Influence of accessory substances,wood density and interlocked grain on the compressive properties of hardwoods

Wood samples of nine tropical hardwoods from Peru and sugar maple wood from Quebec were selected to perform moisture sorption tests associated with parallel-to-grain and tangential compression tests using a multiple step procedure at 25°C. Cold-water and hot-water extractives, sequential cyclohexane (CYC), acetone (ACE) and methanol (MET) extracts, ash content (ASH), wood density and interlocked grain (IG) were evaluated on matched samples too. Wood density corrected for the accessory substances was by far the major factor positively affecting the compressive properties of tropical hardwoods. The total amount of accessory substances is required in order to establish better relationships between physico–mechanical properties and density of tropical hardwoods. For a given wood density, the ultimate stress in parallel-to-grain compression was higher in tropical hardwoods than in temperate hardwoods. However, the compliance coefficients for both types of woods were quite similar. Sequential extraction with organic solvents was the most suitable method for evaluating the effect of extractives on compressive properties of tropical hardwoods. The CYC and ACE fractions did not contribute to variation in these mechanical properties. The substances dissolved in MET affected positively the compliance coefficient s ₁₁ in parallel-to-grain compression and negatively the compliance coefficient s ₃₃ in tangential compression. The IG decreased the compliance coefficient s ₁₁ but also decreased the ultimate stress in parallel-to-grain compression. Finally, variations in compressive properties that were due to changes in equilibrium moisture content (EMC) were clearly influenced by wood density; denser woods were more sensitive to changes in EMC than lighter woods.     

Anisotropy of wood vibrational properties: dependence on grain angle and review of literature data

The anisotropy of vibrational properties influences the acoustic behaviour of wooden pieces and their dependence on grain angle (GA). As most pieces of wood include some GA, either for technological reasons or due to grain deviations inside trunks, predicting its repercussions would be useful. This paper aims at evaluating the variability in the anisotropy of wood vibrational properties and analysing resulting trends as a function of orientation. GA dependence is described by a model based on transformation formulas applied to complex compliances, and literature data on anisotropic vibrational properties are reviewed. Ranges of variability, as well as representative sets of viscoelastic anisotropic parameters, are defined for mean hardwoods and softwoods and for contrasted wood types. GA-dependence calculations are in close agreement with published experimental results and allow comparing the sensitivity of different woods to GA. Calculated trends in damping coefficient (tanδ) and in specific modulus of elasticity (E′/ρ) allow reconstructing the general tanδ-E′/ρ statistical relationships previously reported. Trends for woods with different mechanical parameters merge into a single curve if anisotropic ratios (both elastic and of damping) are correlated between them, and with axial properties, as is indicated by the collected data. On the other hand, varying damping coefficient independently results in parallel curves, which coincide with observations on chemically modified woods, either “artificially”, or by natural extractives.     

Effects of extraneous substances, wood density and interlocked grain on fiber saturation point of hardwoods

Samples of nine tropical hardwoods from Peru and sugar maple wood from Quebec were selected for moisture sorption and swelling tests at 25°C. These tests evaluated the fiber saturation point (FSP) by two methods: following adsorption over distilled water, and from the volumetric swelling intersection point. Cold-water and hot-water extractives, sequential cyclohexane, acetone and methanol extracts, ash content, wood density and interlocked grain were also determined on matched samples. The results indicated that adsorption tests over distilled water were not applicable for determining FSP in all wood species. Condensation of water vapor apparently occurred, even though temperature during adsorption was controlled to the nearest 0.01°C. The volumetric swelling intersection point method was judged more appropriate. FSP ranged from 15 to 25% for tropical hardwoods and was 30% for sugar maple wood. FSP was negatively correlated with wood density, acetone extracted fraction, interlocked grain and ash content. These parameters each exerted similar effects on variability in FSP.     

Effects of extraneous substances,wood density and interlocked grain on fiber saturation point of hardwoods

Abstract

Samples of nine tropical hardwoods from Peru and sugar maple wood from Quebec were selected for moisture sorption and swelling tests at 25°C. These tests evaluated the fiber saturation point (FSP) by two methods: following adsorption over distilled water, and from the volumetric swelling intersection point. Cold-water and hot-water extractives, sequential cyclohexane, acetone and methanol extracts, ash content, wood density and interlocked grain were also determined on matched samples. The results indicated that adsorption tests over distilled water were not applicable for determining FSP in all wood species. Condensation of water vapor apparently occurred, even though temperature during adsorption was controlled to the nearest 0.01°C. The volumetric swelling intersection point method was judged more appropriate. FSP ranged from 15 to 25% for tropical hardwoods and was 30% for sugar maple wood. FSP was negatively correlated with wood density, acetone extracted fraction, interlocked grain and ash content. These parameters each exerted similar effects on variability in FSP.     

Effect of shear deflection on vibrational properties of compressed wood

In this paper, we have investigated vibrational properties of compressed Japanese cedar (Cryptomeria japonica D. Don). The test specimens were compressed in the radial direction at 180°C for 5 h. Compression ratios (the ratio of deformation to the initial thickness) were 33% and 67%, and the vibrational properties were measured by free-free flexural vibration test. The contribution of shear deflection was large when the length-to-depth ratio was small and the Youngs modulus to shear modulus ratio was large. The Youngs to shear modulus ratio increased as the compression ratio increased and was larger under vibration in the radial than in the tangential direction. The loss tangent increased when the contribution of shear deflection to total measured flexural deflection increased.     

Influence of moisture content on the vibrational properties of hematoxylin-impregnated wood

The vibrational property of hematoxylinimpregnated wood was investigated from the aspect of moisture content dependence. The specific dynamic Young's modulus (E/) and loss tangent (tan) of hematoxylin-impregnated wood were determined in the relative humidity (RH) range of 0%–97%, and were compared with those of the untreated and some conventional chemically treated woods. The changes in theE/ and tan of wood with increasing RH were suppressed by acetylation and formaldehyde treatment because of a marked reduction in the hygroscopicity of the wood. Although the hematoxylin impregnation did not significantly affect the hygroscopicity of the wood, its influence onE/ and tan were similar to that of formaldehyde treatment at low RH and of acetylation at medium RH. It was supposed that at low to medium RH hematoxylin restrains the molecular motion of amorphous substances in the cell wall because of its bulkiness and rigidity. On the other hand, at high RH it seems to work as a plasticizer with adsorbed water molecules.     

Real-time measurement of vibrational properties and fine structural properties of wood at high temperature

Vibrational properties and fine structural properties of wood were measured at high temperatures. Sitka spruce (Picea sitchensis Carr.) and Shioji (Japanese ash, Fraxinus spaethiana Lingelsh.) were used as specimens. The specimens, the system to support them, a magnetic driver, and a deflection sensor were in an electric drying oven, where vibration tests were conducted. The heating temperatures ranged from room temperature to 200 °C in 25 °C increments in both heating and cooling processes. X-ray diffractometry was carried out using positive sensitive proportional counter (PSPC) at room temperature to 200 °C in 20 °C increments in both heating and cooling processes. Received 13 December 1999     

Characterisation and categorisation of the diversity in viscoelastic vibrational properties between 98 wood types

Context   

Increased knowledge on diversity in wood properties would have implications both for fundamental research and for promoting a diversification of uses as material.     

Effect of extractives on vibrational properties of African Padauk (<Emphasis Type="Italic">Pterocarpus soyauxii</Emphasis> Taub.)

Extractives can affect the vibrational properties tanδ (damping coefficient) and E′/ρ (specific Young’s modulus), but this is highly dependent on species, compounds, and cellular locations. This paper investigates such effects for African Padauk (Pterocarpus soyauxii Taub.), a tropical hardwood with high extractives content and a preferred material for xylophones. Five groups of 26 heartwood specimens with large, yet comparable, ranges in vibrational properties were extracted in different solvents. Changes in vibrational properties were set against yields of extracts and evaluation of their cellular location. Methanol (ME) reached most of the compounds (13%), located about half in lumen and half in cell-wall. Water solubility was extremely low. tanδ and E′/ρ were very strongly related (R ² ≥ 0.93), but native wood had abnormally low values of tanδ, while extraction shifted this relation towards higher tanδ values. ME extracted heartwood became in agreement with the average of many species, and close to sapwood. Extractions increased tanδ as much as 60%, irrespective of minute moisture changes or initial properties. Apparent E′/ρ was barely changed (+2% to −4%) but, after correcting the mass contribution of extracts, it was in fact slightly reduced (down to −10% for high E′/ρ), and increasingly so for specimens with low initial values of E′/ρ.     

Crack propagation in notched wood specimens with different grain orientations

Summary The cracking patterns of load induced cracks were studied by in-situ testing of compact tension specimens within the specimen chamber of a scanning electron microscrope. The cracks propagated in a generally straight path parallel to the grain, regardless of the orientation of the notch. On a microscopic scale, the crack could not be described as an ideal parallel-walled crack as assumed in fracture mechanics models. Many irregularities such as tortuosities, branching, discontinuities and bridging between the crack walls could be seen. Observations were carried out at the tip of the stable crack and at the same zone after the crack was induced to propagate beyond it. The processes taking place in this zone are discussed. The implication of these observations on the applicability of linear elastic fracture mechanics to wood are also discussed.When this work was carried out, Dr. Bentur was on leave at Purdue University, West Lafayette, Indiana     

Knife slicing of wood across the grain

Summary This paper investigates a method for slicing thick pieces of wood across the grain using a sharp steel knife, and it demonstrates the effectiveness of the knife slicing process at an experimental level. In order to highlight the key parameters facilitating or hindering the thick slicing process, several factors affecting the cutting forces and the cut surface quality have been studied; the blade geometry, the effect of friction, the cutting speed, the boundary load constraints and the wood moisture content. The results indicate that a large proportion of the cutting force may be related to the wedging action of the blade in the wood. Therefore, significant reductions in the cutting energy can be achieved by optimising the blade shape. A simple homogeneous strain model has been developed which quantifies the cutting forces in terms of the blade angle and the blade surface friction. The results from this theoretical model are compared with the experimental findings and discussed in relation to methods for improving the cutting technique.This work was funded by the Foundation for Research Science and Technology. This support is gratefully acknowledged     

Working mechanism of adsorbed water on the vibrational properties of wood impregnated with extractives of pernambuco (Guilandina echinata Spreng.)

To clarify the lowering mechanism of loss tangen (tan) of sitka spruce (Picea sitchensis Carr.) wood impregnated with extractives of pernambuco (Guilandina echinata Spreng. synCaesalpinia echinata Lam.), we examined the vibrational properties of the impregnated wood in relation to the adsorbed water. The results obtained were as follows: (1) The equilibrium moisture content (EMC) of impregnated sitka spruce decreased to some extent compared with untreated wood. (2) Frequency dependencies of tan a about 400–8000Hz showed that impregnated wood has much lower tan than untreated wood at around 9% mois ture content (MC), except for the high-frequency region. At high relative humidity, impregnated wood has a minimum tan (at around 4000Hz); and at other frequency ranges the tan of impregnated wood did not differ considerably from that of untreated wood. (3) The apparent activation energy of the mechanical relaxation process (E) concerned with adsorbed water molecules was higher for impregnated specimens than for untreated ones at moderately high relative humidity, whereas at high relative humidity the difference was not observed. Based on these results, it is thought that the tan of impregnated wood decreased at low rela tive humidity because of the formation of direct hydrogen bonds between impregnated extractives and wood components. However, when the specimen is at higher relativePart of this work was presented at the 48th annual meeting of the Japan Wood Research Society, Shizuoka, April 1998 humidity, the formation of direct hydrogen bonds are disturbed by the existence of a large number of water molecules, and some extractives act as a plasticizer.     

Compression fracture of wood parallel to the grain

Summary A theoretical model of wood is presented consisting of a bonded assembly of long hollow tubes. As increasing load is applied to the tubes in the direction of their length, it is assumed that the bonding material breaks until sufficient free lengths exist for portions of the tubes to buckle. The dependence of the buckling stress and the free length on dimensions and properties of the tubes is calculated and the angle of the band of unstable portions is derived. The behaviour of the model is compared with the measured behaviour of wood. Agreement is encouraging and refinements are suggested to reduce the discrepancies observed.     

A model of anisotropic swelling and shrinking process of wood

To elucidate the origin of the shrinking anisotropy of wood during the drying process, as well as to begin to gain an understanding of the interaction between the moisture and the cell wall components, the shrinking process of a single wood fiber regarding water desorption was simulated by using an analytical model which was developed in the previous report (Part 1). Resulting data were compared with the experimental ones in this paper. The following conclusions were obtained: (1) The matrix substance, as a skeleton in the secondary wall, tends to shrink isotropically. However, the cellulose microfibrils, as a rigid framework of the cell wall, almost did not shrink at all due to the water desorption. As result, wood shrinks anisotropically during a drying process. The microfibril angle in the S2 layer is one of the most important factors related to the degree of shrinking anisotropy of the wood while drying. (2) According to the simulation, the expansive strain caused in the matrix skeleton by the water sorption increases by 15% (= 150,000 micro-strains) from the oven-dried condition to the green condition. Based on this value, the moisture content at the fiber saturation point is calculated to be about 35%, which is close to the experimentally obtained one. These results give quantitative evidences that the hygroexpansion of the wood cell wall is controlled by the mechanism of the reinforced matrix hypothesis. Received: 28 July 1998     

木质仿生智能响应材料的研究进展

智能响应材料需具备3个基本要素,即感知、驱动和控制,在全球新材料研究领域中,仿生智能响应材料是目前世界各国技术战略发展中的竞争热点。木材是一种天然且可再生的生物质材料,具有良好的结构和功能特性。作为人类使用最早的材料,木材具有轻质、美观、生物调节等优良特性,是绿色环境人体健康的贡献者。木材的纤维素、半纤维素和木质素构成了木材精妙的微结构同时提供了许多官能团,为木材仿生智能材料的合成奠定了优良的基础。本文简要介绍了木质仿生智能响应材料的研究进展,综述了pH值、气体、光、机械力、湿度、温度和双重/多重刺激响应木质材料的制备、性能与潜在应用;重点介绍并总结了以木质材料为基材的仿生智能响应材料的发展现状。     

The stress-displacement relation of wood perpendicular to the grain

Summary A non-linear fracture mechanics model, the Fictitious Crack Model (FCM), is utilised for a theoretical study of the Compact Tension (CT) specimen. The complete stress-displacement curve for the material is used in the FCM, even the descending branch after maximum stress. The analysis shows that the computed load-displacement curve is sensitive to modulus of elasticity perpendicular to the grain, tensile strength and fracture energy. The theoretically obtained results agree well with experimentally determined curves.     

The stress-displacement relation of wood perpendicular to the grain

Summary A method for measuring the complete stress-displacement relation in both tension and shear and some experimental results on Nordic redwood (Pinus sylvestris) are presented. The complete stress-displacement relation also includes the post-peak behaviour, i.e. the descending branch of the curve. The influence of annual ring orientation and moisture content on the form of the stress-displacement curve for the fracture process zone has been examined, and the results show that the normalised curve in tension is independent of annual ring orientation and moisture content.     

A comparison of static bending,compression and tension parallel to grain and toughness properties of compression wood and normal wood of a giant sequoia

Summary Compression wood (CW) of the giant sequoia studied had higher values than normal wood (NW) in crushing strength and ultimate stress in tension parallel to grain, in toughness, in modulus of rupture, and in work to maximum load and total work in static bending. In the green condition CW had higher values than NW in stress at the proportional limit and work to the proportional limit, and about the same modulus of elasticity in static bending. In the dry condition CW was about equivalent to NW in work to the proportional limit, but was slightly weaker in stress at proportional limit and modulus of elasticity in static bending. The compression wood of this giant sequoia, even though formed when the tree was suppressed and having relatively narrow rings, can therefore be said to be essentially equivalent to normal wood so far as the mechanical properties tested in this study are concerned.Given at FPRS meeting in Dallas, Texas, June 1972     

油松株内幼龄材与成熟材材性的比较研究

本文就中条山油松株内幼龄材与成熟材材性差异的比较研究,讨论对幼龄期划分的依据。根据木材解剖特征、物理力学性质的径向变异规律,确定其幼龄期为14年。随着树干高度的增加,油松木材幼龄期逐渐缩短、株内幼龄材范围及所占断面上的比例变小。株内幼龄材与成熟材材性差异显著。幼龄材管胞长度短、直径小,胞壁薄,微纤丝角度大,生长轮较宽,晚材率低,浸提物含量高,基本密度较大。幼龄材的力学强度远远小于成熟材。