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The thermal modification of wood is a potential alternative method for improving wood dimensional stability and increasing the resistance of wood to decay. However, during thermal modification, morphological changes occur within the microstructure of the cell, and these confer different properties to the wood. This study investigated the effects of the thermal modification process on the microstructure of radiata pine juvenile wood. Therefore, anatomical measurements were performed via optical microscopy in selected earlywood and latewood samples after each treatment, and the results were compared to untreated wood samples. In this study, two temperatures (190 °C and 210 °C) were considered for the thermal modification process. The results showed that the level of temperature of modification affected to microstructure of cell wall. The cell wall thickness decreased as treatment temperature increased, whereas the average lumen diameter increased slightly as temperature increased. Thermally modified radiata pine showed signs of damage (cracks, broken cells and deformations in the wood cell wall). The proportion of destroyed area increased as temperature increased, and significant differences were evident for the thermal treatment at 210 °C.

The objective of this study was to measure compressive strength properties perpendicular to the fiber of green Eucalyptus nitens specimens. The data was measured for clear 50 × 50 × 150 mm specimens obtained from 17-year-old trees that were harvested at the Ñuble region in Chile, and the data comprised modulus of elasticity (Ec), compressive strength (ƒc), basic density, initial moisture content, and some anatomical characteristics of the wood. The experimental design emphasized the influence of the radial position of the wood within the logs (defined as inner-wood, middle-wood, and outer-wood) on the measurements, and the results showed that the modulus of elasticity, compressive strength, and the basic density all increased from core to bark. The compressive strength of outer-wood showed a strong and positive relation with the basic density. Cell wall thickness and wall cell area showed a significant positive correlation with compressive strength.

In this work, a two-dimensional mathematical model of the moisture transport and stress–strain phenomena during the conventional drying process of Eucalyptus nitens wood is presented. The model consists of a system of partial differential nonlinear second-order equations, where the moisture transport phenomenon is modeled on the concept of effective diffusion, and the stresses are modeled on the hypothesis of viscoelastic deformation, which is strongly linked to the strains by free contraction and mechano-sorption. The drying was assumed as an isothermal process and without effects of sustained loads over time (creep). Experimental tests were also carried out under constant psychometric conditions at 30/25 (°C/°C) in order to obtain transient distributions of moisture and stress. From these experimental data, the physical parameters of the proposed mathematical model were obtained by optimization in a context of inverse problem. The numerical solution of the model was obtained through the method of control volumes combined with finite elements. The results of transient distribution of moisture and stress correlate well with the experimental data.

To evaluate evolution of physical and mechanical properties due to drying and heat modification, a load of radiata pine wood was selected and properties were measured after each drying process. The results revealed interesting correlations between intrinsic factors and properties; the values of density were highly dispersed after drying or thermal treatment and uncorrelated with other parameters, but the minimum density values were kept constant after heat treatment. Moreover, weight loss (WL) and moisture content (MC) were decreasing proportionally to the treatment intensity, due to wood–water interactions, cell wall changes, and thermal degradation of wood fractions. WL and MC were reasonably correlated with the dimensional stability, improving the dimensional stability after drying treatments, but keeping the same order of anisotropy. Regarding the wood stiffness (modulus of elasticity, MOE), it was unaffected by the drying temperature, and the correlations between MOE and MC or WL appear to be...

Collapse and drying stresses are currently induced during the drying of Eucalyptus nitens in solid wood products. The purpose of this study was to investigate these drying stresses by measuring hygromechanical strains during the drying of Eucalyptus nitens boards. Small samples of Eucalyptus nitens wood were oriented in the radial and tangential directions and tested to determine the hygromechanical strains during the drying process. This experimental work consisted of cantilevered bending tests conducted under variable relative humidity conditions. Tests were performed in a conditioning chamber at 30 °C with an equilibrium moisture content ranging from 22 to 12% under four levels of stress: 0, 10, 20 and 30% of the rupture load. The strains were determined using strain gauges, and the total deflection was measured with a linear variable differential transformer. The results show that in hygromechanical strains during the drying of Eucalyptus nitens, both the surface deformation and mechano-sorption strain were found to be proportional to the applied stress and reached their maximum values in the tangential direction. The total deflection increased 0,18 mm/mm with a surface deformation of 0,20 mm/mm, and the mechano-sorptive strain provides a greater contribution with a value of 0,11 mm/mm, thus corresponding to 59% of the total deformation. In attempts to improve the drying schedules of Eucalyptus nitens to develop solid wood products, mechano-sorptive behavior may be applied to relieve collapse and drying stress.

The objective of this work was to simulate the stresses produced during the drying of Eucalyptus nitens wood due to variations in the moisture content. The methodology involved experimental determination and simulation of drying stresses caused by the development of internal moisture content gradients. Modeling of the moisture transport was based on the concept of an effective diffusion coefficient. The mathematical model for stress-strain, and for moisture diffusion into the wood, was constituted by a system of second-order nonlinear partial differential equations with variable coefficients, which were numerically integrated by the control volume based on the finite element method (CVFEM). For validation purposes, tests were realized for evaluating deformations, stress drying, and moisture gradients that were produced during the drying of Eucalyptus nitens. The results showed satisfactory agreement between the experimental and simulated values, indicating an effective simulation.

Collapse is almost certain to occur in the industrial drying of Eucalyptus nitens and, as such this prevents the lumber manufacturing industry in Chile from producing commercial solid wood products from this species. This problem is still unsolved, and different studies to reduce collapse are currently underway. In this exploratory study, shrinkage and collapse after drying of Eucalyptus nitens was measured for boards cut from different radial locations within the stem (core, transition, and outer wood from pith to bark) and having different annual ring orientations (flat-sawn and quarter-sawn). Though exploratory, the results appear to confirm that pieces that were cut from the center of the trees were less susceptible to collapse than pieces cut from the transition zone between the center and the periphery. On average, collapse in transition wood was approximately 50% higher than the collapse observed in wood cut from the central zone of the trees.

The purpose of this paper was to determine the mechano-sorptive strains produced during desorption of moisture from Eucalyptus nitens specimens, from a perspective of modeling wood drying stresses. Eucalyptus nitens samples were tested in the radial and tangential directions. The mechano-sorptive strain was measured by cantilever bending during desorption. The runs were performed in a conditioning chamber at constant low temperature and variable equilibrium moisture content. Four load levels were considered for testing. The results showed that the mechano-sorptive strain during desorption of moisture was proportional to applied stress and reached the maximum value in the tangential direction. Also, the mechano-sorption coefficient depended on wood orientation and reached the maximum value of 5.46 × 10-2 in the tangential direction.

Wood drying is an important process for adding value and manufacturing innovative products. Eucalyptus nitens wood is inherently difficult to dry because of its natural propensity for checking as well as collapse and shrinkage. Lumber recovery after industrial drying of eucalypts is also very low. This study measured the wood quality of E. nitens juvenile wood (13 mm thickness) after radio-frequency vacuum (RFV) drying and wood dried in a conventional kiln dryer (KD). Drying cycles were performed using a radio frequency vacuum dryer with a 3 m3 of capacity and convective kiln-dryer equipment with a 3.5 m3 of capacity. The results showed that the drying time using the radio frequency vacuum method was reduced by 47% when compared to conventional kiln drying. The shrinkage was significantly lower in the RFV than in the conventional KD. The volumetric collapse decreased by approximately 60% in the RFV drying. RFV drying of E. nitens juvenile wood improves the wood quality for solid wood products because the intensity of surface checking and collapse are reduced.