A global rheological model of wood cantilever as applied to wood drying

In the process of wood drying inevitable stresses are induced. This often leads to checking and undesired deformations that may greatly affect the quality of the dried product. The purpose of this study was to propose a new rheological model representation capable to predict the evolution of stresses and deformations in wood cantilever as applied to wood drying. The rheological model considers wood shrinkage, instantaneous stress–strain relationships, time induced creep, and mechano-sorptive creep. The constitutive law is based on an elasto–viscoplastic model that takes into account the moisture content gradient in wood, the effect of external load, and a threshold viscoplastic (permanent) strain which is dependent on stress level and time. The model was implemented into a numerical program that computes stresses and strains of wood cantilever under constant load for various moisture content conditions. The results indicate that linear and nonlinear creep behavior of wood cantilever under various load levels can be simulated using only one Kelvin element model in combination with a threshold-type viscoplastic element. The proposed rheological model was first developed for the identification of model parameters from cantilever creep tests, but it can be easily used to simulate drying stresses of a piece of wood subjected to no external load. It can therefore predict the stress reversal phenomenon, residual stresses and maximum stress through thickness during a typical drying process.     

The variable parameter rheological model of wood

How to establish the rheological model to simulate creep behavior of wood and wood-based composites under change-load has not been solved in research of wood rheology. This paper presents a new model—variable parameter rheological model. The bending creep behavior of small clear poplar specimens under different constant load levels were examined. The load levels within 50% of rupture load of the specimens, and the experimental creep behavior were simulated by the variable parameter Maxwell model. The results show that using only one model of variable parameters may simulate the creep behavior of wood under different constant load levels very well. Applying the generalized Boltzmanns superposition principle, the variable parameter rheological model can be used to simulate the creep behavior of wood under change-loads conveniently and accurately.     

Moisture content adjustments for dynamic modulus of elasticity of wood members

The influence of moisture content (MC) on the dynamic modulus of elasticity of structural lumber was investigated using transverse vibration testing methods. The flexural rigidity (EI) of a transversely vibrating beam was calculated as the modulus of elasticity (E) multiplied by the moment of inertia (I). The increase in E of lumber due to reduction in moisture content was computed by assuming that the flexural rigidity remains constant with changes in moisture content. Reductions in I due to shrinkage were compensated by the increases in E which led to a proposal for a species-dependent MC adjustment model for modulus of elasticity. The model was validated using 38 mm × 89 mm × 4,290 mm western Canadian Spruce–Pine–Fir dimension lumber evaluated in the “as-received” and “dry” conditions. Results obtained from the species-dependent model agreed closely with those from the E adjustment equation for dimension lumber given in ASTM D 1990. The results show that the ASTM moisture adjustment procedures can be used to adjust dynamic E values for changes in moisture content also.     

Dose–response relationships between wood moisture content,wood temperature and fungal decay determined for 23 European field test sites

Scots pine sapwood (Pinus sylvestris L.) and Douglas fir heartwood (Pseudotsuga menziesii Franco) specimens were exposed in double layer field trials at 23 different European test sites under different exposure conditions (in total 27 test sets). The material climate in terms of wood moisture content (MC) and wood temperature was automatically monitored over a period of up to 7 years and compared with the progress of decay. The overall aim of this study was to establish dose–response relationships between climate factors and decay as a basis for the service life prediction of wood. The “Scheffer Climate Index” based on weather data collected at official meteorological stations at the different test sites poorly correlated with the corresponding decay progress and was therefore not a suitable tool for estimating site-specific decay potential. In contrast, the use of the combined material climatic parameters MC and wood temperature led to a feasible dose–response function and turned out to be a useful basis for service life prediction of wood.     

Time dependence of Poisson’s effect in wood I: the lateral strain behavior

To understand the viscoelasticity of wood three dimensionally, a longitudinal tensile creep test for 12 species was conducted to examine the changes with time in the lateral strain and the viscoelastic, i.e., apparent Poisson’s ratio. The changes in the lateral strain (ɛ _T and ɛ _R) were similar to those in the longitudinal strain (ɛ _L). That is, during creep, the absolute value of lateral strain continued to increase with the gradual reduction in the increase rate; immediately after the removal of the load, it recovered abruptly; then, it recovered slowly and finally reached a certain value. The rate of increase in the longitudinal strain during creep was smaller than that in the absolute value of lateral strains. The apparent Poisson’s ratio became large during creep because the lateral strain increased more than the longitudinal strain. The analysis of lateral strain by decomposition into three components, that is, instantaneous strain, delayed elastic strain, and permanent strain, has revealed that the lateral permanent strain in the transverse direction contributes most to the increase in the apparent Poisson’s ratio during creep.     

Effect of load type on failure mechanisms of spruce in compression parallel to grain

Failure mechanisms of small clear specimens (6×6×24 mm) of air-dried black spruce (Picea mariana) under parallel-to-grain compression were investigated by polarised-light microscopy. Fatigue load was used with a peak stress level of 90% static strength, a load frequency of 0.5 Hz, and a square waveform with a duty ratio of 0.50. Matched pure creep and static load tests were carried out. Damage was quantified in terms of the permanent microstructural changes (kinks) in tracheid walls. In static load tests, kinks develop quickly with any increase in stress beyond the limit of proportionality. In creep tests, damage develops mainly from kinks formed during the initial load application. In fatigue tests, damage develops both from kinks formed during the initial load cycle, and kinks formed during subsequent cycles. The number of kinks exhibits a strong relationship with relative cyclic creep or relative creep.     

Time dependence of Poisson’s effect in wood II: volume change during uniaxial tensile creep

To determine the viscoelasticity of wood three-dimensionally, a longitudinal tensile creep test was conducted on 12 species of wood to examine the change in the rate of volume increase (ΔV/V) with time. Immediately after the beginning of creep, ΔV/V was positive, and during creep, ΔV/V decreased rapidly, then more gradually. The decrease in tangential strain was considered to mainly contribute to the decrease in ΔV/V during creep. Immediately after the removal of the load, ΔV/V decreased to a negative value; thereafter, it decreased slowly and finally reached a certain value. The value of ΔV/V during creep tended to decrease with increasing density of wood. Also, there was a negative correlation between wood density and the rate of increase in ΔV/V.     

How dense is my blue gum? prediction of whole-tree basic density of <Emphasis Type="Italic">Eucalyptus globulus</Emphasis>

Accurate prediction of whole-tree wood basic density (BD) from indirect density measures is important for estimating Eucalytpus globulus breeding values and genetic gain. We measured 100 E. globulus trees at 6.5 years of age on each of a high productivity site (Albany) and a low productivity site (Esperance), sampling 16 full-sib families and seven open-pollinated ‘check’ seedlots. On each tree we assessed pilodyn penetration at 1.3 m and BD of: (a) the outer 10 mm which was penetrated by the pilodyn pin of a bark-bark core, (b) a whole bark-bark core at 1.3 m, (c) a cross-sectional disk at 1.3 m and (d) the whole tree, represented by cross-sectional disks at five heights. Site effects and genetic values for families and checks were determined by mixed model analysis. A trend of increasing BD from pith to bark was more pronounced at Albany, where wood formed before age 3.5 years was 12 kg/m³ less dense than at Esperance (P = 0.01) and wood formed from 3.5 to 6.5 years was 27 kg/m³ denser than at Esperance (P < 0.001). Mean whole-tree BD was 19 kg/m³ greater at Albany (P < 0.001). Pilodyn penetration was well-correlated with BD of the 10 mm sample of penetrated wood and this relationship was not greatly affected by site variation. Genetic values for pilodyn penetration formed precise relationships with BD of breast-height discs and whole-tree BD although they were site-specific. Pilodyn penetration was a better predictor for genetic values of whole-tree BD than were bark-bark cores.     

Bending creep of glued laminated timber (glulam) using sugi (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis>) laminae with extremely low Young’s modulus for the inner layers

The main purpose of this study was to establish whether sugi lumber with an extremely low Young’s modulus, which is plentifully produced in southern Japan, can be practically used as laminae for glued laminated timber (glulam) from the viewpoint of long-term performance under loading. Bending creep tests were carried out on sugi (Cryptomeria japonica D. Don) glulam with extremely low Young’s modulus laminae (3–4 kN/mm²) for the inner layers, as were tests on hybrid glulam with Douglas-fir (Pseudotsuga menziesii Franco) laminae (14–15 kN/mm²) for the outermost layer and sugi laminae (including those with a Young’s modulus of 3–4 kN/mm²) for the inner layers. The specimens were eight glulam beams with different compositions that were 105 mm wide, 210 mm deep, and 3980 mm long. The term of the creep test was 4 years. The results are summarized as follows. First, there were no significant differences between the Young’s modulus or bending creep of glulam L30 (laminae with Young’s modulus of 3–4 kN/mm²) and that of glulam L50 (laminae with Young’s modulus of 5–6 kN/mm²) for the inner layers. Second, for asymmetric compositions, the behavior of increases and decreases of relative creep due to atmospheric changes showed opposite behavior for glulam loaded from the side of lower Young’s modulus and from the side of higher Young’s modulus. Third, the required experimental term for the creep test to estimate an accurate long-term curve is 1 or 2 years (with data for the first 6 months excluded) when the power law is used for the estimation. Fourth, the values of relative creep in 50 years obtained from the experimental term were much lower than 2, which is the standard value set by Notification No. 1459 of the Ministry of Construction in Japan, and these values were not affected by the composition of the laminae.     

Growth and photosynthetic responses of Gallesia integrifolia (Spreng.) Harms and Schinus terebinthifolius Raddi seedlings in dense shade

We analyzed the growth and photosynthetic behavior of Gallesia integrifolia (‘pau-d’alho’) and Schinus terebinthifolius (‘aroeirinha’) under shade, seeking to obtain ecophysiological information for introducing seedlings of those species in previously established cacao agroforestry systems. Considering that light intensity under the shade of cacao trees varied between 5 and 10% daylight, 5 months old seedlings were exposed to four irradiance levels (25, 17, 10 and 5% daylight) for 92 days. With shade increase both species displayed trends of decrease leaf mass per unit leaf area, leaf area per plant (LA), relative growth rate (RGR) and net assimilation rate (NAR), and increase leaf area ratio (LAR). The mean values of light-saturated net photosynthetic rate (P _nmax) in 25 and 5% daylight were 12.8 and 8.0 μmol CO₂ m⁻² s⁻¹ for G. integrifolia and 17.9 and 7.4 μmol CO₂ m⁻² s⁻¹, respectively, for S. terebinthifolius. Based on the measurements of photosynthetic photon flux density and estimated values of photosynthetic saturated irradiance (Is) we concluded that, in all shaded conditions, the leaves of both species were under sub optimal light conditions to reach P _nmax. In spite of the lowest P _nmax values, RGR and NAR were significantly higher for G. integrifolia in all irradiance levels. Differences in growth rates can be explained by the higher values of LA, LAR and leaf mass ratio (LMR), as well as by the lower values of Is, photosynthetic compensation irradiance and dark respiration rates observed for G. integrifolia. Even though seedlings of G. integrifolia presented higher capacity to adapt under conditions of dense shade, we concluded that both species were under stress conditions induced by shade in light environments below 25% daylight. On a practical point of view it is possible to conclude that seedlings of both species should be introduced in light gaps, formed after the fall of big trees, or in places in which cacao trees are cultivated using large plant spacing.     

Bending creep behavior of hot-pressed wood under cyclic moisture change conditions

This study examined the bending creep behavior of hot-pressed wood during cyclic moisture changes. Sugi (Cryptomerica japonica D. Don) specimens were pressed in the radial direction under six combinations of nominal compressive strain (33% and 50%) and press temperatures (140°C, 170°C, 200°C). Creep tests were conducted at 20°C with three cyclic relative humidity changes between 65% and 95% under 25% of short-breaking stress. The effect of moisture content (MC) change on elastic compliance and mechanosorptive (MS) compliance was investigated. The relation between MS compliance and thickness swelling was studied. The results indicated that total compliance increased over the history of cyclic moisture changes; and its behavior was closely related to the changes in MC and thickness swelling. The total compliance increased during adsorption and decreased during desorption. Elastic compliance increased linearly with MC and was dependent on press temperature and compression. With increasing MC change, MS compliance increased during adsorption and decreased during desorption. The first adsorption led to greater MS compliance than did the subsequent adsorption with the same amount of MC change. In general, the elastic parameterK _E and the MS parameterK _Mincreased with compression and decreased as the press temperature increased. The MS parameterK _M was apparently greater than the elastic parameterK _E. The MS parameterK _M increased with swelling coefficient K_SW of the hot-pressed specimen during adsorption and decreased with an increasing shrinkage coefficientK _SH during desorption.     

Experimental determination of the convective heat and mass transfer coefficients for wood drying

The knowledge of the convective heat and mass transfer coefficients is required for the characterization of the boundary conditions of the heat and mass transfer equations of a wood drying model based on water potential. A new experimental method for the determination of the convective mass transfer coefficient is presented. This method is based on the measurement of the moisture content, and indirectly the water potential, at the surface of a wood specimen at different drying times. Drying experiments were performed on red pine (Pinus resinosa Ait.) sapwood from nearly saturated to dry conditions at 56 °C, 52% relative humidity and air velocities of 1.0, 2.5 and 5.0 m s⁻¹. The results show that the convective mass transfer coefficient is constant until the wood surface moisture content reaches about 80% and then decreases more or less gradually as the moisture content decreases further. The convective mass transfer coefficient increases with air velocity. A regression analysis shows that there is no significant improvement in considering the water potential gradient near the wood surface when the difference in water potential between the surface and the surrounding air (ψ_s − ψ_∞) is used to determine the convective mass flux at the surface. Also, ψ_s − ψ_∞ is more appropriate than the water vapour pressure difference (pv_s − pv_∞) as the responsible driving force of the moisture flux leaving the wood surface. The convective heat transfer coefficient was determined during the same experiments. A plateau is observed at high values of moisture content corresponding to the constant drying rate period. Received 27 February 1998     

Nonisothermal moisture movement in wood

In order to analyze the effect of temperature gradient on moisture movement during highly intensive drying, such as microwave-vacuum drying, the profile of the temperature and moisture content in sealed wood whose opposite faces were subjected to temperature gradient for a short time was measured. The ratio of the moisture content (MC) gradient to the temperature gradient (dM/dT) was calculated and the factors influencing moisture movement under nonisothermal conditions were discussed. The results indicate that moisture moved in wood from the warm surface to the cold one even if opposite faces of the sealed wood assembly were exposed continuously to different but constant temperatures for a short period. The moisture content on the cold surface was higher than that on the warm surface. The moisture content gradient opposite to the temperature gradient was established, and the dM/dT was below 0.9%/°C. The temperature in the sample and the distance from the hot surface of the sample was strongly linearly correlated. With an increase in temperature, initial moisture content and experimental time, the dM/dT was significantly increased. __________ Translated from Journal of Beijing Forestry University, 2005, 27(2): 96–100 [译自: 北京林业大学学报, 2005, 27(2): 96–100]     

Genetic variation and genotype-environment interactions in short rotation <Emphasis Type="Italic">Populus</Emphasis> plantations in southern Europe

Growth and production in the first year, as additional selection criteria, were assessed for nine poplar clones to be used as short rotation woody crops (SRWC) in the production of biomass for energy purposes. In order to identify the most promising clones in terms of growth and yield and also to assess their stability, trials were established at different locations in Spain. The majority of these clones, which form part of the European list of base materials, have frequently been used in plantations aimed at timber production but not for biomass in Mediterranean conditions. Others, such as those selected in Italy specifically for biomass production (currently provisionally admitted), are being tested for the first time under different soil and climatic conditions in Southwest Europe. The early selection of clones for rapid juvenile growth provides a valuable additional input to the clonal selection process, especially where very short rotations are desired (no more than 3 years). In any case, determining clonal stability in terms of growth is of great use not only when deciding on the clones to be used in plantations but also when developing breeding programs. ANOVA and Genotype plus Genotype × Environment (GGE) biplot analyses were used to analyse the growth and stability of the clones, which were then ranked according to mean performance and stability. Differences were detected between clones as well as between the different environments tested. The biplot analysis allowed different groups of clones to be identified according to their performance and degree of interaction displayed, thus providing useful information for the selection process. The production of aboveground biomass in the first vegetative period ranged from 1.7 to 8.0 Mg DM ha⁻¹ at the different sites. ‘Monviso’, ‘Guardi’, ‘AF2’ and ‘2000 verde’ were the most productive clones whereas ‘Unal’, ‘Pegaso’ and ‘USA 49-177’ were the least productive. The stability analysis identified ‘AF2’, ‘Guardi’, ‘I-214’ and ‘MC’ as more stable clones while ‘Monviso’, ‘2000 verde’, ‘Unal’, ‘Pegaso’ and ‘USA 49-177’ were found to be specifically adapted to certain environments. This implies that where information on site conditions is not available, the ‘AF2’ and ‘Guardi’ clones offer greater assurance of successful establishment and higher initial growth. The growth of ‘Monviso’ ‘2000 verde’ ‘Unal’ ‘Pegaso’ and ‘USA 49-177’ clones is highly dependent on site conditions during the establishment phase. Similarly, the SH (Shore Henares river) and LT (La Tallada) sites were identified as the most highly discriminative environments for the set of clones while CS (Cubo de la Solana) and AR (Atarfe) were identified as those where performance levels were average.     

Future harvesting pressure on European forests

We provide quantitative insight in the spatial distribution of the future supply of wood as a raw material from European forests (27 countries) until 2060. This supply is tested for two scenarios: ‘projection of historical management’ and ‘new management trends’ and compared against a benchmark scenario. The new management trends scenario incorporates influences of issues as nature-oriented management, carbon credits and increased demand for bio-energy. The results of these projections provide insight in the state of the European forests and indicate that under the ‘new management trends’ supply can still increase to 729 million m³ by 2060 in Europe, whereby almost throughout Europe we allow harvest to be higher than increment for some time. Without linking countries dynamically through international trade, we identify regions where harvesting pressure is highest. Under the new management trends scenario, the harvested volume is reduced with 82 million m³/year (compared to ‘projection of historical management’) because of stricter management constraints. However, the management regimes as parameterised here allow harvesting pressure to remain highest in Central Europe and some Scandinavian countries, notably Finland and Norway.

Creep of the beam in Japanese conventional structures composed of green and kiln-dried timber II: predictive model for relative deflections

We conducted creep tests to evaluate creep behaviors of conventional Japanese framing (jikugumi) structures as reported in a previous article. We measured beam deflections of two structures: one of them was composed of only green timbers (G) and the other with only kiln-dried timbers (D). Besides the two structures, we prepared green and kiln-dried beams to measure moisture content (MC), weight, and dynamic Young’s modulus (E _f) by the longitudinal vibration method. We attempted to predict deflections of beams in the structures by using experimental data for single beam specimens. The proposed simple predictive model was derived from two equations: a relation between MC and equilibrium moisture content calculated with temperature and relative humidity, and a relation between MC change and relative deflection change. Beam deflections were traced for 2.5 years, while the predictions were based on experimental data from loading to the 11th day of the test. It was assumed that sensitivity of deflection change to MC should differ during desorption or adsorption. Although annual cyclic changes were observed in E _f, there was no obvious relationship between E _f and beam deflection. Part of this article was presented at the Annual Meeting of the Architectural Institute of Japan, Kyushu, September 1998     

Predicting field performance of five irrigated tree species using seedling quality assessment in Burkina Faso,West Africa

Five exotic tree species (Acacia angustissima (Mil.) Kuntze, A. mangium Wild, Gliricidia sepium (Jacq.) Alp., Leucaena hybrid (L×L), and Leucaena leucocephala (Lam.) de Wit) were investigated to determine whether parameters of nursery seedling stock quality could be used to predict their field performance in a plantation irrigated with treated waste-water to produce fodder and wood. Plants were grown in the nursery in two contrasting rooting substrates (ordinary nursery soil and sand), predicted to have different effects on resource allocation. Three categories of morphological indicators were measured, i.e., plant dimensions (height, diameter, root length), plant weights (shoot, root and whole plant weights) and indices (sturdiness quotient ‘SQ’, shoot:root dry weight ratio ‘SRR’ and Dickson’s quality index ‘DQI’). In the nursery, all species performed better in the ordinary nursery soil for all growth parameters except root length. Thus ordinary nursery substrate appeared superior to sand in terms of plant quality. However, a follow up at plantation phase revealed that only some morphological attributes or ratios were suitable to predict field performance for the five tested species in irrigated plantation. In addition, the effect of the substrate observed at the nursery stage had disappeared 12 months after out planting due to the availability of water and nutrients provided by the treated waste water used for the irrigation. The results showed that root collar diameter and DQI appeared to be the most appropriate indicators to predict the outplanting performance of the five tested species in a short-rotation irrigated plantation in semi-arid Burkina Faso. The former measure is simpler and non-destructive.     

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.     

Vessel size and number are contributors to define wood density in cork oak

Cork oak (Quercus suber L.) has a dense wood that allows high-quality uses. In the present work, we study the influence of vessel characteristics, measured through image analysis and optical microscopy, on wood density, measured using X-ray microdensitometry, on 40-year-old trees. Vessel area increases with cambial age (5403–33064 μm²), while wood density decreases (1.229–0.836 g/cm³). The number of vessels is relatively constant at 6 vessels/mm², while vessel proportion in cross-section increases from 3.3% near the pith to 20.5% near the bark. In growth rings closest to the pith, with high wood density and low vessel area, the relationship between the two variables is linear (R ² = −32.1%, P < 0.01) but with increasing tree age and vessel size, the wood density remains rather constant, suggesting that decreases in density might compromise mechanical support of the tree at a stage when the increase in cross-sectional area alone might not provide mechanical stability. Other anatomical characteristics not considered in this study, like large xylem rays that increase with cambial age, may be responsible for the constant density.     

Effects of salt-alkali stress on active oxygen metabolism in roots of Spiraea × bumalda ’Gold Mound’ and Spiraea × bumalda ’Gold Flame’

Under artificially-simulated complex salt-alkali stress, the levels of active oxygen metabolism in roots were studied using three-year-old cutting seedlings of Spiraea × bumalda ’Gold Mound’ and Spiraea × bumalda ’Gold Flame’. The present study aimed at exploring the antioxidant capacity in roots of spiraeas and revealing their adaptability to salt-alkali stress. Results indicate that the oxygen free radicals contents, electrolyte leakage rates and MDA contents in roots of Spiraea × bumalda ’Gold Mound’ and Spiraea × bumalda ’Gold Flame’ show an increasing tendency with the increases of the salinity and pH value, whereas the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) all increased firstly and then decreased. With the increase in intensity of salt-alkali stress, the CAT activity in roots of Spiraea × bumalda ’Gold Flame’ is higher and the increasing extents in the oxygen free radicals contents, electrolyte leakage rates as well as MDA contents are lower compared with Spiraea × bumalda ’Gold Mound’, indicating that Spiraea × bumalda ’Gold Flame’ has a stronger antioxidant capacity.