Fiber Saturation Research Articles

Influence mechanism of radio frequency heating on moisture transfer and drying stress in larch boxed-heart square timber

Radio frequency heating combined with convection (RF/C) drying of larch boxed-heart square timber and its influence on drying kinetics such as rate, moisture content distribution, and stresses was explored. Results revealed that RF heating increased the drying rate and in RF/C drying was twice as high as in conventional drying. Below fiber saturation point, RF heating reduced internal moisture gradients, especially around moisture content of 20%. The effect of RF heating on moisture transfer was strongly associated with moisture content. Specifically, above the fiber saturation point, RF heating played a minor role in moisture transfer however, it reached maximum around fiber saturation point and thereafter, it largely decreased with moisture content. RF heating relieved some drying stresses during RF/C drying and reduced residual stresses in the timber surface layers. Furthermore, it changed the original development pattern of drying stresses in conventional drying.

Drying influence on the development of cracks in Eucalyptus logs

The aim of this study was to evaluate the effect of the moisture loss, diameter, and wood density on the appearance of surface cracks and end splits in Eucalyptus urophylla logs. The drying and emergence of defects were evaluated in 108 logs with diameters ranging from 60 mm to 150 mm from the initial moisture content to the equilibrium moisture content. The defects were measured during this time. Smaller diameter logs dried faster than larger diameter logs and took less time to reach the fiber saturation point. Surface cracks tended to develop during the loss of bound water. End splits developed during free water loss and continued to appear during bound water loss. Smaller diameters presented higher percentages of surface cracks compared with larger diameters, while larger diameters had a tendency for higher percentages of end splits compared with smaller diameters. The density did not influence the total emergence of end splits, but it did influence the total emergence of surface cracks, indicating a possibility for the selection of genetic material with lower tendency for cracking. Overall, the results reinforce the need to control drying at its beginning in order to decrease the risk of defects.

New formula and conversion factor to compute basic wood density of tree species using a global wood technology database.

Basic wood density is an important ecological trait for woody plants. It is used to characterize species performance and fitness in community ecology and to compute tree and forest biomass in carbon cycle studies. While wood density has been historically measured at 12% moisture, it is convenient for ecological purposes to convert this measure to basic wood density, i.e., the ratio of dry mass over green volume. Basic wood density can then be used to compute tree dry biomass from living tree volume. Here, we derive a new exact formula to compute the basic wood density Db from the density at moisture content w denoted Dw , the fiber saturation point S, and the volumetric shrinkage coefficient R. We estimated a new conversion factor using a global wood technology database where values to use this formula are available for 4022 trees collected in 64 countries (mostly tropical) and representing 872 species. We show that previous conversion factors used to convert densities at 12% moisture into basic wood densities are inconsistent. Based on theory and data, we found that basic wood density could be inferred from the density at 12% moisture using the following formula: Db = 0.828D12 . This value of 0.828 provides basic wood density estimates 4-5% smaller than values inferred from previous conversion factors. This new conversion factor should be used to derive basic wood densities in global wood density databases. Its use would prevent overestimating global forest carbon stocks and allow predicting better tree species community dynamics from wood density.

Moisture state variety in poplar lumber with moisture content above fibre saturation point during hot-press drying

Hot-press drying has gained attention due to its excellent drying rate and surface strengthening effect. The moisture state in poplar lumber (Populus tomentosa) with moisture content (MC) above fibre saturation point (FSP) was analyzed during hot-press drying. The effect of heating platens’ temperature on the temperature and pressure inside poplar lumber was investigated. The moisture state changes inside the poplar lumber during hot-press drying were further analysed in accordance with the measured pressure and theoretical pressure calculated with the measured temperature inside poplar lumber. Results were as follow: When the heating platens’ temperature increased from 120 to 140 °C, the maximum values of temperature and pressure also increased by 14.5 and 26.2%, respectively. However, a delay occurred between the maximum values of pressure and temperature. The moisture at the center layer of poplar lumber with MC above FSP was in liquid state, i.e. unsaturated water under overpressure condition in hot-press drying process. Flashing occurred to the unsaturated water in poplar lumber during the opening period of heating platens and resulted in the decrease in MC. This phenomenon is the main moisture transfer mode in wood hot-press drying process with MC above FSP.

Investigations on the Effects of Seasonal Temperature Changes on the Electrical Resistance of Living Trees

In order to use the electrical resistance method to accurately and timely detect and evaluate the internal decay defects of living trees, the effects of the seasonal temperature and moisture content on the electrical resistance of standing trees were investigated. At the Northeast Forestry University Experimental Forest Farm, Harbin, Heilongjiang Province of China, Populus simonii Populus simonii Carr. and Larix gmelinii (Rupr.) Rupr. were selected as the objects and the electrical resistance of standing trees was tested through different seasons from December 2016 to December 2017. Meanwhile, the effects of changes in the seasonal temperatures (−20 to −10 °C, −10 to −5 °C, −5 to 0 °C, 0 to 5 °C, 5 to 10 °C, 10 to 15 °C, 15 to 25 °C) as well as changes in the moisture content (MC) (Populus simonii, MC ≥ 103%; Larix gmelinii, MC ≥ 77.5%) on the electrical resistance in the cross-sections of living trees were studied. The influence of temperature at different moisture contents, the moisture content at different temperatures, and their combined effects on electrical resistance were analyzed, following which a regression model was also established. The obtained results indicated that ambient temperature had a significant effect on the average value of electrical resistance in the cross-section of living trees when temperatures were below the freezing point. There was a sudden discontinuity near the freezing point, and logR (logarithm value of electrical resistance) in the cross-sections of sound trees and decayed trees changed in a similar trend with variations in the temperature. While the effect of moisture content on logR in the cross-sections of threes was insignificant at different temperatures because of the moisture content above FSP (fiber saturation point). It indicated that the temperature and moisture content had interactive effects on logR in the cross-sections. The binary linear regression model between moisture content, temperature, and logR was highly fitted with a correlation coefficient (R2) higher than 0.8. The outcome of this investigation indicates that when non-destructive testing is performed on living trees using electrical resistance at different seasonal temperatures, the measured results need to consider both the temperature and moisture content. For practical work, it is not recommended to consider testing living trees near the freezing point temperature using the electrical resistive tomography. Below the freezing point, the electrical resistance changes with temperature greatly relative to the normal temperature. Therefore, when performing the detection of electrical resistance, it is necessary to calibrate the effects of temperature

Longitudinal and Tangential Coefficients of Chip Compression in Orthogonal Wood Cutting

The complexity of the cutting zone makes it impossible to establish a reliable, simple, and comprehensive mathematical model for orthogonal wood cutting. Thus far, the science of cutting materials have used only approximate and simplified engineering theories that attempt to explain the mechanisms of chip shaping. The research of wood processing by chip separation is mostly based on models developed for metal processing. By applying Astakhov's model to orthogonal wood cutting, the total cutting power is divided into four components, and each cutting force component can be theoretically related to the coefficients of chip compression. In the present study, longitudinal, and tangential coefficients of chip compression were determined experimentally during orthogonal cutting of wood in the longitudinal (0 - 90) direction depending on wood moisture content (MC) and cutting speed. The direction of the fibers, cutting speed, and MC influenced the coefficient of chip compression. The MC of wood above the fiber saturation point significantly changed the impact of the cutting speed on the tangential coefficient of chip compression. In order to explain and better understand the obtained results of chip compression coefficients, the modulus of elasticity (MOE) of wood samples with different MC was determined. The MOE was calculated from the compression tests on small specimens in tangential and longitudinal direction.

Bamboo Conditions for Processing Bamboo Fiber with Combing Method

The bamboo fibre is a new kind of natural fibre from bamboo with machine method, and is in the frontier of bamboo timber processing, new composite and textile research field. The combing method is mainly one in processing bamboo fiber. The plasticity and strength of bamboo were required in processing bamboo fiber by machine method. The aim of this work is to explore changes of mechanical properties -lengthways tensile properties of bamboo after alkali treatment and focuses on a relation between lengthways tensile properties of alkali treated bamboo and the moisture content. Mechanism of plastic deformation of bamboo was initially revealed in this paper. Experiment shows that: above fibre saturation point, the lengthways tensile strength and elasticity modulus of bamboo decreased with increase of moisture content with 2.5%NaOH soaking for 24h, and bamboo plasticity is better when its moisture content is between 40% and 90%. According to the principle of bamboo fiber processing by combing, the moisture content of bamboo should be controlled from 50% to 70% after 24h soaked with 2.5%NaOH. Bamboo fiber processing test shows that the conclusion is correct.

Swelling restraint of thermally modified ash wood perpendicular to the grain

The paper reports on creep of ash wood (Fraxinus excelsior L.) thermally modified at 180 and 200 °C, and subsequently subjected to compression in tangential and radial directions and simultaneously wetted, from the moisture content (MC) of 6% to above the fibre saturation point (FSP). The compressing load made 0.00, 0.25, 0.50 and 0.75 of impact stress at the proportional limit (Rc). The compression stress needed to restrain the swelling of wood, the so-called swelling pressure, was indirectly determined from isochrones of mechano-sorptive creep. The most important finding was that thermal modification reduces the strain of ash wood subjected to compression perpendicular to the grain to a degree proportional to the mass loss. The compression stress needed to restrain the swelling of thermally modified wood is ca. 10 and 20% smaller in the tangential and radial directions, respectively. This effect leads to a reduction in the anisotropy of swelling pressure of thermally modified wood perpendicular to the grain. Moreover, although upon thermal modification the mass loss of wood takes place, at the MC of 6% it shows practically the same modulus of elasticity (MOE) and Rc as the unmodified wood. After wetting to MC higher than the FSP, the thermally modified wood at 200 °C shows significantly higher MOE and Rc than the wood modified at 180 °C and untreated wood. Reduction of wood hygroscopicity, an inevitable effect of thermal modification, also reduces the range of changes in mechanical properties of wood caused by the increase in its MC to the FSP.

Development and Validation of a Simulation Model for the Temperature Field during High-Frequency Heating of Wood

In the process of applying high-frequency heating technology to wood drying, controlling the material temperature affects both drying speed and drying quality. Therefore, research on the heat transfer mechanism of high-frequency heating of wood is of great significance. To study the heat transfer mechanism of high-frequency heating, the finite element method was used to establish and solve the wood high-frequency heating model, and experimental verification was carried out. With a decrease in moisture content, the heating rate decreased, then increased, and then decreased again. There was no obvious linear relationship between the moisture content and heating rate; the simulation accuracy of the heating rate was higher in the early and later drying stages and slightly lower near the fiber saturation point. For the central section temperature distribution, the simulation and actual measurement results matched poorly in the early drying stage because the model did not fully consider the differences in the moisture content distribution of the actual test materials. In the later drying stage, the moisture content distribution of the test materials became uniform, which was consistent with the model assumptions. Considering the changes in heating rate and temperature distribution, the accuracy of the model is good under the fiber saturation point, and it can be used to predict the high-frequency heating process of wood.

An Approach for Computing the Heat Sources in Logs Subjected to Freezing

Abstract This study suggests an approach for computing the specific energies of the internal heat sources in logs subjected to freezing. The approach maximally considers the physics of the freezing processes of both the free and the bound water in wood. It reflects the influence on the mentioned energies of the wood density above and below the hygroscopic range. It also considers the icing degrees formed separately by both the free and bound water in the logs, as well as the influence of the fiber saturation point of each wood species on its respective amount of non-frozen water depending on temperatures below 272.15 K. Mathematical descriptions of the specific heat energies Qv-fw and Qv-bw released in logs during free water freezing in the range from 0 °C to −1 °C and of the bound water below –1 °C, respectively, have been executed. These descriptions are introduced in own 2D non-linear mathematical model of the freezing process of logs. For the solution of the model and computation of the energies Qv-fw and Qv-bw, a software program based on the suggested approach and mathematical descriptions was prepared in FORTRAN, which was input into the calculation environment of Visual Fortran. With the aid of the program, computations were completed to determine the energies Qv-fw and Qv-bw and their sum, Qv-total of a beech log subjected to freezing. The beech log had a diameter of 0.24 m, a length of 0.48 m, an initial temperature of 20.5 °C, a basic density of 683 kg·m−3, and a moisture content of 0.48 kg·kg–1 during its 30 hours in a freezer at approximately −30 °C.

Axial tensile properties and flexibility characteristics of elementary units from multidimensional bamboo-based composites: radial and tangential moso bamboo slivers

Abstract Bamboo sliver (BS) is a potential elementary unit for multidimensional bamboo-based composites (MBBCs). Axial tensile and flexibility characteristics of thin radial (R) and tangential (T) BSs (BSR and BST with a thickness of 0.5–2.0 mm) have been studied. Axial tensile strength (AxTS) and modulus of elasticity (AxMOE) were positively correlated with the vascular tissue ratio (VTR), and the tensile properties of slices with 43–51% VTR increased more rapidly than in the VTR range of 20–36%. In axial tensile tests, cracks propagated along a V or Z route in BSR, while fibers were pulled out in the central section in case of BST. AxTS and AxMOE values increased with decreasing moisture content (MC) at 6.2–16.6% below 75°C. Flexural flexibility (FF) was enhanced with increasing MC and VTR, and the effect of MC on FF was amplified with MCs below the fiber saturation point (FSP). In situ scanning electron microscope (SEM) observation during flexural tests revealed a deformation resistance of vascular tissue (VT) and the deformation disposition of parenchyma.

Influence of processing parameters and wood properties on the edge gluing of green Eucalyptus grandis with a one-component PUR adhesive

The predominant plantation hardwood species in South Africa Eucalyptus grandis is, despite relatively good strength and stiffness properties, seldom used for lumber production due to low dimensional stability, splitting, and cracking of the wood. As critical defects often develop or aggravate during the drying process, the edge gluing of Eucalyptus grandis lumber in its wet state, before kiln-drying, was considered a potential inhibiting factor for this behaviour. The objective of this study was to determine how certain material and processing variables influence the bond quality of unseasoned, edge-glued, Eucalyptus grandis, using a moisture-curing one-component polyurethane adhesive. The study considered the effect of four parameters, namely wood density, moisture content, adhesive spread rate and pressure, on the shear strength of bondlines. Micro computed tomography scanning was used to observe adhesive penetration behaviour. Overall, the bonding quality was satisfactory proving the feasibility of edge gluing of this wood in the wet state. Multiple ANOVA showed that generally better results were obtained for samples with a higher moisture content of roughly 60% compared to specimens with lower moisture content around fibre saturation point. Results indicated that the lower spread rate tested (150 g m−2) should be preferred to the higher one (250 g m−2), since it will likely give more stable shear strength results and it would be preferable from an economic point of view. Increasing pressure did not increase the shear strength significantly.

Influence of timber moisture content on wave time-of-flight and longitudinal natural frequency in coniferous species for different instruments

AbstractNon-destructive techniques (NDTs) are well suited for rapid estimation of timber properties, but NDT results are affected by several factors, the most important of which is the moisture content (MC) of wood. Much of the research in this context was limited to ultrasound measurement of a few wood species, mainly to Norway spruce. The present paper investigates the MC influence on the NDT results obtained by instruments based on ultrasound (two devices), impact stress waves (one device) and longitudinal vibrations (two devices). A hundred large cross-section specimens of four timber species were tested, namely: radiata pine, Scots pine, Salzmann pine and maritime pine. The influence of MC on velocity was found to be stronger below the fiber saturation point (FSP) than above FSP. MC adjustment factors below FSP are proposed for these wood species.

Investigating the water vapor sorption behavior of bamboo with two sorption models

Studying the hygroscopic behavior of bamboo is important for industrial applications because it influences the dimensional stability and mechanical properties of the final bamboo products. In this study, the water vapor sorption behavior of 14 bamboo species was investigated using a dynamic vapor sorption apparatus and the results were analyzed using the Guggenheim–Anderson–de Boer (GAB) and Hailwood–Horrobin (H–H) models. The different bamboo species exhibited varying sorption isotherms and degrees of hysteresis. The reasons for this may be related to the different sorption isotherms of the two main cell units of bamboo, namely the fiber and parenchyma cells, and the chemical composition of bamboo. The GAB and H–H models provided good fits to the experimental data and meaningful physical parameters regarding the monolayer capacity, especially the water accessible specific surface area can be obtained from the GAB parameters. The fiber saturation point values were also determined using the GAB and H–H models, which ranged from 16.37 to 27.91% for the different bamboo species.

Influence of pressing parameters on dimensional stability and density of compressed beech wood

European beech (Fagus sylvatica) wood specimens were subjected to thermo-mechanical densification, and the influence of pressing parameters on dimensional stability, mean density and density profiles was evaluated. The specimens with two initial moisture contents, 18% and the fibre saturation point, were compressed by 20 and 40% at temperatures of 160, 180, 200, and 220 °C for several times, specified in the Methods, ranging from 6 to 12 min. Dimensional changes were observed immediately after the removal from the hot press and subsequent conditioning (20 °C, 65% relative humidity). The properties of the compressed beech wood were influenced by all the pressing parameters. This influence, however, varied based on the factor and property. The analysis showed as most appropriate: an initial moisture content ranging from 17 to 20%, a pressing temperature ≥ 180 °C and pressing time ≥ 10 min for the given thickness. As for the compression degree, it is necessary to consider the purpose of compression and the wood properties required.

Influence of moisture content on the results of penetration and withdrawal resistance measurements on softwoods

Abstract Needle penetration resistance (NPR) and screw withdrawal resistance (SWR) are widely used techniques for density estimation of woods integrated in timber structures. The moisture content (MC) influences these measurements and correction coefficients are needed to ensure the accuracy of results. The goal of the present paper was to scrutinize the relation between NPR and SWR measurements and MC in case of radiata pine, Scots pine, Salzmann pine, and maritime pine usually used in wood constructions, from which 25 specimens from each species were probed. The specimen’s MC ranged from 65.1 to 8.3%. Results show that NPR depth has a positive linear relationship with MC while the SWR force a negative one below the fiber saturation point (FSP). Above the FSP, the MC influence is less pronounced and less regular. MC correction factors of measurements below the FSP are proposed for the species studied.

Evaluation of compressive strength characteristics of structural-sized Apa (<i>Afzelia bipindensis</i>) and Opon (<i>Lannea schimperi</i>) timber species columns found in Nigeria

The work centers on the compressive strength characteristics of Nigerian Apa (Afzelia bipindensis) and Opon (Lannea schimperi) timber species columns of nominal lengths 200, 400, 600 and 800mm and a nominal width and thickness of 50mm by 50mm. The data revealed that, Apa and Opon have an average density of 652.74 and 472.60kg/m3 respectively. The mean moisture content (MC) of both species were less than the fibre saturation point (FSP) recommended value of 25-30% and the average strength at yield of Apa and Opon are 35.65 and 14.00N/mm2. The derived continuous equations for design of Apa column and Opon column are σ = 47.882-0.009λ and σ =17.211-0.007λ respectively. The results of the reliability analysis show that Apa and Opon timber species have reliability index of 0.64 and 0.65 respectively for a service life of 50 years, assuming other serviceability conditions are met. This design procedure is distinct and more effective than the usual procedure of classifying compression members as short, intermediate and long using their slenderness ratios. The paper therefore recommends the adoption of these equations for the design of compression members from these timber species in Nigeria.Keywords: Apa, Compressive strength, Opon, Reliability, Structural-sized

Properties of Eucalyptus umbra Wood for Timber Structures

This study involved the study of physical and mechanical properties of Eucalyptus umbra wood for its use in timber structures. Two physical and fourteen mechanical properties tests were performed according to Brazilian code ABNT NBR 7190:1997. Two moisture contents were considered: above the Fiber Saturation Point (30%) and standardized point 12%. Decreasing the moisture content, this study presented an increase in eight mechanical properties. Oppositely, five mechanical properties presented decreasing of values, and one single variable persisted stable. But statistical perspective supported by the t-test, only three mechanical properties (ft90, fH0 and W) increased significantly. According to results obtained in this study, it is possible to use Eucalyptus umbra wood as raw material for structures.

The Effect of Full-Cell Impregnation of Pine Wood (Pinus sylvestris L.) on Changes in Electrical Resistance and on the Accuracy of Moisture Content Measurement Using Resistance Meters

The impact of the full-cell impregnation of pine wood was investigated with respect to changes in electrical resistance and the accuracy of moisture content measurement. This study compared the resistance of impregnated and untreated pine timber harvested from the northern part of Poland (Pomeranian region). The wood was impregnated by the vacuum-pressure method. The preservative (TANALITH E 3475) and coloring (TANATONE 3950) agents were based on copper salts. The results showed a dependence of wood resistance as a function of the moisture content. Impregnated and not treated wood samples were used. This result reflects the greater conductivity of the impregnate solution (based on copper salt) than the water. This phenomenon became more distinctive as moisture content value was above the Fiber Saturation Point (FSP).

Tribological Behavior of Microtextured Cemented Carbide in Contact with Wood

Abstract We used microtextures to develop a novel antifriction technology for wood cutting tools. Northeast China ash (Fraxinus spp.) was selected as the research species, and the influence of the microtexture on the friction coefficient of cemented carbide samples with and without the texture was studied using various loads and wood moisture content. The results showed that a reasonable microtexture form (d = 40 μm) can effectively reduce the friction coefficient between wood and cemented carbide regardless of the moisture content. The moisture in the wood is conducive to reducing the friction coefficient between the cemented carbide and the wood surface. Its mechanism of action is affected by the wood moisture content, the load used, and other factors. The friction coefficient significantly decreased when the wood moisture content was above the fiber saturation point.