非接触式测试木材干燥应力方法的研究

提出并建立了非接触式测试木材干燥应力的方法及装置，并对水曲柳木材干燥过程中的应力进行了测试。结果表明：此方法简单方便，可在线连续地定量测试木材干供应力。     

非接触式无损检测木材干燥应力的方法

木材干燥应力是干燥过程中产生干燥缺陷的主要因素。文章根据数字散斑相关方法(DSCM)检测应力的原理设计出了一种非接触式无损检测木材干燥过程中应力的方法,用木材干燥表面测点位移的变化速率即应变速率来表示木材干燥过程中的应力状态,初步提出了建立木材干燥应力评价体系的设想。研究结果表明:木材表面测点位移变化量与时间存在着对数关系,与含水率变化量则呈线性关系。     

木材干燥应力研究现状与趋势

木材干燥应力相关研究是木材科学的重要组成部分,对木材实际加工过程具有重要指导作用,也是制定和优化干燥工艺的依据。本文从木材干燥应力研究的主要试验手段和模型方法 2方面入手,对比分析各种方法的优势和不足,系统总结干燥应力研究现状,以期为木材干燥应力相关研究提供借鉴,推动干燥应力研究向更深层次发展,为木材高质量干燥奠定基础。试验研究方面,传统的叉齿法和切片法依旧是木材干燥应力、应变检测的常规试验方法,数字图像、近红外光谱等现代化技术手段为干燥应力、应变研究提供了新的契机,但至今仍没有一种方法具备绝对的检测精度和实际推广应用条件。模型研究方面,木材流变学理论模型日趋完善成熟,对干燥应力的模拟进入一个瓶颈阶段。随着计算机技术的不断发展以及数值分析软件的逐渐强大,有限元法被用于木材干燥应力的模拟研究,该方法在处理木材这种各向异性材料方面具有一定优势,目前处于快速发展阶段。根据现阶段木材干燥应力研究现状,试验手段和模型方法对于干燥应力研究均具有不可替代的作用,二者结合是全面研究干燥应力的关键,试验探寻能够快速、精准、连续检测木材干燥应力的新装置或新技术依然是未来的研究重点。有限元法是木材干燥应力模型研究最具潜力的数值分析方法,未来适用性广泛的模型构建要立足于木材自身的多尺度、多层级结构特征,利用先进仪器设备获取精准的模型参数,并将所建模型纳入干燥设备控制系统,指导和服务于实际生产。     

木材干燥窑智能化控制系统的设计

推出了一种新型的全自动木材干燥智能控制系统，以取代常规木材干燥控制设备，具有结构简单、功能齐全、投资少、性能可靠，能对木材干燥过程中的各参数进行在线测量，并能根据所选定的工艺基准对木材干燥的全过程进行自动监测和控制。该系统适合我国国情，有利于促进我国经济发展，提高木材工业的地位。     

木材干燥微机控制系统

木材干燥是决定木制品质量的重要工艺手段。为了提高木制品的产品质量，应该选择经济合理的木材干燥工艺方法。在木材干燥过程中，影响木材干燥质量及成材率的因素很多，如：材种、材型、木材含水率、窑内温、湿度及热风循环速度等。实践表明，目前大量使用的常规及半自动控制的木材干燥方法，很难获得理想的干燥效果，不能满足人们对木材干燥质量的要求。因此，有必要研究新的木材干燥控制方法。全自动木材干燥控制方法是较理想的干燥控制方法，它可对木材干燥全过程进行自动控制，这不但可以降低工人的劳动强度，而且还可以提高产品质量及…     

木材高温干燥过程中的弹性应变

本文对美洲黑杨锯材高温干燥过程中的弹性应变进行了研究。结果表明：高温干燥使得木材表层产生较大的塑化变定,并对干燥后期木材内层的正常收缩造成障碍。心层的拉伸应力与木材内裂有关,当心层拉伸应力超过木材横纹拉伸极限强度时,木材产生内裂。因此,在用高温干燥工艺快速干燥杨木类软阔叶树材时,在干燥后期应注意由于干燥应力转向而引起的干燥缺陷。     

电阻式木材含水率仪温度自动补偿的方法及其应用

１电阻式木材含水率仪温度自动补偿的意义木材干燥是木材加工过程中的关键工序，它直接影响到木制品的质量及制作成本等，而木材干燥质量的好坏、干燥成本的高低及干燥周期的长短，与干燥过程中含水率的检测精度直接相关。为了减轻木材干燥过程中操作工的劳动强度，减少操...     

木材干燥应力的研究方法与进展

木材干燥应力的研究对丰富和完善木材干燥理论、建立和完善干燥工艺、提高干燥质量和经济效益具有重要意义,许多研究者从不同角度对木材干燥应力应变进行了试验测定及数值计算.本文对其中的主要研究方法进行了较详细的归纳,并对今后的研究提出了看法和建议.     

木材流变学特性对板材常规干燥开裂、变形的影响

简述了木材干燥应力数学模型的研究背景,建立了木材常规干燥的干燥应力应变数学模型,模型主要考虑了干燥收缩应变、弹性应变、黏弹性蠕变应变和机械吸附蠕变应变四种因子.通过干燥应力应变数学模型与干燥扩散数学模型的联合分析求解,可以解释板材物理力学特性(如干缩率、含水率、干缩各向异性、横纹静曲弹性模量等)对干燥应力、干燥应变的影响,分析干燥应力的产生、发展和释放的机理,为科学制订木材干燥工艺提供理论依据.     

水热预处理对枫香木材干燥性能影响的研究

为探索水热预处理对枫香木材干燥性能影响规律,枫香木材先通过水热处理,再利用常规与降温干燥方法对其进行干燥,通过扫描电子显微镜对比分析预处理材与未处理材的微观组织结构变化,评价不同干燥方法的干燥速度与干燥质量。研究结果表明:干燥初期,预处理后枫香木材的常规干燥速度与降温干燥速度较未处理木材的干燥速度分别提高9.2%和12.8%。干燥中后期,常规干燥速度与降温干燥速度分别提高28.7%和15.5%。同时,预处理枫香木材厚度方向含水率偏差及残余应力指标均低于未处理木材。     

A global rheological model of wood cantilever as applied to wood drying

In the process of wood drying inevitable stresses are induced. This often leads to checking and undesired deformations that may greatly affect the quality of the dried product. The purpose of this study was to propose a new rheological model representation capable to predict the evolution of stresses and deformations in wood cantilever as applied to wood drying. The rheological model considers wood shrinkage, instantaneous stress–strain relationships, time induced creep, and mechano-sorptive creep. The constitutive law is based on an elasto–viscoplastic model that takes into account the moisture content gradient in wood, the effect of external load, and a threshold viscoplastic (permanent) strain which is dependent on stress level and time. The model was implemented into a numerical program that computes stresses and strains of wood cantilever under constant load for various moisture content conditions. The results indicate that linear and nonlinear creep behavior of wood cantilever under various load levels can be simulated using only one Kelvin element model in combination with a threshold-type viscoplastic element. The proposed rheological model was first developed for the identification of model parameters from cantilever creep tests, but it can be easily used to simulate drying stresses of a piece of wood subjected to no external load. It can therefore predict the stress reversal phenomenon, residual stresses and maximum stress through thickness during a typical drying process.     

Moisture-induced stresses in engineered wood flooring with OSB substrate

Engineered wood flooring (EWF) is a multilayer composite flooring product. The cross layered structure is designed to give good dimensional stability to the EWF under changing environmental conditions. However, during winter season in North America, the indoor relative humidity could decrease dramatically and generate an important cupping deformation. The main objective of this study was to characterize the interlaminar stresses (σ ₃₃, σ ₁₃ and σ ₂₃) distribution at free-edges in EWF made with an OSB substrate. A three-dimensional (3D) finite element model was used to predict the cupping deformation and to characterize stresses developed in the EWF. The finite element model is based on an unsteady-state moisture transfer equation, a mechanical equilibrium equation and an elastic constitutive law. The physical and mechanical properties of OSB substrate were experimentally determined as a function of the density and moisture content. The simulated EWF deformations were compared against the laboratory observations. For both simulation and experimental results, the cupping deformation of EWF was induced by varying the ambient relative humidity from 50 to 20% at 20°C. A good agreement has been found between the numerical and experimental EWF cupping deformation. The stress distribution fields generated by the model correspond to the delaminations observed on the OSB substrate in the climate room. Delamination in EWF can occur principally under the action of the tension stress or a combination of tension and shear stresses. Finally, simulated results show that the levels of interlaminar stresses are maximal near the free-edges of EWF strips.     

Evaluation of non-destructive methods of measuring growth stress in Eucalyptus globulus: relationships between strain, wood properties and stress

High levels of growth stress are implicated in causing end splitting of logs, deflection during sawing and deformation of boards as stresses are released during sawing operations. Level of stress is a function of strain and the elastic modulus of the wood (MOE). Levels of peripheral strain can be measured on standing trees and, if the MOE is known, stresses can be estimated. The validity of using peripheral strain measurements relies on underlying theoretical models that relate strain to expected patterns of stress distribution and levels of board deflection. This study evaluates these theoretical relationships by determining relationships of stress and strain with board deflection, end splitting and a range of wood properties.

Peripheral strain levels were extremely variable within the bottom log and little evidence was found for consistent patterns of variation, although measurements generally increased with increasing height above ground. Sampling on two sides of the standing tree at breast height appeared to be a suitable strategy, with the mean for these strain readings having a correlation (r) of 0.86 with the average strain in the bottom log.

Growth strain was not a reliable predictor of board deflection and cannot be recommended as a non-destructive sampling method. Overall there was a poor relationship between growth strain and board deflection. No consistent relationships were found between a range of wood properties and growth strain or board deflection across both sites. Stress levels were calculated for each tree as the product of growth strain and modulus of elasticity and the relationship between calculated stress and mean board deflection determined. No relationship was found at either site with correlations being very close to zero.

The underlying theoretical relationships between stress and strain were examined and several questions raised about the validity of such models.     

Osmotic stress in wood — Part I: The analogy between thermal and swelling stress

Summary The analogy between thermal stress and swelling (drying) stress in wood makes it possible to give mathematical formulae for calculating the drying and swelling stresses in many situations. The models allow for viscoelastic and plastic phenomena.Valuable discussions with Dr. P. U. A. Grossman are gratefully acknowledged.     

Evidence of residual stress in small sections of ordinary green Eucalyptus regnans

Summary Conclusive evidence is offered to show the existence of significant residual elastic stress in the form of alternate layers of tension and compression in relatively small pieces of normal green eucalyptus wood. A method of detecting residual stress in small samples of wood is described, and the stress distribution approximated from deflection measurements following a formula widely used in the science of metallurgy.The authors wish to acknowledge the assistance of Mr. J. W. P. Nicholls in providing the X-ray density scans, and Mr. A. Jones in performing the required wood machining.     

Stress relaxation of chemically treated wood during processes of temperature elevation and decline

In order to clarify the effect of drying on structural changes of DMSO swell treated and DEA-SO2-DMSO decrystallization treated Chinese fir （Cunninghamia lanceolate） wood, the stress relaxation of treated oven-dry specimens during the processes of temperature elevation and reduction and that of treated wet specimens at constant temperature were determined. A stress decrease process and a stress increase process were observed in all stress ratio curves of wood during the processes of decreasing temperature. Untreated wood, during the process of temperature reduction under higher initial temperature conditions and during the process of temperature elevation, has a larger stress decrease than treated woods. In a wet state this trend is reversed. It indicated that the drying set made treated woods have a smaller increase in fluidity of wood constituents with increasing temperature. Some bonding between decrystallization reagents and wood molecules may occur.     

Use of Leicester's “rheological model for mechano-sorptive deflections of beams”

Summary The model for mechano-sorptive deformation proposed by Leicester is used to predict stress relaxation of wood drying under constraint from apparent creep of drying wood under constant load. The predictions are compared with measurements on four pairs of beams, one beam in each pair being tested under constant load, the other at constant deflection. Agreement was sufficiently close to demonstrate the value of the model.     

Wood as a natural smart material

The dominant feature of artificial smart materials is the “shape memory” effect. This phenomenon is based on frozen strains (FS). They were detected in wood fastened specimens during drying in the early 1960s. The integral law of wood deforming under loading and moisture content and/or temperature changes was subsequently formulated. This law takes into account the forming of FS. It was applied for the calculation of wood drying stresses. Stress memory and strain memory effects for wood were discovered. Wood has the ability to recollect the type of loading (tension or compression) which it had undergone. The difference between the free and restrained shrinkage is named “frozen shrinkage” (FSh). In calculations of drying stresses, it is more justified to use the FSh concept than “mechano-sorptive creep” (MSC). The MSC phenomenon is observed at cyclical change of moisture content in loaded wood. “Hygrofatigue” that reduces wood stiffness plays the main role in this process.     

A microwave applicator for on line wood drying: Temperature and moisture distribution in wood

Summary An especially designed open microwave applicator was analysed using wood as the material to be heated and dried. The idea was to develop an on line microwave construction consisting of several small open applicators, each fed by a small standard magnetron (for example 1.4 kW main power). The process was analysed by measuring the wood temperature during heating using an IR-camera and detecting the moisture distribution during drying by CT-scanning. Pine and birch wood samples were used in the experiments, mainly 40 mm in thickness. The experiments show that the power distribution differs between dry wood and moist wood. The analysis of the temperature fields captured by the IR-camera during the first minutes allows a rather accurate determination of the MW power. Consequently, the drying proceeds unevenly in the wood specimens, especially in the longitudinal direction. The dimensions of the applicator and its relation to the wood dimension are very important. However, the wood was not destroyed, the temperature and moisture gradients did not affect the wood in terms of checks or deformations. The drying rate in different positions of the specimen varied between 0.30 and 0.80 percentage moisture content/min. The uneven energy, meaning temperature and field distribution, is to be compensated in the future by a moving wood load and by alternating the position of each applicator in a larger scale microwave pilot plant. Received 25 February 1997