Effects of temperature and moisture content on selected wood mechanical properties involved in the chipping process

The effects of temperature and moisture content on selected mechanical properties associated with the chipping process were evaluated. In chipping, mechanical properties such as shear parallel to the grain, cleavage, and bending are involved. Matched samples of heartwood and sapwood were obtained from freshly harvested logs of black spruce and balsam fir to determine the variation of the studied mechanical properties between ?30 and 20 °C, at intervals of 10 °C. Moisture content (MC), basic density (BD), and annual ring width (RW) were measured for each sample. For both wood species, temperature had a significant effect on all mechanical properties under freezing conditions (below 0 °C). This effect was more important for sapwood than for heartwood, which was explained by the difference in MC between these two types of wood. Between 0 and 20 °C, temperature and type of wood did not show any significant effect on the mechanical properties. Multiple regression models were obtained to predict the mechanical properties. These regressions showed that MC was the most important factor to explain the mechanical properties below 0 °C. However, for temperatures of 0 °C and higher, BD was the principal factor to predict the mechanical properties. RW was not a significant factor to predict any mechanical property. Cleavage was the most sensitive one to changes in temperature followed by shear, modulus of rupture, and modulus of elasticity. These results could be of great importance in the chipping process.     

Stiffness of normal, opposite, and tension poplar wood determined using micro-samples in the three material directions

Mechanical tests on micro-samples were performed in the three material directions in normal, opposite, and tension wood collected from a poplar tree. Two custom micro-devices were designed and built in the laboratory to test samples under pure tension in the transverse direction and under 4-point bending conditions in the longitudinal direction. Both devices were designed to handle samples with a small transverse section (a few square mm), which allowed to select zones with homogenous anatomical features. The results indicate a very high longitudinal stiffness in tension wood (up to 35 GPa compared to an average of 18 GPa for normal wood). Considering wood density, the value represents a specific modulus that is nearly 70 % crystalline cellulose. However, tension wood is slightly less stiff in the tangential and radial directions (1,150 vs. 1,500 MPa for normal wood in the radial direction and 430 vs. 530 MPa in the tangential direction).     

The effect of the G‐layer on the viscoelastic properties of tropical hardwoods

Context and aim

This study aimed to examine the effect of the tension wood G‐layer on the viscoelastic properties of wood.

Methods

Tension wood and opposite wood samples were obtained from six French Guianese tropical rainforest species (Sextonia rubra, Ocotea guyanensis, Inga alba, Tachigali melinoni, Iyranthera sagotiana and Virola michelii); the tension wood of the former three of these species had a G‐layer, whilst the tension wood from the latter three had no G‐layer. Tensile dynamic mechanical analysis (DMA) was performed on green never dried wood samples in the longitudinal direction with samples submerged in a water bath at a temperature (30°C) and frequency (1 Hz) representative of the conditions experienced by wood within a living tree. Then, DMA was repeated with samples conditioned to an air-dried state. Finally, samples were oven-dried to measure longitudinal shrinkage.

Results

Tension wood did not always have a higher longitudinal storage (elastic) modulus than opposite wood from the same tree regardless of the presence or absence of a G‐layer. For the species containing a G‐layer, tension wood had a higher damping coefficient and experienced a greater longitudinal shrinkage upon drying than opposite wood from the same species. No difference was found in damping coefficients between tension wood and opposite wood for the species that had no G‐layer.

Conclusion

It is proposed that the different molecular composition of the G-layer matrix has an influence on the viscoelasticity of wood, even if a biomechanical gain is not yet clear. This study shows that rheological properties and longitudinal shrinkage can be used to detect the presence of a G‐layer in tension wood.

     

针叶树木材细胞力学及纵向弹性模量计算——纵向弹性模量的理论模型

本文是关于针叶树木材弹性特性研究的理论分析。设定针叶树木材细胞主要由管胞和射线组成 ,根据管胞和射线细胞的解剖构造特征 ,建立了两端劈尖、矩形截面、中空的管胞模型与长方体状、中空的射线细胞模型。沿用MP层 (胞间层 初生壁 )内与 3S层 (次生壁的S1、S2、S3层 )内力学性质相同 ,而MP与 3S层间力学性质相异的假定 ,利用管胞和射线细胞模型 ,导出了管胞和射线细胞纵向弹性模量的计算公式。同时 ,根据管胞和射线细胞在针叶树木材中的排列规律 ,结合线形弹性体串并联的特性 ,进一步给出了计算针叶树木材试件宏观纵向弹性模量的方法。本项研究从木材细胞的结构和弹性特性出发 ,分析研究针叶树木材的宏观弹性行为 ,一方面可以获得关于针叶树木材弹性特性的认识 ,另一方面可以用于针叶树木材纵向弹性模量的计算和预测。     

针叶树木材细胞力学及纵向弹性模量的计算--试件纵向弹性模量的预测

依据针叶树木材管胞和射线细胞的结构模型。使用计算机抽样模拟解剖结构参数。以及使用针叶树木材纵向弹性模量计算公式和方法,计算人工林杉木,马尾松幼龄材和成熟材试件纵向弹性模量,计算结果与常温条件下气干试件测定结果十分符合。在试件晚材率和管胞解剖结构参数改变的条件下。计算预测了人工林杉木,马尾松幼龄材和成熟材纵向弹性模量的变化。结果表明：试件纵向弹性模量随晚材率,管胞长度,管胞壁厚度的增加而增加,而试件纵向弹性模量随管胞直径增加而减小。本文提出的纵向弹性模量计算的预测方法,对于运用现代生物技术控制和改变针叶树木材的材质,材性有实际意义。     

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.     

Comparison of physical and mechanical properties of tension and opposite wood from ten tropical rainforest trees from different species

On 10 trees from 10 species of French Guyana tropical rainforest in a clear active process of restoring verticality growth strains were measured in situ in order to determine the occurrence of tension wood within samples. Wood specimens were cut in the vicinity of the growth strains measurements in order to measure some mechanical and physical properties. As suspected, tensile growth strains was very much higher in tension wood zone, because longitudinal modulus of elasticity was slightly higher. Longitudinal shrinkage was also much higher in tension wood than in opposite wood.     

Ring-level models for predicting wood and fibre properties of Abies balsamea

Wood and fibre properties such as wood density, microfibril angle, and modulus of elasticity are industrially relevant factors in determining the mechanical properties of wood. Radial ring-level predictive models of these properties were developed using balsam fir data from a long-term trial in New Brunswick (Canada), where precommercial thinning was applied 8 years after the site was harvested. The mixed effects models developed accounted for most of the variability in wood density (68 %), microfibril angle (94 %), and modulus of elasticity (77 %) with low RMSE. This study shows that balsam fir wood and fibre properties are strongly related to distance from the pith, particularly in the first 20 mm, and much less to annual ring width. Disk height and selected weather variables related to air temperature during the growing season significantly improved the models, whereas the effect of precommercial thinning was not significant. These equations can be incorporated into computer models, such as Optitek, that simulate mill recovery and wood properties to obtain accurate information on wood products. The unexplained variation in these models is likely related in part to between-tree genetic variation, which is unknown in this study.     

Wood properties and their among-family variations in 10 open-pollinated families of Picea jezoensis

This study aimed to evaluate radial and among-family variations of wood properties in Picea jezoensis. A total of 174 trees were randomly selected from 10 open-pollinated families in a progeny trial for measuring stem diameter, dynamic Young’s modulus of log (DMOE_log), annual ring width (ARW), air-dry density (AD), modulus of elasticity (MOE), and modulus of rupture (MOR). Mean values of DMOE_log, AD, MOE, and MOR were 9.60 GPa, 0.41 g/cm³, 9.44 GPa, and 76.6 MPa, respectively. Significant differences among families were observed in all properties. F values obtained by analyzing variance in wood properties were higher than those generally observed in growth traits. In addition, F values in wood properties remained relatively higher from the 1st to 25th annual ring from the pith, although F value in ARW rapidly decreased with each increase in annual ring number. These results indicate that genetic factors largely contributed to the variance in wood properties compared with the growth traits.     

Radial variations of wood properties of an endangered species, <Emphasis Type="Italic">Pinus armandii</Emphasis> var. <Emphasis Type="Italic">amamiana</Emphasis>

A dead tree of Pinus armandii Franch. var. amamiana (Koidz.) Hatusima (abbreviated to PAAm) was obtained from a natural habitat on Tanega-shima Island and various properties of its wood were investigated. Grain angle was measured and soft X-ray analysis was undertaken to obtain the density in each annual ring. Unit shrinkage and dynamic properties were measured by shrinkage, bending, and compression tests. Variations of wood properties in the radial direction, relationships of wood properties to density, and annual ring width were examined. Roughly speaking, variations in the radial direction of the grain angle, twist angle by drying, Young’s modulus and strength in static bending, absorbed energy in impact bending, compressive Young’s modulus, compressive strength, and compressive proportional limit corresponded to the variation of annual ring width. As a result, it was determined that if PAAm is afforested artificially for the purposes of lumber production and conservation, the annual rings of logs should not be too widely spaced. Wood properties of PAAm were similar to those of Japanese black pine (Pinus thunbergii Parl.), which is another representative pine on Tanegashima Island. This study was presented in part at the 56th Annual Meeting of the Japan Wood Research Society, Hiroshima, August 2007     

Indented rings (hazel growth) of Norway spruce reduce anisotropy of mechanical properties

This study presents three-point bending test results of Norway spruce clear wood specimens loaded on the radial-longitudinal plane in two different load cases. The tested samples were graded as resonance wood for instrument making and were characterised by narrow annual rings and relatively low density. The modulus of elasticity (MOE) and the corresponding modulus of rupture (MOR) are illustrated separately for the samples with straight grain and the group showing the specific growth pattern of indented rings (‘hazel growth’). With the longitudinal wood anatomical direction parallel to span width, the fibre deviation caused by the indents reduces MOE and MOR values, whereas a ‘reinforcing’ effect of the indents could be observed for the load case with span width parallel to the radial direction. Both aspects lead to a reduction in anisotropy for hazel-growth Norway spruce (anisotropy MOE: indented rings 11.6, straight grain 14.7, anisotropy MOR: indented rings 6.9, straight grain 8.9), which partly explains the exceptional position of this growth pattern for the construction of high-class musical instruments with outstanding mechanical and acoustical performance.     

Longitudinal hardness and Young's modulus of spruce tracheid secondary walls using nanoindentation technique

Summary Using a mechanical properties microprobe, measurements of hardness and elastic modulus of tracheid walls in the longitudinal direction of spruce wood were obtained by continuously measuring force and displacement as a diamond indenter impressed a cell wall. Maximum mechanical properties were found at the edges of the walls of angular shaped tracheids. Both the hardness and elastic modulus of latewood cell walls were higher than cell walls in the earlywood. The high spatial resolution of this new concept of mechanical testing allows a direct comparison with ultrastructural and microchemical parameters of lignified cells which opens a wider area of applications for the understanding of intrinsic wood properties.This work was conducted while the senior author was a Visiting Scientist at the Oak Ridge National Lab, Oak Ridge, TN 37831, USA partly with joint fundings from the Austrian Science Foundation (Schrödinger scholarship J799-BIO)     

Effect of fatigue and annual rings’ orientation on mechanical properties of wood under cross-grain uniaxial compression

The mechanics of fresh wood with and without a fatigue pre-treatment that mimics a mechanical pulping process was experimentally studied. The mechanical properties of Norway spruce samples under compression are considered with the macroscopic stress–strain data and from local strain properties via digital image correlation technique. The results highlight the effects of the orientation of the wood annual rings compared to the loading direction and of the pre-fatigue. The wood presents a low yield point when the annual rings are tilted compared to the load axis, but the Young’s modulus and yield stress are higher when the annual rings are either parallel or perpendicular to the load direction. In the last case, buckling of softest layers occurs. The fatigue treatment makes the wood less stiff as deduced from the decreases of Young’s modulus and yield stress, whatever the orientation of annual rings. Secondly, it creates a thin and localized softened layer.     

毛竹竹材物理力学性能研究

为了解不同竹龄毛竹生材含水率、线性干缩率、气干密度、抗弯强度、抗弯弹性模量和顺纹抗压强度等物理性能,对其加工应用的影响,笔者以2-7年生毛竹为材料进行研究,结果表明：竹材的生材含水率、气干干缩率（弦向、径向、纵向）和全干缩率（弦向、径向、纵向）随着竹龄的增加呈减小的趋势;从基部到梢部竹材的生材含水率、线性干缩率均减小;竹材线性干缩率弦向＞径向＞纵向.竹材气干密度、抗弯强度、抗弯弹性模量和顺纹抗压强度均随着竹龄的增加呈增大的趋势,尤其是3年生竹材的这些物理力学性能与2年生差异显著,但3年后生竹材差异不大;从基部到梢部竹材的气干密度、抗弯强度、抗弯弹性模量和顺纹抗压强度逐渐增加.综合考虑毛竹的物理力学性能和竹林的经济效益,适合采伐的是3年后生竹材,锯截之后的竹材也应根据部位不同进行区分,以便于加工应用过程中合理利用,提高产品的理化性能和质量的稳定性.     

Genetic variation analysis and comprehensive evaluation of wood property traits of 20-year-old Chinese fir clone

This work focuses on the genetic variation of Chinese fir clones in the radial direction. Four 20-year-old Chinese fir clones were selected in Kaihua Forest Farm, Zhejiang, China, and the differences and genetic characteristics were studied. Clonal repeatability, genotype coefficient of variable, and phenotypical coefficient of variable were calculated to light on the genetic variation on the radial direction. Results showed significant differences (P?<?0.05) between the clones and annual rings of the same clone. The clonal repeatability of the ten wood properties is ranged from 0.5 to 0.8, wherein the fiber length, elastic modulus, and total dry density are higher than 0.7. For all the measured traits, the phenotypic variation of clones is greater than that of the genetic variation. Overall, the measured wood traits, especially the mechanical and physical properties, are strongly controlled by heredity, which means they are potentially helpful for genetic improvement of wood properties and optimal breeding. Furthermore, the clonal repeatability of the measured traits varies between annual rings, and the physical and mechanical properties are relatively stable. The principal component analysis results reveal that the clone of Kai 13 has the highest score.

     

The influences of boiling and drying treatments on the behaviors of tension wood with gelatinous layers in <Emphasis Type="Italic">Zelkova serrata</Emphasis>

This study examined how boiling and drying treatments influenced various physical properties of the tension wood with gelatinous fibers (G-fibers) of a 29-yearold Zelkova branch. By boiling treatment, tension wood with numerous G-fibers contracted considerably in the longitudinal direction and the longitudinal Young’s modulus decreased in spite of the water-saturated condition. The drying treatment caused green tension wood and boiled tension wood with numerous G-fibers to shrink longitudinally and increased their longitudinal Young’s moduli. These specific behaviors in tension wood were highly correlated with the proportion of G-fibers in a specimen and were probably caused by the microscopic behavior of cellulose microfibril (CMF) in the gelatinous layers (G-layers). The longitudinal shrinkage of tension wood due to drying suggests the existence of a hygro-sensible, noncrystalline region in the CMF, which is abundant in the G-layer. Furthermore, the noncrystalline region in the CMF softens during boiling treatment, resulting in the reduction of the longitudinal Young’s modulus in tension wood. The longitudinal contraction of tension wood with G-fibers by boiling might be caused by the tensile growth stress remaining in green G-layers. However, no changes were detected in the 004 d-spacing of cellulose crystal in tension wood from the boiling and drying treatments, regardless of the proportion of G-fibers.     

Microscopic tensile tests in the transverse plane of earlywood and latewood parts of spruce

This paper presents a method which makes it possible to measure elastic properties of a small group of tracheids in the transverse plane. The method is based on tensile tests under microscope that are performed with the assistance of an image analysis system. The calculation of the strain field is based on a global comparison of the grey levels between each deformed image and the initial image. All tests were carried out within one annual ring of spruce: • radial and tangential Young's modulus and Poisson's ratio can be measured in earlywood with a good accuracy, • radial Young's modulus and Poisson's ratio of tracheids in latewood can be estimated with good confidence, • two tests of very thin samples allowed the evaluation of the tangential elastic modulus in latewood. The small size of the sample together with the local measurement of the strain field permitted us to perform several measurements along one single annual ring. Consequently, it was possible to reveal a good relationship between the mechanical properties and the local density determined by microdensitometry. Received 27 October 1997     

Comparison of mechanical properties of tension and opposite wood in <Emphasis Type="Italic">Populus</Emphasis>

In this paper we focused on the differences of mechanical properties of tension and normal wood of 1-year-old poplar trees, artificially tilted. Elastic and fracture properties have been measured and linked to the anatomy. Tension wood is well known because it prevents good surface finishing and leads to difficulties with sawing. We studied three main mechanical properties: young modulus, energy of cutting and longitudinal residual strain of maturation (with strain gauges) because of their importance in wood technology. Moreover, this work takes place in a larger project of study, the phenomena of axes re-orientation in trees (allowing by the production of reaction wood), where these data are required for biomechanical modelling. The results show that tension wood has a higher young modulus, needs a higher energy to be cut and exhibited a higher level of longitudinal residual strain of maturation than those of normal wood. The results suggest that these differences require deeper analysis of the wood than anatomy: measurement of microfibril orientation in the S2 layer and also the lignin composition in monomeric units.     

Mechanical and ecophysiological significance of the form of a young Acer rufinerve tree: vertical gradient in branch mechanical properties

Most tree biomechanics models assume uniformity of mechanical properties within a tree, and only a few studies have focused on differences in mechanical status among branches. We examined mechanical properties of 49 branches of two 10-year-old trees of Acer rufinerve Sieb. et Zucc. For each branch, bending moment due to its own fresh mass, elastic modulus, section modulus and flexural stiffness were obtained. Elastic modulus of the branch was correlated with the density and thickness of the fiber cell wall and decreased with crown depth, indicating that branches at lower positions were more elastic than branches at upper positions. Compared to lower branches, upper branches were less inclined, possessed thicker growth rings, more long shoots and were subject to smaller stresses. The leaf arrangement in the upper branches might be effective in transmitting more light to the lower branches. In contrast, the lower branches were more inclined toward the horizontal and subject to greater stresses than the upper branches. Lower branch inclinations were attributed to smaller dry matter investment in diameter growth. Upper and lower branch inclinations were slightly greater and smaller, respectively, than those predicted by beam theory. The alleviation in inclination of the lower branches is probably caused by negative gravitropic responses such as tension wood formation or upward shoot elongation, or both. The horizontal display of leaves in the lower branches would be effective in light interception. The reduction in cost of the lower branches can be adaptive because they have a shorter life expectancy than the upper branches. The results showed that an adaptive tree form is realized by a vertical gradient in branch mechanical properties.     

Influence of heating history on dynamic viscoelastic properties and dimensions of dry wood

To obtain new information about the mechanical and physical properties of dry wood in unstable states, the influence of heating history on viscoelastic properties and dimensional changes of dry wood in the radial, tangential, and longitudinal directions was studied between 100° and 200°C. Unstable states of dry wood still existed after heating at 105°C for 30 min and were modified by activated molecular motion in the first heating process to temperatures above 105°C. This phenomenon is thought to be caused by the unstable states reappearing after wetting and drying again. Dry wood components did not completely approach the stable state in the temperature range tested, because they did not entirely surpass the glass transition temperatures in most of the temperature range. In constant temperature processes at 135° and 165°C, E′ increased and E″ decreased with time regardless of the direction. This indicated that the unstable states of dry wood components were gradually modified with time at constant temperatures. On the other hand, anisotropy of dimensional change existed and dimension increased in the longitudinal direction, was unchanged in the radial direction, and decreased in the tangential direction with time at constant temperatures. Part of this report was presented at the 13th Annual Meeting of the Chubu Branch of the Japan Wood Research Society, Shizuoka, August 2003