水曲柳顺纹压缩率和PDR的变化规律研究

利用木材弹塑性的特性对经过不同条件水热软化处理后水曲柳幼龄材和成熟材分别进行顺纹压缩,分析顺纹压缩率和PDR之间的关系和顺纹压缩过程中试材弹性恢复的状况,为后续的弯曲试验提供前期基础。结果表明:在沸水软化时间为140～150 min时,试材可以获得较好的顺纹压缩性能;顺纹压缩率和PDR呈正相关;顺纹压缩恢复过程可以分为弹性、弹塑性和塑性3个阶段,弹性阶段瞬时即可恢复,弹塑性阶段较缓慢地恢复,塑性阶段不再恢复,出现永久的塑性变形。     

榆木顺纹压缩率和PDR的变化规律研究

对不同质量分数氢氧化钠碱液软化处理的榆木幼龄材和成熟材试材分别进行顺纹压缩试验,对其顺纹压缩率和永久变形率(PDR)的变化规律以及顺纹压缩后的恢复情况进行分析,试验结果表明:NaOH的质量分数为6%～7%,改性剂的质量分数为10%～15%时,获得最大顺纹压缩率和PDR值;相同顺纹压缩率时,溶液浓度和PDR值呈正相关,且幼龄材的PDR值大于成熟材;顺纹压缩后的恢复曲线分为弹性变形恢复阶段、弹塑性变形恢复阶段和塑性变形阶段;幼龄材的恢复速率小于成熟材。     

复配碱液处理榆木顺纹压缩应力-应变本构关系

采用复配碱液处理榆木后,对榆木的幼龄材和成熟材顺纹压缩,通过建立的榆木顺纹压缩应力-应变本构关系得出:在榆木顺纹压缩初始阶段,符合线形的弹性虎克定律;在弹塑性阶段,也基本符合线形的力学关系。分析榆木幼龄材在顺纹压缩中体现出较大的差异性、顺纹压缩弹性模量和应力、应变的原因是木材化学组分的降解和抽出;得出复配碱液处理后木材顺纹压缩弹性、弹塑性变化的基本规律,木材在弹塑性阶段呈一段平滑的曲线,细胞壁在此阶段形成褶皱。     

大花序桉木材顺纹抗压强度变异研究

以18 a生的11个大花序桉种源木材为研究对象,分析了种源间、单株间、树干高度上的木材顺纹抗压强度的变异情况,结果表明.大花序桉部分种源间、株间及高度间木材顺纹抗压强度均有显著差异,B85号种源的顺纹抗压强度最大,为83.4 MPa;12195号种源的最小,为69.8 MPa,并显著小于B85、17008等6个种源.各种源木材顺纹抗压强度沿树干高度的变化并不相同,有的呈递增趋势,有的呈递减趋势,有的先增后减,有的先减后增.因此,在种源选择基础上,应进一步进行单株选择,以便在更大程度上提高大花序桉木材顺纹抗压强度改良效果.大花序桉种源木材顺纹抗压强度和树高、胸径等生长性状相关性不明显,可相互进行独立改良,因而可望同时获得生长快和木材顺纹抗压强度高的改良成效.     

水热-微波处理对榆木软化和顺纹压缩及弯曲的影响

采用水热-微波处理方法对榆木成熟材进行软化处理,通过顺纹压缩率、单维和多维弯曲最小曲率半径表征木材软化效果;以XRD,FYIR等方法测定木材表面组成、结晶度变化,分析软化处理对木材顺纹压缩的影响.结果表明:水热-微波处理可显著增强木材的软化性能,处理后木材中的抽提物几乎完全抽出(1.73%～0.47%),半纤维素发生明显降解,使木质素相对含量得到增加,木材表面羟基数量显著增加;非结晶区微纤丝趋于有序,相对结晶度提高,结晶区表面微纤丝羟基裸露,氢键键合增强,结晶区宽度增加.当采用处理方案为B时,顺纹压缩率最大,单维和多维弯曲时的曲率半径最小.较高的结晶度和适宜的羟基数量是试样B表现出较高软化性能的主要原因.     

水热-微波软化处理对水曲柳弯曲的影响

采用水热-微波处理方法对水曲柳幼龄材和成熟材进行软化处理,通过顺纹压缩率、单维和多维弯曲最小曲率半径表征木材软化效果。以XRD,FTIR等方法测定木材表面组成、结晶度变化,分析软化处理对木材顺纹压缩和弯曲曲率的影响。结果表明:水热-微波处理可显著增强木材的软化性能,使木材表面羟基数量显著增加,非结晶区微纤丝趋于有序,相对结晶度提高,结晶区表面微纤丝羟基裸露,氢键键合增强,结晶区宽度增加。当水煮处理时间为140～150min、微波处理时间为350s时,试材顺纹压缩率最大,单维和多维弯曲时的曲率半径最小。     

低温环境下桦木顺纹抗压强度的研究

为揭示不同水分状态木材在低温环境下的力学强度变化规律,研究了5种水分状态(饱水、生材、纤维饱和点、气干、绝干)桦木木材在0~-196℃低温环境下的顺纹抗压强度,并与室温环境下(20℃)的5种水分状态木材顺纹抗压强度进行比较。结果表明,在低温(0~-196℃)环境下,随着测试环境温度的降低,木材顺纹抗压强度增加,在-196℃环境下,5种水分状态的桦木顺纹抗压强度比室温环境下分别增加821.24%,718.05%,632.87%,223.75%和95.28%。木材顺纹抗压强度与温度呈线性关系,其斜率代表了顺纹抗压强度随温度变化的增加率,即木材含水率越高,随着低温温度的降低,顺纹抗压强度增速越大。在低温环境下,木材细胞中水分形成的冰柱,是木材顺纹抗压强度增加的主要原因。对绝干材而言,木材细胞壁上纤维和纤维胶着物质发生硬化,是其顺纹抗压强度增加的主要原因。     

榆木顺纹压缩弯曲技术

木材从力学角度上看是一种粘弹性材料 ,从结构上看又是一种多孔材料 ,木材的这个特性 ,可以使其进行简单的弯曲 ,但是要获得较小的弯曲曲率半径 ,首先应强化木材软化机理的研究 ,增大木材的塑性 ;其次研究木材软化处理后的顺纹压缩机理和压缩技术 ,使木材在顺纹压力的作用下 ,细胞壁中微纤丝之间产生滑移 ,导致木材细胞壁的壁层纵向产生褶皱 ,木材在弯曲力矩的作用下 ,弯曲时的受压面形成褶皱 ,受拉面形成展皱 ,在允许的形变范围内获得较小的弯曲曲率半径。本文研究结果表明 ,试件尺寸为 2 80mm× 1 6 5mm× 1 6 6mm时 ,经过水热软化处理后 ,顺纹压缩率达到 2 4 % ,试件回弹后的长度最小为 2 6 1 74mm时 ,弯曲的曲率半径最小为 4 2 5mm ,测试的试件平均值为 5 6 94mm。     

湿地松与马尾松人工林木材物理力学性质的比较研究

通过对湿地松和马尾松人工林木材物理力学性质的测定和比较分析,结果表明:湿地松人工林木材密度、干缩系数和除冲击韧性以外的力学强度均稍大于马尾松人工林木材,而冲击韧性则恰恰相反.经差异显著性t检验表明:湿地松和马尾松木材物理力学性质指标中顺纹抗压强度、抗弯强度、弦面顺纹抗剪强度和冲击韧性差异极显著,气干密度、基本密度、径面顺纹抗剪强度和端面硬度差异显著,除此之外的其余指标差异不显著.     

样品尺寸对竹材顺纹压缩力学性能的影响研究

以毛竹为研究对象,通过对不同尺寸的样品进行顺纹压缩测试,分析样品尺寸对竹材顺纹压缩模量和顺纹抗压强度的影响。结果显示：竹材的顺纹压缩模量与竹壁厚度、样品高度、弧度呈正相关关系,顺纹抗压强度与竹壁厚度、样品高度、弧度呈负相关关系。在弹性变形阶段,样品尺寸的增加会提高维管束的组织比量而增大竹材顺纹压缩模量的计算值,在塑性变形阶段,样品尺寸和弧度会导致提前失稳而降低竹材的顺纹抗压强度。     

Longitudinal shrinkage behaviour of compression wood in radiata pine

The behaviour of longitudinal shrinkage was investigated in the corewood of a swept, 17-year-old New Zealand radiata pine stem. Wood categories in terms of normal wood, mild compression wood and severe compression wood were identified microscopically using autofluorescence of lignin. Average longitudinal shrinkage was collated according to corewood location and wood category within corewood in the leaning and the vertical parts of the stem, and then maximum radial difference of longitudinal shrinkage within growth ring was examined. The results show that the average longitudinal shrinkage is significant (2.4%) in the corewood of the leaning part of the stem. Among wood categories, severer compression wood displays the highest (2.9%) average longitudinal shrinkage. In the context of this study, growth rings may consist of one of three types of wood: (1) only normal wood; (2) a single compression wood type; and (3) mixed-type wood. Where multiple compression woods co-existed with normal wood, the maximum radial difference of longitudinal shrinkage within the growth ring was found to be 4.0%. A strong correlation (R ² = 0.90) between average MFA and average longitudinal shrinkage suggests a significant influence of the average MFA on average longitudinal shrinkage across the three growth ring types.     

Generation process of growth stresses in cell walls: Relation between longitudinal released strain and chemical composition

Summary To advance the discussion on the evolution mechanism of tree growth stresses, the relation between released strain and the chemical components was investigated experimentally. The expansive released strain in the longitudinal direction assumed large values as the lignin content increased in the compression wood, but there was no relation between released strain and lignin content in the normal wood region. The contractive released strain assumed large values as the cellulose content and its crystallinity increased. Their correlation was very high and clear. From these facts, it is considered that the lignin deposition plays an important role in the generation of the growth stresses in compression wood but is not important in normal or tension wood regions. Cellulose microfibrils contract along their longitudinal axis during cell maturation, and the stress included by the contraction creates a longitudinal growth stress in normal and tension woods.The authors thank Prof. R. R. Archer, University of Massachusetts, for his valuable advice. A part of this research was supported by a grant under the Monbusho International Scientific Research Program     

木材弯曲工艺研究综述

木材弯曲制品因优美流畅的曲线造型而备受人们的欢迎, 然而木材弯曲制品的成品率较低。文中对木材弯曲的软化、形变固定工艺和机理及其他影响因素进行总结归纳, 并讨论当前木材弯曲工艺中存在的问题和未来的发展方向。复杂的弯曲工艺、众多的影响因素和对工人较高的操作技能要求是造成木材弯曲制品成品率低下的主要原因, 而纵向预压缩技术因其可在很大程度上解决传统木材弯曲中的问题, 可能成为木材弯曲新的发展方向。     

The generation of longitudinal maturation stress in wood is not dependent on diurnal changes in diameter of trunk

A hypothetical mechanism for the generation of maturation stress in wood was tested experimentally. The hypothesis was that the maturation stress could partly originate in a physical mechanism related to daily changes in water pressure and associated diurnal strains. The matrix of lignin and hemicellulose, deposited in the cell wall during the night, would be put in compression by the effect of water tension during the next day. The cellulose framework, crystallizing during the day, would be put in tension by the decrease in tension at night and subsequent cell-wall swelling. This was tested on young saplings of sugi and beech. Half of the saplings were submitted to continuous lighting, which canceled diurnal strains. Saplings were tilted 40 degrees, and their uprighting movement was measured. The uprighting movement is directly due to the production of reaction wood and the concomitant development of large longitudinal maturation stress. It occurred in the continuously lighted plants at least as much as in control plants. We conclude that the generation of longitudinal maturation stress in tension or compression wood is not directly related to variations in water pressure and diurnal strains.     

木材液体渗透性的改善方法

归纳了影响木材液体渗透性的因素(木材结构、抽提物、微生物、含水率和干燥方法)及其改善方法(化学法、生物法、物理法),着重介绍了压缩法,并提出今后改善液体渗透性的研究方向.     

Variations in physical and mechanical properties between tension and opposite wood from three tropical rainforest species

Growth strains were measured in situ in nine trees of three species from a French Guiana tropical rainforest in a clearly active verticality restoration process. The aim was to detect tension wood within the samples. Wood specimens were cut in the vicinity of the growth strain measurements in order to determine the microfibril angle and some mechanical and physical properties. As suspected, tensile growth strain was much higher in tension wood zones, as shown by the slightly higher longitudinal modulus of elasticity. Conversely, tension wood showed reduced compression strength. Longitudinal shrinkage was much higher in tension wood than in opposite wood. Clear relationships between the microfibril angle and longitudinal properties were noted in comparison (i) with those observed in gymnosperm compression wood and (ii) with expected relationships from the organization of wood fibres cell wall structure.     

Methods for the very early selection of Pinus radiata D. Don. for solid wood products

? Introduction

There has been an increasing interest in very early selection of radiata pine to reduce the breeding cycle for solid wood products. For such selection, new approaches are required to assess wood quality in wood from very young stems.

? Methods

Nursery seedlings of clones of radiata pine were grown in leant condition using two leaning strategies for 18–20 months. Opposite wood and compression wood were isolated from the leaning stems and tested for dynamic modulus of elasticity, density, longitudinal shrinkage, volumetric shrinkage and compression wood area using new methods evolved for testing small size samples quickly and reliably. The methods were tested for their efficiency in differentiating clones by their wood properties.

? Results

Leaning of stems provided distinct opposite and compression wood for testing. Automated image analysis method used for compression wood area assessment was found to be a quick and effective method for processing large number of samples from young stems. Compression wood was characterised by high basic density, high longitudinal shrinkage and low volumetric shrinkage than that of opposite wood. Acoustic velocity in opposite wood had a strong negative association with longitudinal shrinkage. The study signifies the importance of preventing mixing of opposite wood with compression wood while assessing wood quality in young stems thus making leaning a critical strategy. The comparison of wood properties of opposite wood revealed significant differences between clones. Opposite wood of the clone with the lowest dynamic modulus of elasticity exhibited the highest longitudinal shrinkage.

? Conclusion

Significant differences in measurable wood properties between clones suggest the prospects of early selection for solid wood products.     

Failure in timber part II: The angle of shear through the cell wall during longitudinal compression stressing

Summary The angle at which the slip plane traverses the wall of cells subjected to longitudinal compression has been studied in detail and appears to be a function of the angle of the microfibrils in the middle layer of the secondary wall together with the ratio of the modulus of elasticity in the longitudinal and radial planes. These parameters can adequately explain the observed variations in slip plane angle that occurred between species, between early and late wood, and at different temperatures. Significant differences were absent in the comparison of radial and tangential walls, normal and compression wood, and samples at different moisture content.     

Behavior of the cellulose microfibril in shrinking woods

We measured the longitudinal and tangential shrinking processes in wood specimens from Chamaecyparis obtuse Endl. with different microfibril angles (MFAs). The shape of the shrinking curve was compared with the MFA. Only the longitudinal shrinking process of specimens with a small MFA clearly showed nonlinearity, and the degree of nonlinearity increased as the MFA decreased. In contrast, the tangential shrinking process and the longitudinal shrinking process of compression wood with a large MFA were linear. The nonlinearity is probably caused by the longitudinal shrinkage of the noncrystalline region of the cellulose microfibril (CMF) in regions of low moisture content during water desorption. When the moisture content is high, the matrix substance in the cell wall begins to dry; however, the shrinkage in the chain direction is restrained by the rigid CMF. As the wood dries further, the noncrystalline region of the CMF embedded in the matrix substance begins to shrink. Because the longitudinal mechanical behavior of wood with a small MFA is greatly affected by a rigid CMF, longitudinal shrinkage increases suddenly at about 10% moisture content; as a result, the shrinking process shows nonlinearity.     

4种不同类型栓皮栎软木物理力学性能研究

以厚皮深裂、厚皮浅裂、薄皮深裂与薄皮浅裂4种类型栓皮栎(Quercus variabilis)软木为材料,通过测定皮层厚度、密度、含水率、吸湿率、吸水率、硬度、抗压性和压缩回弹性等主要物理力学参数,对软木特性进行比较分析。结果表明,厚皮浅裂型软木的综合性能较优,皮层厚度为0.34 mm;密度为0.26 g/cm^3;含水率为4.91%;吸湿率为3.69%;吸水率为16.24%;硬度为63.42 HA;抗压指数(纵向为3.29 MPa,横向为2.44 MPa);24 h压缩回弹率(纵向为87.50%,横向为87.20%),更适于栓皮栎的培育及深加工利用。