木材拉压一体蠕变自动监测系统的研发

为解决变温变湿状态下木材拉伸和压缩蠕变试验过程中普遍存在的试验环境难以控制、蠕变变形量难以测量、数据采集烦琐、设备长期运行稳定性较差等问题,依据木材拉伸及压缩力学性能的测试基本原理和相关国内外标准研制了 一套木材拉伸及压缩一体化蠕变自动监测系统.系统主要由软件系统和硬件系统两个部分组成,软件系统基于VB语言和查表算法进...     

木材多尺度结构的干缩湿胀研究进展

木材干缩湿胀是影响木材加工利用的关键特性，与木制品尺寸和形状的稳定性密切相关。木材宏观到微观层次分明的多尺度分级结构导致木材干缩湿胀行为的复杂性。将木材干缩湿胀研究从宏观尺度引向介观和微观尺度，建立三者之间的有机联系，有助于阐明木材干缩湿胀行为的发生机制和演变机理，对深入认识木材性质，合理、高效利用木材具有重要理论价值和实践意义。本研究综述木材宏观尺度（无疵试样）、介观尺度（生长轮）和微观尺度（木材细胞）3种层级单元的干缩湿胀行为及其滞后现象，分析木材不同尺度结构的干缩湿胀行为发生和演变规律，介绍基于多尺度结构的木材干缩湿胀测试方法和技术手段。同时，对今后开展木材多尺度结构干缩湿胀行为研究提出建议：1）在介观和微观尺度上系统开展针叶材和阔叶材基于生长轮和木材细胞的干缩湿胀行为研究，建立木材多尺度结构与干缩湿胀行为之间的构效关系；2）揭示基于各向异性、非均质性和细胞类型多样性引发的干缩湿胀行为差异化响应规律及其互作机制；3）阐明木材干缩湿胀行为的时间滞后和湿胀滞后现象；4）引入相关理论模型，如有限元法的应用，构建木材多尺度结构的自由缩胀可视化有限元模型。     

循环解吸-吸湿处理对尾巨桉胀缩性的影响

采用人工模拟大气湿度变化环境,对尾巨桉木材进行循环解吸-吸湿处理,并测定木材的弦向胀缩性.结果表明:在解吸过程中,初含水率对尾巨桉木材弦向干缩率影响显著;在吸湿过程中,循环处理次数对尾巨桉木材弦向湿胀率影响显著.循环处理可以使尾巨桉木材弦向胀缩率达到稳定的时间缩短,有利于其尺寸稳定.     

木质材料变参数流变模型的研究

如何建立木材在变载荷下蠕变的数学模型，是木材流变学在理论研究和生产应用中迄今尚未解决的问题。本文突破传统的常参数建模方法，提出一种变参数流变模型的概念，并以两单元的变参数Maxwell模型拟合了杨木的蠕变性质。结果表明：采用变参数流变模型不仅能以较少的弹、粘单元组合代替常参数的多个弹、粘单元组合模型，使流变参数便于求解，且对不同形式变载荷下的蠕变变形，均有很高的拟合精度。     

杨木高温干燥过程中表层流变特性的研究

本文讨论了干燥温度分别为８５℃、１０５℃、１１５℃和１２５℃时板材表层厚度为３ｍｍ处的木材横纹流变特性。结果表明：干燥过程中的总应变为干缩应变、弹性应变、粘弹性蠕变应变和机械吸附蠕变应变的代数和。木材在长时间、高温高湿的联合作用下，不仅表现出弹性、塑性，而且还表现出粘弹性。作者根据木材干燥应力模型理论，提出了在同一块板方材上能同时测定所建模型各应变参数的试验方法。     

加筋膨胀土挡墙蠕变试验研究

通过模拟试验研究了加筋膨胀土挡墙的蠕变特性,考虑了含水量对膨胀土的特殊作用.通过对试验结果进行分析后认为加筋膨胀土挡墙在含水量和温度升高时其蠕变变形会变大;膨胀土的湿胀干缩特性使加筋土中筋材并非一直处于受拉状态,可以减缓筋材的蠕变性质;在较低的极限荷载比例下,膨胀土体中含水量的变化对蠕变起主要作用,在较高的极限荷载比例下,加筋膨胀土的恒载对蠕变起主要作用.     

白蜡木、黑胡桃木、橄榄木的干缩湿胀特性研究

实木制品的干缩湿胀性直接影响其尺寸稳定性,对白蜡木、黑胡桃木和橄榄木的干缩湿胀特性研究结果表明：三种木材全干状态下的体积干缩率分别为13.88%、10.85%、9.89%,气干状态下的体积干缩率分别为4.89%、4.71%、4.14%,气干至饱水状态下的体积湿胀率分别为18.28%、12.82%、11.54%,全干至气干状态下的体积湿胀率分别为5.94%、4.20%、3.96%。三种木材中白蜡木的干缩湿胀性最强,黑胡桃木次之,橄榄木最小。研究结果对实木制品用材含水率的控制具有一定的指导意义。     

木材普通蠕变和机械吸湿蠕变研究概述

木材是一种具有不同细胞类型的天然纤维复合材料,其细胞壁的壁层结构与化学组分具有多样性。在静态恒定应力作用下,木材形变随时间延长而逐渐增大,即产生蠕变。木材蠕变是影响木制品和工程构件质量以及结构设计安全性的一个关键特性。根据木材中的水分状态,木材蠕变可以划分为"含水率平衡态时的普通蠕变"和"含水率非平衡态时的机械吸湿蠕变"。本文分别阐述了木材普通蠕变和木材机械吸湿蠕变的现象和特点;围绕木材的普通蠕变,归纳了木材组织结构、含水率、温度和应力水平等因素的影响;针对木材的机械吸湿蠕变,重点从分子水平、细胞壁层结构、物理老化等方面解释其发生机制,并论述微纤丝角、化学组分、含水率和温度等因素对机械吸湿蠕变的影响。此外,本文还总结了应力/应变本构方程和Kelvin-Voigt模型模拟木材蠕变的研究进展,分别概述了普通蠕变和机械吸湿蠕变的数值模拟过程及其应用。木材蠕变特性受其自身复杂的组织构造和环境条件的影响,且各因素之间又存在一定程度的交互作用,使得木材蠕变的研究仍有较多方面尚未涉及。建议今后的研究重点从以下4个方面展开:1)针对木材正交各向异性的构造特点,采用不同的载荷类型和形变模式进行组合测试,系统表征木材的正交异向蠕变特性;2)解明湿热条件下,水分、温度以及二者的交互作用对木材组分以及蠕变行为的影响,探索温湿度场中蠕变柔量与泊松比的同步实时测定;3)明确单根纤维(管胞、木纤维细胞)的机械吸湿蠕变行为规律,并确定木材组分,尤其是半纤维素在木材机械吸湿蠕变中的作用机制,从分子水平上解释氢键作用对木材机械吸湿蠕变的影响机制;4)构建能够充分解释木材蠕变特性内在机制和外在影响的蠕变模型。     

控制锯材变形的方法

针对影响锯材变形的因素。从降低原木残余生长应力、对干缩湿胀和干缩应力的干燥工艺处理，到下锯法的合理选择，以及通过木材改性达到尺寸稳定等几方面进行了全面探讨，为提高锯材和木材品产品质量供参考。     

进口辐射松木材主要物理性能径向变异研究

为了提高进口辐射松木材加工利用效率,扩展其使用范围,参照国家标准对新西兰辐射松木材的气干密度、干缩性、湿胀性径向变异进行了研究。结果表明:辐射松木材近树皮、中部和近髓心的气干密度分别为0.502,0.480和0.452 g/cm~3,自树皮至髓心气干密度呈现出逐渐减小的趋势,并且在0.05水平达到了显著差异,平均气干密度为0.478 g/cm~3,属于低密度木材;辐射松木材在气干状态下,其径向、弦向及体积干缩率自近树皮至近髓心呈现出逐渐减小的趋势,并且弦向和径向干缩率在0.05水平上达到了显著差异,体积干缩率在0.01水平上达到了极显著差异;辐射松木材在全干至气干状态下,其径向、弦向及体积湿胀率自近树皮至近髓心呈现出逐渐减小的趋势,并且径向湿胀率在0.05水平达到了显著差异,弦向和体积湿胀率在0.01水平达到了极显著差异。在同一环境条件下,辐射松径向的干缩率和湿胀率均小于弦向。研究结果为进口辐射松木材的加工利用及径向不同部位木材尺寸稳定性能评价提供了重要的数据支撑。     

Cantilever experimental setup for rheological parameter identification in relation to wood drying

Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed by authors for rheological parameter identification.     

Effect of moisture content on the longitudinal tensile creep behavior of wood

In our previous report, we investigated the effect of the microfibril angle (MFA) in the middle layer of the secondary wall (S₂) on the longitudinal creep behavior of a thin homogeneous earlywood specimen sugi. In the present study, we investigated the role of moisture on the tensile creep behavior of wood. We discuss the creep behavior of the wood cell wall from the viewpoint of the composite structure of the cell wall and the properties of the constituent materials. A microtomed thin specimen of earlywood of sugi (Cryptomeria japonica D.Don) was used for the longitudinal tensile creep test. Creep tests were conducted at three moisture stages (oven-dry, air-dry, fiber saturation point) over a broad range of MFA. Results showed that the longitudinal tensile creep behavior was highly dependent on both the moisture content and the MFA. With a small MFA, the variation in the creep function among the three moisture states was very small. For a large MFA, the variation in the creep function was larger. At low moisture contents, the magnitude of the creep function was very small, while at high moisture content, it was very large except for the case of specimens with very small MFA. Those results show that the longitudinal tensile creep behavior was directly affected by the fine composite structure and the internal properties of the cell wall constituents.     

木材蠕变模拟研究概述

综述树种和材性变异、应力水平、温度、含水率等因素对木材蠕变的影响,介绍分析含水率稳定和变化条件下木材蠕变模拟预测等方面的研究进展情况.我国木材蠕变特别是木材的机械吸附蠕变研究尚缺乏系统性,应综合考虑不同的影响因素对木材蠕变进行充分研究,以建立不同条件下木材蠕变预测的模型,为国内木材蠕变研究及木结构建筑的安全和可靠性设计提供基础.     

Effect of microfibril angle on the longitudinal tensile creep behavior of wood

In this report, we undertook studies of the viscoelastic properties of wood from the viewpoint of the fine structure and properties of the constituent materials in the wood cell wall. To measure the mechanical properties of the wood as the behavior of the cell wall, it is required to perform the longitudinal tensile test using a homogeneous specimen. In this study, microtomed specimens of sugi (Cryptomeria japonica D.Don) earlywood were used for the creep test, which were conducted at the fiber saturation point. The substantial creep compliance of the cell wall was simulated using a simplified viscoelastic model consisting of a Voigt element and an independent spring in series. Based on the experimental results, the values of the parameters were optimized. The results were as follows: (1) the longitudinal tensile creep deformation tends to increase with the elapsed time, similar to the bending creep behavior; (2) the magnitude of the longitudinal creep function increases with MFA; and (3) each parameter in the simplified viscoelastic model is markedly affected by the MFA. Based on these results, the mechanism of the longitudinal tensile creep deformation of wood is discussed.     

Moisture content and strain relation in wood by Bragg grating sensor and unilateral NMR

The paper describes a low-invasive experimental apparatus designed for the accurate determination of mechanical deformation and moisture content (MC) relationship on macroscopic wood samples. The device is particularly indicated for monitoring wooden handwork whose mechanical deformation is especially critical in relation to its role such as, for example, works of art or architectural works. The MC of wood is measured with a portable single-sided NMR probe and mechanical deformation by a fiber Bragg grating optical sensor. The data obtained are of high accuracy, despite the dimensions of the sample. The methodology provides an effective tool for investigating the dynamic relation between environmental relative humidity, MC, and shrinking–swelling of wood. Adsorption results collected for longitudinal deformation in silver fir (Abies alba Mill.) are presented to show the sensitivity of the optical sensor. Interesting findings include the detection of two different mechanisms of elongation and the time evolution of water mobility versus hydration and strain.     

木结构用胶黏剂蠕变性能的评价

以红松、落叶松、杉木和桦木为试验材料,采用苯酚-间苯二酚-甲醛树脂胶(RPF)和酚醛树脂胶(PF)进行胶合,使用专用卡具,在长期、恒定的荷栽作用下,研究胶合试件在各种使用条件下的蠕变性能.结果表明:RPF树脂的抗蠕变性能优于PF树脂,落叶松、红松试件的抗蠕变性能优于桦木和杉木.     

Deformation kinetics based rheological model for the time-dependent and moisture induced deformation of wood

Summary A new general rheological model for the calculation of the creep of wood is presented. The flow equation derived in the theory of molecular deformation kinetics is adjusted to account for creep flow, moisture content change induced swelling/shrinkage and their combined effect by making an assumption that both of these processes activate the same bond breaking and reforming process. The rheological model is built by making the dashpots in a generalised Maxwell material model to obey the adjusted flow equation and by placing an additional swelling/shrinkage component to each parallel Maxwell element. Two calculated examples of the performance of the model with comparisons to experiment results are given. It seems that an explanation for the mechano-sorptive effect can be found at the coupling of the creep deformation process and moisture swelling/shrinkage and the non-linearity of the phenomena.This work has been financed by the Academy of Finland and VTT (Technical Research Centre of Finland)     

刨花板厚度方向变形研究Ⅱ.刨花板厚度方向变形模型的建立及释因

本文基于我们以往所做的众多的实验事实 ,从刨花板厚度方向的结构出发 ,全面考虑影响刨花板厚度膨胀率的因素 ,经过论证和筛选 ,建立了刨花板厚度膨胀率的方程模型 :TS(t) =F(t) f(M .C ,t) V(t) ,即刨花板的厚度膨胀率主要取决于人造板内木材的粘弹变形恢复、人造板内木材的吸湿膨胀和胶接点破坏引起的变形 ;当刨花板的含水率存在较大变化时 ,需再加上机械吸附蠕变项A(M .C ,t)     

Creep of Chinese fir wood treated by different reagents

In order to investigate the effect of different reagents on changes of the crystalline region and amorphous region (Matrix) in wood cell walls, the creep behavior of Chinese fir (Cunninghamia lanceolata) wood treated with dimethyl sulfoxide(DMSO) and diethyl amine, sulfur dioxide and dimethyl sulfoxide mixture (1)EA-SO2-DMSO), and the untreated wood at oven-dried, air-dry and water-saturated states during adsorption and desorption processes were all examined in air or in water. The measurements were carded out at ambient temperature and atmospheric pressure. The load is constant with 62 g or 0.607 6 N. The results obtained were as follows: 1) The instantaneous compliance J0 and the creep compliance J of specimens decrystallized with DEA-SO2-DMSO solution were bigger than those of DMSO swollen wood, and the latter was still much bigger than those of untreated wood. 2) For untreated wood, J0 and J increased with equilibrium moisture content (EMC) of wood, but there was not apparent correlation between wood EMC and the relative compliance. 3) Specimens treated with DMSO and DEA-SO2-DMSO mixture were recrystallized after immersion in water, and the degree ofrecrystallization of the former was larger. 4) For oven-dried specimens, the creep compliances in water were bigger than those in air. But for fiber-saturated and water-saturated specimens they were nearly equivalent to each other.