Mechanical properties and their interrelationships for medium-density European hardwoods,focusing on ash and beech

ABSTRACT

The usage of hardwoods for engineered wood products, such as glulam, requires defined mechanical properties reflecting the actual tensile strength of the material. Currently, the European strength class system EN 338 only covers profiles for hardwoods tested in bending. In this study, the material properties of medium-density hardwoods are analysed with the focus on a total of 3663 European ash (Fraxinus excelsior) and European beech (Fagus sylvatica) specimens tested in different loading modes (tension, compression, bending, and shear). The relationships between the material properties—tensile strength, stiffness, and density—are analysed on grouped data of both graded and ungraded specimens. As a result, a tailored ratio of tensile strength to tensile MOE and density is given, which allows to utilize a higher tensile strength of hardwoods (f_t,0,k over 30?N/mm²) compared to softwoods. Furthermore, the relationship of the test values and the derived values is checked. The equations for deriving the compression and bending strength from tensile strength are verified based on available data. For tensile and compression strength perpendicular to the grain and for shear strength of both beech and ash, higher strength values than the ones listed in EN 338 are possible. The relationship between the mechanical properties are combined to tensile strength profiles for hardwoods.     

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.     

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.     

A NMR study of water distribution in hardwoods at several equilibrium moisture contents

The water state of one tropical (Robinia coccinea) and two temperate (Acer saccharum and Fagus grandifolia) hardwoods was determined at different equilibrium moisture contents (EMC) during desorption at 25°C. NMR technique was used to separate different components of water in wood. The species studied presented different structures, which were apparent on the spin–spin relaxation T₂ values. Three different water components were separated: slow T₂ (liquid water in vessel elements), medium T₂ (liquid water in fiber and parenchyma elements) and fast T₂ (bound or cell wall water). The NMR results showed that even at equilibrated conditions a region exists where loss of liquid water and bound water takes place simultaneously. This region will vary according to the wood structure. Finally, liquid water was present at EMC lower than the fiber saturation point, which contradicts the concept of this point when considered as a bulk property of wood.     

Chemical characterization of wood and extractives of fast-growing Schizolobium parahyba and Pinus taeda

This study aims to evaluate the chemical composition of wood and extractives of Pinus taeda and Schizolobium parahyba (guapuruvu) as potential feedstock for new applications in the biorefinery industry. For this purpose, their content of α-cellulose, hemicellulose, insoluble lignin, hot water solubility, NaOH_1% solubility, inorganic materials (ash), and monomeric sugars by high-performance liquid chromatography was quantified. Attenuated total reflectance infrared spectroscopy and thermogravimetric analysis were also used to complete the physicochemical characterization of the studied woods. The extractives were obtained by soxhlet extraction with ethanol:toluene and dichloromethane and identified with pyrolysis-gas chromatography/mass spectroscopy technique. The results showed that guapuruvu wood has the higher amount of hemicellulose (16%) when compared to pine wood (10%), which resulted in higher solubility in alkali solution. Furthermore, in relation to other biomasses, the two woods presented more percentage of lignin and minor content of hemicelluloses. The P. taeda wood presented the highest percentage of extractives mainly composed of fatty acids and aromatic hydrocarbons, while guapuruvu wood had a higher percentage of phenolic compounds and also fatty acids. Both the materials have low content of extractives with dichloromethane and were mainly composed of lipophilic compounds.     

Identification of anisotropic vibrational properties of Padauk wood with interlocked grain

Grain deviations and high extractives content are common features of many tropical woods. This study aimed at clarifying their respective impact on vibrational properties, referring to African Padauk (Pterocarpus soyauxii Taub.), a species selected for its interlocked grain, high extractives content and uses in xylophones. Specimens were cut parallel to the trunk axis (L), and local variations in grain angle (GA), microfibril angle (MFA), specific Young’s modulus (E′ _L /ρ, where ρ stands for the density) and damping coefficient (tanδ_L) were measured. GA dependence was analysed by a mechanical model which allowed to identify the specific Young’s modulus (E′₃/ρ) and shear modulus (G′/ρ) along the grain (3) as well as their corresponding damping coefficients (tanδ₃, tanδ_G). This analysis was done for native and then for extracted wood. Interlocked grain resulted in 0–25° GA and in variations of a factor 2 in E′_L/ρ and tanδ_L. Along the grain, Padauk wood was characterized, when compared to typical hardwoods, by a somewhat lower E′₃/ρ and elastic anisotropy (E′/G′), due to a wide microfibril angle plus a small weight effect of extracts, and a very low tanδ₃ and moderate damping anisotropy (tanδ_G/tanδ₃). Extraction affected mechanical parameters in the order: tanδ₃ ≈ tanδ_G > G′/ρ > > E′₃/ρ. That is, extractives’ effects were nearly isotropic on damping but clearly anisotropic on storage moduli.     

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.     

Heterogeneity in formation of lignin

Summary The formation of lignin in the cell wall of compression wood of Pinus thunbergii was examined by selective radio-labeling of specific structural units in the lignin and visualization of the label in the different morphological regions by microautoradiography. Deposition of lignin in the tracheid cell wall of compression wood occurred in the order: p-hydroxyphenyl, guaiacyl and syringyl lignin, which is the same order as observed in normal wood. However, the period of lignification in the compression wood was quite different from those of normal and opposite woods. The p-hydroxyphenyl units were deposited mainly in the early stage of cell wall formation in compound middle lamella in normal and opposite woods, while in compression wood, they were formed in both the compound middle lamella and the secondary wall. The most intensive lignification was observed during the formation of the S₂ layer, proceeding from the outer to inner S₂ layers for a long period in compression wood. In the normal or opposite woods, in contrast, the lignification became active after formation of S₃ had begun, then proceeded uniformly in the secondary wall and ended after a short period.A part of this report was originally presented at the 1989 International Symposium on Wood and Pulping Chemistry at Raleigh, NC, U.S.A.     

Mechanical properties assessment of Cunninghamia lanceolata plantation wood with three acoustic-based nondestructive methods

The objectives of this study were to establish the method of evaluating wood mechanical properties by acoustic nondestructive testing at standing trees and at logs of a Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantation, and to compare three acoustic nondestructive methods for evaluating the static bending modulus of elasticity (MOE), modulus of rupture (MOR), and compressive strength parallel-to-grain (σ_c) of plantation wood as well. Fifteen Chinese fir plantation trees at 36 years of age were selected. Each tree was cut into four logs, for which three values of dynamic modulus of elasticity, i.e., E _sw, of the north and south face based on stress waves to assume the measuring state of the standing tree, E _fr, longitudinal vibration, and E _us, ultrasonic wave, were measured in the green condition. After log measurements, small specimens were cut and air-dried to 12% moisture content (MC). Static bending tests were then performed to determine the bending MOE and MOR, and compressive tests parallel-to-grain were made to determine σ_c. The bending MOE of small clear specimens was about 7.1% and 15.4% less than E _sw and E _us, respectively, and 11.3% greater than E _fr. The differences between the bending MOE and dynamic MOE of logs as determined by the three acoustic methods were statistically significant (P < 0.001). Good correlation (R = 0.77, 0.57, and 0.45) between E _sw, E _fr, and E _us and static MOE, respectively, were obtained (P < 0.001). It can be concluded that longitudinal vibration may be the most precise and reliable technique to evaluate the mechanical properties of logs among these three acoustic nondestructive methods. Moreover, the results indicate that stress wave technology would be effective to evaluate wood mechanical properties both from logs and from the standing tree.     

Swelling of wood

Summary The rate and maximum swelling of several North American wood species in water have been obtained with a computer interfaced linear variable displacement transformer. Since wood swells extremely fast in water even at room temperature, this apparatus made it possible for the first time, to obtain accurate rate data on the swelling of wood in water. The strict linear dependence of swelling on the temperature suggests a chemical mechanism. The activation energies obtained from Arrhenius plots ranged from 32.2 KJ/mole for sitka spruce to 47.6 KJ/mole for sugar maple. Although the two hardwoods exhibited greater maximum tangential swelling compared with the two softwoods, the maximum swelling appears to be correlated with the wood density. Generally both the rate and maximum swelling of the woods were increased by removal of extractives and the activation energies were reduced.     

Radial and between-family variations of the microfibril angle and the relationships with bending properties in Picea jezoensis families

With emphasis on tree breeding for wood quality in Picea jezoensis, we aimed to evaluate radial and between-family variations in the microfibril angle (MFA) of the S₂ layer in the latewood tracheids in 10 open-pollinated families of 43-year-old P. jezoensis trees. In addition, the relationships between MFA/wood density with the modulus of elasticity (MOE) or modulus of rupture (MOR) were investigated. Significant differences in MFA between families were found from the pith toward the bark. MFA showed higher values around the pith area, although some families showed relatively lower values than others around this area. In addition, due to a larger coefficient of variations of MFA near the pith, the potential for juvenile wood MFA improvement may be greater compared with mature wood. MOE was correlated with MFA in juvenile wood and with wood density in mature wood, whereas MOR was mainly correlated with wood density at radial positions in both woods. Therefore, to improve the MOE and MOR of P. jezoensis wood, both MFA and wood density would be factors to consider in both juvenile and mature woods. On the other hand, there are indications that, only wood density would be an important criterion for improving mature wood properties.     

The calculation of EMC for the analysis of wood/water relations in Fagus sylvatica L. modified with 1,3-dimethylol-4,5-dihydroxyethyleneurea

Beech wood (Fagus sylvatica L.) was modified with 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU). The equilibrium moisture content (EMC) of wood modified with DMDHEU calculated on a dry modified basis is reduced. Previous results have shown that the modification with DMDHEU does not alter the capillary condensation; therefore, the reduction in EMC seems exaggerated. The equilibrium constants of the Hailwood–Horrobin model (K _d and K _h) and the molecular weight of a hypothetical polymer of modified wood capable of adsorbing one molecule of water (W _i) were calculated from the EMC on a dry modified wood basis (M) and on a dry wood basis (M _R). The hypothetical polymer was also calculated by stoichiometry (W _c) and compared to W _i to estimate the number of operative OH groups. The number of operative OH groups decreased when M was used, in contradiction with the previously obtained results of differential heat of adsorption (∆H _s). Therefore, the use of M _R is recommended for the analysis of moisture sorption in wood modified with DMDHEU.     

Changes in surface properties of tropical wood species exposed to the Indonesian climate in relation to mold colonies

Changes in mold populations and genera on the exposed surfaces of tropical hardwoods — albizia (Paraserianthes falcata), kapur (Dryobalanop lanceolata), mahoni (Switenia macrophylla), nangka (Artocarpus heterophyllus), puspa (Schima wallchii) — were investigated. The wood specimens were exposed to the Indonesian climate for 32 weeks. Properties including mass loss, wettability, mold growth (colony-forming units), and mold genera were evaluated. The change in properties after exposure was significantly affected by the wood species, but there was no clear relation between mass loss and the initial chemical components or between wettability and wood density. The number of mold populations was different by exposure period and wood species, but there was no significant effect of climate conditions, such as rainfall and ultraviolet radiation. Of the genera identified,Aureobasidium, Cladosporium, andPenicillium were dominant molds on the exposed wood surfaces.     

Effect of compression on the liquid absorption of Chinese fir wood with different heartwood-to-sapwood ratios

In this study, the Chinese fir (Cunninghamia lanceolata Hook.) specimens with different ratios of heartwood thickness to sapwood thickness (HS) were radially compressed at different compression speeds, and then absorbed amine copper quat-type D (ACQ-D) preservative solution under the negative-pressure produced by the recovery of compression deformation. The liquid uptake (Al), the recovery rate of compression deformation (Rs) and the chemical absorption (Ac) of samples were determined, as well as the overall distribution of density and effective component of ACQ-D (i.e., copper in wood), the mechanical properties such as surface hardness were also measured. The Al, Rs, Ac values of compressed samples including the whole heartwood ones were higher than those of uncompressed samples, showing that radial compression had an obvious positive effect on improving the liquid absorption of heart-wood. Higher compression speed of 3 mm·min–1 is preferable since the samples with that speed could reach the highest Al and Ac; in addition, more deformation fixation has been produced possibly because of the faster heat and moisture transmission at the higher compressed speed, and more bonds of hydrophobic nature were formed, leading to the higher surface hardness and density. A consistent tendency of the density distribution and the copper retentions along the thickness direction could be explained that the layers with higher density have smaller volumes of void areas, and more chemicals were absorbed and fixed, resulting in the higher copper retentions.     

Extractives of muirapiranga (<Emphasis Type="Italic">Brosimun</Emphasis> sp.) and its effects on the vibrational properties of wood

The potential of muirapiranga (Brosimun sp.) as a substitute material for violin bows was estimated in terms of vibrational properties, and the influence of extractives on the vibrational properties was examined. The loss tangent of muirapiranga was somewhat higher and the specific dynamic Young’s modulus was rather lower than the respective values for pernambuco, which is regarded as the best material for violin bows. Therefore, muirapiranga is a poorer bow material in terms of vibrational properties. Impregnation of crude extractives from muirapiranga decreased the loss tangent of other wood specimens. The main compounds of the extractives were identified as xanthyletin and luvangetin. Impregnations of isolated xanthyletin and commercially available methoxsalen, which was tested as an analogue of luvangetin, markedly decreased the loss tangent of other wood specimens. Methoxsalen and xanthyletin differ from conventional loss tangent-decreasing substances, namely protosappanin B and hematoxylin, in terms of water insolubility and the absence of hydroxyl groups. From the similarity in molecular characteristics of loss tangent-decreasing substances found so far, restriction of molecular motion due to an impregnated substance in the wood matrix is suggested as one loss tangent-decreasing mechanism.     

Theoretical and experimental deflections of glued laminated timber beams made from a tropical hardwood

Abstract

Nondestructive testing (NDT) can play an important role in improving the quality and reliability of tropical hardwood as an engineering material. By means of these methods, the stiffness of the material can be determined and the information used to improve its structural performance. Although, it is a usual approach for qualifying the material used to manufacture engineered wood products made mainly from softwoods, it is not so common for tropical hardwoods. Additionally, the lack of information regarding properties of glulam beam made from these kinds of wood is evident. In this context, the paper aimed at evaluating the theoretical and experimental deflection of glulam beams made from the Brazilian hardwood louro-vermelho (Sextonia rubra). Initially, the stiffness of each lamina was determined nondestructively using transverse vibration method (E _dtv), which has been demonstrated to be the most suitable method for this wood species. Then, ten 5-lamina glulam beams were assembled according to descending lamina E _dtv values. The experimental evaluation was performed using a four-point bending schedule. In general, the theoretical values of deflection were 2% higher than the experimental ones. The transverse vibration showed to be a suitable method to both measure lamina stiffness and predict glulam beam deflection.     

Effect of heat treatment intensity on some conferred properties of different European softwood and hardwood species

Effect of heat treatment intensity on some conferred properties like elemental composition, durability, anti-swelling efficiency (ASE) and equilibrium moisture content (EMC) of different European softwood and hardwood species subjected to mild pyrolysis at 230 °C under nitrogen for different durations has been investigated. Independently of the wood species studied, elemental composition is strongly correlated with the mass losses due to thermal degradations which are directly connected to treatment intensity (duration). In all cases, an important increase in the carbon content associated with a decrease in the oxygen content was observed. Heat-treated specimens were exposed to several brown rot fungi, and the weight losses due to fungal degradation were determined after 16 weeks, while effect of wood extractives before and after thermal treatment was investigated on mycelium growth. ASE and EMC were also evaluated. Results indicated important correlations between treatment intensity and all of the wood conferred properties like its elemental composition, durability, ASE or EMC. These results clearly indicated that chemical modifications of wood cell wall polymers are directly responsible for wood decay durability improvement, but also for its improved dimensional stability as well as its reduced capability for water adsorption. All these modifications of wood properties appeared simultaneously and progressively with the increase in treatment intensity depending on treatment duration. At the same time, effect of extractives generated during thermal treatment on Poria placenta growth indicated that these latter ones have no beneficial effect on wood durability.     

Distribution of lignin interunit bonds in the differentiating xylem of compression and normal woods of Pinus thunbergii

The lignification process and lignin distribution at different stages of cell wall differentiation in the secondary xylem of compression and normal woods of Pinus thunbergii were investigated by thioacidolysis and subsequent desulfuration. We prepared 50-µm-thick, contiguous tangential sections of pine shoots, cut from the cambial zone through to mature xylem. In compression wood, uncondensed guaiacyl (G) and p-hydroxyphenyl (H) lignins were deposited simultaneously from early to late stages of lignification. The various types of G-G, G-H, and H-H dimers were detected in compression wood, and the ratio of G-H and H-H dimers to total dimers increased as lignification proceeded. In contrast, uncondensed and condensed H units were detected in trace amounts in normal wood. Significant differences in the relative distributions of lignin interunit linkages were not observed between compression and normal woods or between differentiating and mature xylems in either compression or normal woods.Part of this report was presented at the 10th International Symposium on Wood and Pulping Chemistry, Yokohama, June, 1999     

Spectroscopic analysis of the role of extractives on heat-induced discoloration of black locust (Robinia pseudoacacia)

Abstract

To investigate the role of extractives on heat-induced discoloration of wood, samples of black locust (Robinia pseudoacacia) wood flour were extracted with various solvents prior to heat-treatment. Analysis of their color parameters and chromophoric structures showed that the chroma value of the unextracted sample decreased while that of the extracted sample increased after heat-treatment. Both samples showed broad diffuse reflectance UV-Vis (DRUV) absorption bands with maxima around 360–380 nm after heat-treatment due to the formation of conjugated double bonds, carbonyl functionalities, and quinoid structures. Compared with the unextracted sample, the dominant chromaticity of the extracted samples hypochromatically shifted and the peak became narrower. This result showed that extractives contribute mostly to the reduction in the light reflection on heat-treated wood. In addition to extractives, lignin and hemicellulose also contributed to the formation of color substances upon heat-treatment. The increase in C₃/C₂ ratio in X-ray photoelectron spectroscopy (XPS) spectra signified the oxidation reactions in the heating process. The increase in O₁/O₂ for extracted sample after heat-treatment and changes in DRUV and Fourier transform infrared spectroscopy (FTIR) spectra support the hypothesis that discoloration can also arise from the degradation of hemicellulose and the condensation reactions of lignin.     

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