曲原木拱形部分边皮作为废材时，锯切用弯曲原木出材率表达式为Vc=1-1/360[arccos（1-2C2）＋2arccos（1-C2）]＋1/π[（1-2C2）√C2（1-C2）＋2（1-C2）√1/2C2（1-1/2C2）]即，锯切用弯曲原木的出材率是弯曲度C2的函数，而不是弯曲率C2的函数。     

浅谈实木弯曲家具生产中的实木弯曲成型技术

实木弯曲家具以其优美的曲线造型、精细的制作工艺、高雅的款式,深受人们欢迎,且部分曲线的弧度符合人体工程学原理,可以让使用者更加舒适,减轻疲劳感.实木弯曲成型技术是一种木材无屑成型加工技术,它是通过对木材进行软化处理后实现弯曲变形,再干燥定型,从而实现弯曲成型.按木材软化和加压弯曲方式,可将实木弯曲成型技术分为:传导加热式实木弯曲成型技术、微波加热式实木弯曲成型技术、纵向压缩后实木弯曲成型技术及横向压缩实木弯曲成型技术等[1-5].     

矩形截面对称板材弯曲成型和铣削成型的强度分析

实木弯曲是最早应用于家具制造业中的木材弯曲成型技术,早在160多年前,德国木匠Michael Thonet就制造了第一张实木弯曲成型的椅子。从那时起,这种工艺在家具制造业中就得到了广泛的应用,第二次世界大战时,这种弯曲木还应用于飞机制造业和高要求的建筑构件上(刘绪林,1999)。木材软化和弯曲机制也成为研究的重要内容,我国目前主要采用蒸煮法软化木材,然后用手工或机械法弯曲成规定形状,再进行干燥和定型(李军,1998)。弯曲后构件的强度是决定其应用的主要因素,本文将通过与铣削成型的弯曲木构件对比以证明其高强度性。弯曲形状的木制品构件一…     

弯曲枕资下锯图的研究

本文应用计算机辅助设计弯曲枕资下锯图，建立了辅助设计的数学模型，并编制了Ｃ语言程序，为合理加工弯曲枕资，提高出枕率、综合出材率和产品经济效益提供了理论依据。     

弯曲原木材积测定的实用近似公式探讨

本文采用柱坐标,以椭圆—阿基米德螺线包络组成解析曲线椭圆柱体的弯曲原木的数学模型。定义它为C型弯曲。并以材长和弯曲弓背长度定义,提出了一个实测参数的弯曲原木材积计算公式。定量地解决了弯曲造成原木材积统计时的损失。这种方法和理论,可以解决弯曲原木最佳出材率和弯曲原条最佳截断问题。为制材的数控软件编制提供了理论基础。     

早材管孔分布对环孔材栎木蠕变特性的影响

在采用生物图像软件(IPP 6.0)表征环孔材栎木横切面管孔组织比量与分布特性的基础上,探究不同温度条件下早材管孔分布对环孔材栎木板材弯曲蠕变行为的影响,利用Burger模型和五参数模型对其蠕变特性进行模拟预测,为木材弯曲加工成型、木质材料热压成型和木结构设计提供参考.利用动态热机械分析仪测试不同温度条件下环孔材栎木试...     

红松原木不同断面几何形状与枕木出材率的关系

一、前言在枕木生产中,合理下锯是节约木材、提高成材出材率和产品质量,降低成本的重要措施之一。为了做到合理下锯,需要考虑的因素很多。如弯曲原木和径差较大的5米枕资合理截锯的问题,合理留钝稜的问题,注意剔除原木的天然缺点合理配置主副产品的问题以及充分利用原木断面形状的特点提高枕木出材率的问题等。而过去在枕木生产中工人接触最频繁,潛力较大,而又尚未被人们所重视的因素就是原木断面形状与枕木出材率的关系问题。     

连江县湿地松人工林单木地径出材率表的研究

应用连江县湿地松人工林伐倒木造材数据,以地径为自变量,经济材、薪材、废材出材率分别为因变量,采用多方程拟合对比的方法建立材种出材率模型,据以编制湿地松人工林单木材种出材率表.经检验地径出材率表的平均系统误差小于3％,平均相对误差小于10％,在林业实践上有应用价值.     

确定合理出材率的方法

出材率大小直接影响森林资源的利用率,以往采用的方法确定的出材率大都与实际出材率有出入。本文介绍了确定合理出材率的方法是利用树高、胸径与出材率的关系曲线,确定出材率。此方法简便,与实际出材率很接近。     

基于GIS的木材运输计划决策支持系统

制定木材运输计划需要大量的空间和非空间数据。基于GIS的木材运输计划决策支持系统 ,对线性规划和GIS技术进行集成 ,以运材费与集材费的综合费用为寻找最小费用路径和最佳楞场空间位置的目标函数 ,建立线性规划模型。运输费、可采木材材积和木材定货量等模型参数以MPS文件格式存储 ,实现LINDO与系统间的数据交换。系统是制定木材运输计划的有效工具 ,也为编制作业计划提供重要的基础数据     

Determination of elastic properties of clear wood by the homogenization method in two dimensions

A two-dimensional homogenization model for determining elastic properties of clear wood is described. It is based on the elastic properties of the chemical constituents of the wood and the known geometry of the fibers. The model is valid for clear wood and can be used to determine the state of stresses at both micro and macrostructural levels. Stress and displacement analysis were made with the finite element method. Experiments in bending, compression parallel- and perpendicular-to-grain were made with six different Mexican wood species, including softwoods, temperate climate hardwoods and tropical hardwoods using small clear wood specimens. Such tests were made at various moisture content levels and the results of the model match closely with experimental data. The analytical technique can be readily extended to three dimensions. The data generated with this model can be used in further studies for material and structures optimization purposes.     

GIS-based decision support system for wood logistics

A wood logistics system was combined with a linear programming (LP) method utilizing GIS-based techniques on the platform of GIS soflware-ARC/INFO. The combined costs of road and off-road transport were taken as the objective function to find the least cost route and the optimal landing locations of wood transportation. Then transport costs and allowable wood volume of stands were calculated. An LP model was developed to allocate timber resources among mills in order to minimize the wood logistics costs from harvesting sites to mills. The parameters of the LP model, including the transport costs, allowable wood volume and wood orders, were written into a text file in MPS format which were then accessed by LINDO to solve the LP problem. The system is an effective tool to manage logistics, information and funds together in order to increase the speed of wood logistics and reduce the cost.The benefits and efficiency of mill cluster can be improved. The focal firm in the cluster can be competitive.     

Strength and stiffness capacity utilisation of timber members in roof truss structures

The main objective of this study was to determine which property, of the six strength and stiffness properties used in structural timber design, was the most influential in the design of nail-plated roof trusses. Thirty recently completed nail-plated roof truss designs were randomly selected from three roof truss manufacturing plants and a total of 8 758 individual truss members were analysed for bending stress, shear stress parallel to grain, tensile stress parallel to grain, compression stress parallel to grain and deflection. The mean strength and stiffness capacity utilised of all the different design properties was well below 50% for all of the different dimension classes. Of all the individual strength properties, the mean bending strength capacity utilised per member was found to be the highest. The results of this study can be used for decision support related to wood property evaluation through? out the structural lumber value chain where roof truss members are the end products.     

Thermal degradation of bending strength of plywood and oriented strand board: a kinetics approach

The construction industry has relied heavily on wood and wood-based composites, such as oriented strand board (OSB) and plywood for timber frame construction. Therefore, it is highly imperative to categorize the response of wood-based composites when exposed to elevated temperatures for a sustained period of time. The essence of fire-resistant structural design is to ensure that structural integrity be maintained during and after the fire, prevent collapse and maintain means of egress. Another aspect is to assess post-fire structural integrity and residual strength of existing structure. The objective of this project was (a) to study the effect of exposure time on bending strength (MOR) of OSB and plywood at elevated temperatures, (b) to interpret any relationships between different temperature and time of exposure using a kinetics model for thermal degradation of strength, and (c) to develop a master curve representing temporal behavior of OSB and plywood at a reference temperature. As much as 1,152 samples were tested in static bending as a function of exposure time and several temperatures. Strength (MOR) of both OSB and plywood decreased as a function of temperature and exposure time. These results were fit to a simple kinetics model, based on the assumption of degradation kinetics following an Arrhenius activation energy model. The apparent activation energies for thermal degradation of strength were 54.1 kJ/mol for OSB and 62.8 kJ/mol for plywood. Furthermore, using the kinetics analysis along with time–temperature superposition, a master curve was generated at a reference temperature of 150°C which predicts degradation of strength with time on exposure at that reference temperature. The master curves show that although plywood has a higher initial strength, OSB performs better in terms of strength degradation after exposure to elevated temperature.     

Comparison between modulus of elasticity values calculated using 3 and 4 point bending tests on wooden samples

Several data banks on wooden properties of different species contain mechanical characteristics of which the bending modulus of elasticity. This modulus can be calculated using different test methods, the more ordinary used are the 3 point and 4 point bending tests. The values obtained by one method cannot be directly compared with those of other methods. So the bending properties read in a data bank have to be converted before using them and correctly compared with other data from different references. The aim of this study is to make an analytic formula of a crossing coefficient between 3 point and 4 point bending concerning the longitudinal modulus of elasticity measured following the French standards (NF 1942; NF 1987). This formula includes a study of the shear force influence, and a study of supports and loading head indentation effect, in a 3 point bending test. The analytical study and the experiences have shown that the supports and loading head indentation effect are not negligible but have the same influence as the shear effect. The indentation is the result of the competition between two physical phenomena which are the wood stiffness and the load level applied on the piece of wood during a bending test. The practical result of this study is the development of a crossing analytic formula from a 3 point bending modulus of elasticity to a 4 point bending one, verified by the experimentation. Received 26 June 2000 The C.I.R.A.D.-Forêt team and especially M. Bernard Thibaut, M. Gilles Calchera, and M. Joseph Grill from Laboratoire de Mécanique et de Génie Civil (L.M.G.C.) are gratefully acknowledged for their precious help during this research.     

Fresh-wood bending: linking the mechanical and growth properties of a Norway spruce stem

To provide data and methods for analyzing stem mechanics, we investigated bending, density and growth characteristics of 207 specimens of fresh wood from different heights and radial positions of the stem of one mature Norway spruce (Picea abies L. Karst.) tree. From the shape of each stress-strain curve, which was calculated from bending tests that accounted for shear deformation, we determined the modulus of elasticity (MOE), the modulus of rupture (MOR), the completeness of the material, an idealized stress-strain curve and the work involved in bending. In general, all mechanical properties increased with distance from the pith, with values in the ranges of 5.7-18 GPa for MOE, 23-90 MPa for MOR and 370-630 and 430-1100 kg m(-3) for dry and fresh wood densities, respectively. The first three properties generally decreased with stem height, whereas fresh wood density increased. Multiple regression equations were calculated, relating MOR, MOE and dry wood density to growth properties. We applied these equations to the growth of the entire stem and considered the annual rings as superimposed cylindrical shells, resulting in stem-section values of MOE, MOR and dry and fresh densities as a function of stem height and cambial age. The standing tree exhibits an inner stem structure that is well designed for bending, especially at a mature stage.     

Thermal degradation modeling of flexural strength of wood after exposure to elevated temperatures

Abstract

Wood is being used heavily in single-family residential dwellings. Therefore, it is important to categorize their response when exposed to elevated temperatures for a sustained period of time. An important aspect of structural fire design is to assess postfire residual strength of existing structures. This study addresses this issue by developing models to predict strength degradation of wood after exposure to elevated temperature. The objectives were to (a) study the effect of exposure time on bending strength [Modulus of Rupture (MOR)] of wood at elevated temperatures, (b) interpret any relationships between different temperature and time of exposure using kinetics and a statistical approach, and (c) compare the two approaches. Two hundred thirty-two samples in total were tested in flexure as a function of exposure time and several temperatures. MOR of wood decreased as a function of temperature and exposure time. Rate of degradation was higher at higher temperatures. These results were fit to a simple kinetics model, based on the assumption of degradation kinetics following an Arrhenius activation energy model with apparent activation energy of 37.4 kJ/mol. A regression-based statistical model was also developed. The kinetics-based model fit the data better with one less parameter and predictions consistently matched the observed values, making the model preferred over the regression approach.     

Three-dimensional finite element modelling of heat transfer for linear friction welding of Scots pine

Finite element numerical analyses were performed to determine three-dimensional heat flux generated by friction to wood pieces during linear friction welding. The objective was to develop a computational model to explain the thermal behaviour of welded wood material rather than experimental methods, which are usually expensive and time consuming. This model serves as a prediction tool for welding parameters, leading to optimal thermo-mechanical performance of welded joints. The energy produced by the friction welding of small wood specimens of Scots pine (Pinus sylvestris L.) was determined by thermocouples and used as input data in the model. The model is based on anisotropic elasticity and the thermal properties were modelled as isotropic. This numerical simulation gave information on the distribution of the temperature in the welding interface during the entire welding process. A good agreement between the simulation and experimental results showed the appropriateness of the model for planning welded wood manufacture and prediction of thermal behaviour of wood during other mechanically induced vibration processes. The specimens presented in this model required a heat flux of 11 kW/m² to achieve a satisfactory welding joint.