Acoustic emission related to strain energy during drying of Eucalyptus regnans boards

Summary During timber drying, surface differential shrinkage within a board causes a high surface tensile stress and associated instantaneous strain. Acoustic Emission (AE) is generated when localised irreversible stress release events act to reduce the strain energy imparted to the material. A nonlinear one-dimensional drying model was used to calculate through-thickness moisture, stress and strain profiles during drying. The surface stress and instantaneous strain were used to calculate the strain energy at the surface. AE was measured during drying and the relationship between strain energy and the cumulative count (total ringdown counts) was investigated. The cumulative count is related to the unrecoverable strain energy rather than the elastic or recoverable strain energy. The cumulative count is not a useful measure of the propensity for surface checking. However the peak AE rate values are closely related to the surface instantaneous strain.Symbols E Young's Modulus (GPa) - instantaneous strain - _m mechano-sorptive effect - _c creep strain - _u unconfined shrinkage - _n net shrinkage - _o set (residual) strain - _pl proportional limit strain - W_in strain energy (kJ/m³) - U_u unrecoverable strain energy (kJ/m³) - U_r recoverable strain energy (kJ/m³) - W _in ^* nondimensional strain energy - U _u ^* nondimensional unrecoverable strain energy - U _r ^* nondimensional recoverable strain energy - stress (MPa) The authors are pleased to acknowledge the assistance of Emeritus Professor A. R. Oliver, University of Tasmania, the Australian Furniture Research and Development Institute and the Tasmanian Timber Promotion Board.     

Collapse and internal checking in the latewood of Eucalyptus regnans F.Muell

Summary Tangential shrinkage was measured on longitudinal-tangential slices of separate earlywood and latewood from one board of Eucalyptus regnans F.Muell. at various temperatures. Large amounts of collapse shrinkage were measured in the latewood slices, and lesser amounts in the earlywood slices. Collapse shrinkage in the earlywood was found only when the slices were dried at temperatures above a minimum temperature (the collapse threshold temperature). End-coated board sections approximately 200 mm long were rapidly dried at dry-bulb temperatures below the collapse threshold temperature for earlywood. Incipient internal checks were found in the latewood of these boards. Board sections dried at higher temperatures showed internal checks starting in both early and latewood. A non-linear drying simulation model was modified to take the heterogeneous nature of wood into account. This model predicted that internal checks would form in this wood even if it were dried sufficiently slowly to avoid surface checking.Symbols D Diffusion coefficient - e_c Creep strain - e_i Instantaneous strain - e_m Mechano-sorptive strain - e_n Net strain - e_u Unconfined shrinkage strain - EW Earlywood - FSP Fibre saturation point - LW Latewood - L-T Longitudinal-tangential slice - MC Moisture content (kg water/kg dry wood) - q Moisture concentration - R-T Radial-tangential slice - t Time - y Depth in board The author is pleased to acknowledge the assistance of Emeritus Professor A. R. Oliver, Associate Professor P. E. Doe, University of Tasmania, and the Australian Furniture Research and Development Institute     

The applicability of a color acetate film for estimating photosynthetic photon flux density in a forest understory

The suitability of a color acetate film for estimating photosynthetic photon flux density (PPFD) in a forest understory was examined. The fading ratio of the film (F), the total PPFD (PPFD_total) to which the film was exposed, and the average daily maximum temperature during exposure (T) were obtained from measurements at multiple sampling points throughout an entire year within a natural secondary forest (n = 42). The ranges of the recorded values were as follows: F 35%–99%, PPFD_total 1.4–28.3molm^–2, and T 6°–32°C. PPFD_total was regressed by F and T with a high r ² (=0.94; P < 0.0001): PPFD_total = (100 – F)/(1.085 + 0.051T). The absolute error (|estimated PPFD_total – measured PPFD_total|) averaged 1.3molm^–2 with a maximum of 5.7molm^–2, indicating a good fit. These results indicated broad applicability of the film, both spatially and temporally, for estimating forest understory PPFD.     

Wood drying and Fick's second law

Summary The diffusion equation (sometimes referred to as Fick's second law) is derived in terms of water movement under the action of capillary forces. The mass diffusivity is thereby expressed in terms of the capillary diffusion coefficient. A numerical calculation is given for yellow poplar.Notations C diffusion coefficient for water in wood with capillary pressure as the driving force, kg/msPa - D diffusion coefficient for water in wood with moisture content as the driving force, kg/ms - F mass flux, kg/m²s - p_c capillary pressure, Pa - p_cf capillary pressure extrapolated linearly to fibre saturation, Pa - T absolute temperature, K - t time, s - x distance ordinale in the direction of flow, m - mass diffusivity, m²/s - density of liquid water, kg/m³ - _g basic density (dry mass/green volume), kg/m³ - _w density of wood substance, kg/m³ - moisture content of wood - _cls moisture content at continuous liquid saturation - _cs moisture content at complete saturation - _f moisture content at fibre saturation     

On the activation energy of diffusion of water in wood

Summary The diffusion equation for water in wood is expanded in terms of temperature and moisture gradient on the assumption that the driving force for the diffusion of water in wood is the partial pressure of water vapour. An analytic expression is then developed for the activation energy of diffusion in terms of enthalpy and entropy changes associated with the sorption process. The expression is compared with another published curve and some similarity was observed.Symbols C water concentration, kg/m³ - D diffusion coefficient for water vapour in wood with vapour pressure as the driving potential, kg/ms Pa - D_c diffusion coefficient for water vapour in wood with water concentration as the driving potential, m²/s - D_c a constant value of D_c, m²/s - E activation energy of diffusion, J/kg - F flow density, kg/m² s - f h/l - h specific enthalpy, J/kg - L l/R T - l latent heat of vapourization of free water, J/kg - l_s latent heat of vapourization of sorbed water, J/kg - p partial pressure of water vapour, Pa - p_s pressure of water vapour at saturation, Pa - R specifc gas constant for water, J/kg K - r relative humidity - s specific entropy, J/kg K - w dry basis moisture content - x length coordinate, m - a constant temperature equal to 6,800 K - -/ln r - _w density of wood (dry mass/moisture volume) at a given moisture content, kg/m³ - s/R - L style as 2 lines above - free water relative to sorbed water The author is grateful to the Editorial Board in relation to the use of (4)     

Sound absorption capability and mechanical properties of a composite rice hull and sawdust board

Rice hull–sawdust composite boards were manufactured for sound-absorbing boards in construction. The manufacturing parameters were target density (400, 500, 600, and 700?kg/m³) and rice hull content as percent weight of rice hull/sawdust/phenol resin (10/80/10, 20/70/10, 30/60/10, and 40/50/10). Commercial gypsum board and fiberboard were also used as comparative sound-absorbing materials. The average modulus of rupture (MOR) of the board with a density of 700?kg/m³ and rice hull mixing ratio of 10% was 8.6?MPa, and that of the board with a 400?kg/m³ board density and a rice hull mixing ratio of 40% was 2.2?MPa. The MOR increased with increasing board density or decreasing rice hull mixing ratio. The sound absorption coefficients of some boards (400?kg/m³ and 10%, 500?kg/m³ and 30%, and 500?kg/m³ and 40%) were better than those of the commercial 11-mm-thick gypsum board. Thus, it is concluded that rice hull–sawdust composite boards may be implemented as sound-absorbing barriers in construction due to their high sound absorption coefficients.     

Interception of photosynthetic photon flux density in a mangrove stand of <Emphasis Type="Italic">Kandelia candel</Emphasis> (L.) Druce

The canopy structure and interception of photosynthetic photon flux density (PPFD) in a 10-year-old Kandelia candel (L.) Druce stand were investigated before and after artificial defoliation. Leaf and wood areas for different layers were measured through area–weight relationships of subsamples. PPFD was measured at specified heights before and after leaf clipping. The leaf area index (LAI) and wood area index (WAI) were 4.501m²m^–2 and 1.412m²m^–2, respectively. There was a strong linear relationship between the cumulative wood area © and leaf area (F) densities from the top down to a given depth of the canopy, C = aF (r ² = 0.950), with a proportional constant a of 0.096 ± 0.008 (mean ± SE). The PPFD relative to that above the canopy (relative PPFD; I _R) at a given depth of the canopy was assumed to be given by the equation I _R = e^–(KCC+KFF ) = e^–KF , where the apparent light extinction coefficient K (= K _F + aK _C , where K _F and K _C are respectively the light extinction coefficient of leaves and woody organs) was calculated to be 0.502 ± 0.041 (mean ± SE) m^–2m² before leaf clipping. After leaf clipping, I _RC = e^–KCC is satisfied. As a result, the value of K _C was estimated to be 0.785 ± 0.046 (mean ± SE) m^–2m². The light extinction coefficient of leaves K _F was calculated to be 0.427m^–2m² using the indirect method, K _F = K – aK _C, and 0.432 ± 0.026 (mean ± SE) m^–2m² using the direct method, I _R/I _RC = e^–KFF . Of the total PPFD intercepted by the canopy, the fraction K _F/K due to leaves alone was estimated to be 85.0%–86.1% and the rest was contributed by woody organs.     

An experimental assessment of driving forces for drying in hardwoods

Summary An investigation has been carried out into whether the internal moisture movement inside Australian hardwood timber is best described by a diffusion model with driving forces based on gradients in moisture content or in partial pressure of water vapour. Experimental data from two sets of drying schedules applied to timber from three species of Australian hardwoods (yellow stringybark, spotted gum and ironbark) reported in Langrish et al. (1997) have been used to assess the use of the two driving forces, and the standard error has been used as the criterion for goodness of fit. Moisture-content driving forces have fitted the data better than a model based on vapour-pressure driving forces alone. The use of moisture-content driving forces with diffusion parameters obtained from data from one drying schedule is also better in predicting the drying behaviour with another schedule than vapour-pressure driving forces for yellow stringybark and ironbark. These results may be due to the complexity of the moisture-movement process through timber, with more than one moisture-transport mechanism being active, so that the use of only one driving force for moisture movement is at best only an approximation to the true behaviour.Symbols D diffusion coefficient, m² s^–1 (moisture-content gradient), m³ s kg^–1 (vapour-pressure gradient) - D_e activation energy, K - D_r pre-exponential factor m² s^–1 (moisture-content gradient), m³ kg^–1 (vapour-pressure gradient) - J mass flux of water divided by density, m s^–1 - t time, s - x position, m - X moisture content, kg kg^–1 This work has been supported by the Australian Research Council, the Ian Potter and George Alexander Foundations, and The University of Sydney Research Grant Scheme.     

Modeling internal temperature changes of timber poles during ACA treatment

Summary The temperature-time-location relationships during steam conditioning and pressure treatment of timber poles using ammoniacal copper arsenate (ACA) have been studied and a new mathematical model that incorporates both the thermal properties of the poles and the parameters of the treatment process is discussed. Prediction equations and charts are presented that show the minimum required steaming time to satisfy the 1982 Rural Electrification Authority (REA) purchase specification, i.e. a center temperature above 150 °F (65.5 °C) for 2 hours. A six hour steaming time, commonly used for ACA treatment, has been found to be too short to bring poles with diameters larger than about 40 cm to the required sterilization conditions. Therefore longer steaming times, predicted using the methods given here, are recommended. The temperature of the preservative used does not appear to be a major factor in determining the maximum temperature achieved at the center of a pole, but it can influence the length of time the pole is above 65.5 °C.Symbols D pole diameter, cm - k₁ rate of surface temperature change during vacuuming, °C/h - K₂ rate of surface temperature change early in the pressure period, °C/h - J₀ Bessel function of the first kind of order zero - J₁ Bessel function of the first kind of order one - r radial location in a pole measured from the center, cm - R pole radius, cm - T interior temperature of a pole during preservative treatment, °C - T_f final preservative temperature, °C - T_mid average of the steam and the final preservative temperatures, °C - t₀ initial temperature of a pole, °C - T_s temperature of the steam used for conditioning the poles, °C - T₁ surface temperature at the end of the vacuum period, °C - t time measured from the opening of the steam valve for steaming, h - t_above time the center of a pole remained above 65.5°C during preservatives treatment, h - t_c1 total time required for the surface temperature to reach T_f, °C - t_pr time elapsed at which initial vacuuming stops and pressure is applied, h - t_stm minimum required steaming time used, h - t_vc time elapsed at which initial vacuuming starts, h - thermal diffusivity, cm²/h - _n Roots of a particular characteristic equation - _d the time delay period between initial flow of steam and surface temperature response, h     

Stress model of a wood fibre in relation to collapse

A wood fibre cell from a Tasmanian Eucalypt is typically cylindrical in shape with a length to diameter ratio of approximately 501. Early in the process of seasoning for solid timber, when the fibre lumens are still saturated, internal tension within a fibre can rise to a value high enough to cause it to physically flatten, or collapse. A stress model of a fibre cell has been developed which predicts the stress and strain distributions within the fibre wall as a function of temperature, moisture content, and fibre wall strength properties and size in the early stages of drying. This model will be used together with measurement of the behaviour of collapse prone timbers to determine conditions which will avoid collapse during seasoning.The author is pleased to acknowledge the assistance of Emeritus Professor A. R. Oliver, Associate Professor P. E. Doe, University of Tasmania, and the Australian Furniture Research and Development Institute     

On movement of water through wood — The diffusion coefficient

Summary It is demonstrated that there can be only one driving potential for the movement of water through wood and this will be a function of wood state. On the assumption that the driving potential is the partial pressure of water vapour, a theoretical expression is derived for the diffusion coefficient. Such expression is fitted to diffusion coefficients for Scots pine and a remarkably good fit is obtained.Symbols a reciprocal mean radius of curvature of a capillary meniscus; also taken to be the radius of the corresponding exposed liquid surface, m - b spacing between flow paths in the cell wall, m - D diffusion coefficient for water in wood with vapour pressure as the driving potential, kg/ms Pa - D_a diffusion coefficient for water vapour through air, kg/ms Pa - D diffusion coefficient for water in wood with the driving potential - D diffusion coefficient for water in wood with the driving potential - D₀ diffusion coefficient for water in wood with vapour pressure as the driving potential, which is associated with leakage paths through the wood, kg/ms Pa - D_f diffusion coefficient for water in wood with vapour pressure as the driving potential, corresponding to fibre saturation and with no leakage paths, kg/ms Pa - D_c diffusion coefficient for water in wood with vapour pressure as the driving potential, which is associated with the constriction of the vapour flow as it approaches the cell wall, kg/ms Pa - D diffusion coefficient for water in wood with moisture content as the driving potential, kg/ms - diffusivity for water vapour in air, m²/s - F flux of water, kg/m² s - p partial pressure of water vapour, Pa - R specific gas constant for water, J/kg K - r fractional relative humidity - T temperature, K - x length coordinate in direction of flow, m - the dimensionless ratio D_f/D_c evaluated at r=1/e - arbitrary driving potential for movement of water in wood - cell spacing in the direction of water flux, m - density of liquid water, kg/m³ - coefficient of surface tension, N/m - arbitrary driving potential for movement of water in wood - fractional moisture content     

Applications of the capillary isotherm

The effect of temperature on the capillary isotherm is accounted for in a modified derivation. Some new equilibrium moisture content data for E. regnans are presented and fitted by the capillary isotherm. Some earlier data for Klinki pine are also fitted. It is shown precisely how reductions in the shear modulus of the cell wall material with increasing temperature give rise to reductions in equilibrium moisture content for a given relative humidity.Symbols A G₀/R, K - a₁ external radius of annulus, m - a₂ internal radius of annulus, m - a_f a₂ at fibre saturation, m - a a constant length, m - B a constant of integration - b₁, b₂ temperature parameters, K^1- - G rigidity of wood substance, Pa - G₀ G for dry wood, Pa - G_f G at fibre saturation, Pa - h isosteric heat, J/kg - latent heat, J/kg - p capillary pressure, Pa - P_s pressure of water vapour at saturation, Pa - R specific gas constant for water, J/kg K - r relative humidity - r_i inflection intercept - r_t tangent intercept - T temperature, K - t temperature, °C - X see equation (18) - x see equation (28) - , , ₁, ₁ coefficients, equations (27), (37) - y₁, y₂ see equations (25), (26), K - parameter, equation (9) - parameter, equation (33) - density of water, kg/m³ - _W density of wood substance, kg/m³ - equilibrium moisture content - _0.2 at r = 0.2 - _0.5 at r = 0.5 - _0.9 at r = 0.9 - _f at fibre saturation     

Field measurements of gas exchange in Avicennia marina and Bruguiera gymnorrhiza

Diurnal gas exchange characteristics were measured simultaneously in two mangrove species, Avicennia marina and Bruguiera gymnorrhiza, over 7 d in summer (February–March), to compare their productivity. The study was undertaken in the Beachwood Mangroves Nature Reserve, Durban, South Africa, using fully expanded leaves of young and mature trees at the top of the canopy. Gas exchange was strongly influenced by photosynthetic photon flux density (PPFD), leaf temperature and the accompanying leaf to air vapour pressure deficit ( w). Carbon dioxide exchange was saturated at a PPFD of about 600 mol m^-2s^-1 in B. gymnorrhiza compared to 800 mol m^-2s^-1 in A. marina. Maximal CO₂ exchange occurred between 12h00 and 14h00 and was consistently greater in A. marina (8.8 mol m^-2s^-1) than in B. gymnorrhiza (5.3 mu;mol m^-2s^-1). Mean internal CO₂ concentrations ( c_i) were 260 l l^-1 in A. marina and 252 l l^-1 in B. gymnorrhiza. Photorespiratory activity was 32% in A. marina and 30% in B. gymnorrhiza. Mean water use efficiency (WUE) was 8.0 mol mmol^-1 in A. marina and 10.6 mol mmol^-1 in B. gymnorrhiza. Diurnal leaf water potentials ranged from –0.8 to –3.5 MPa and were generally lower in A. marina.     

Process of absorption and desorption of water in a wood board,with 3-dimensional transport beyond the FSP

Summary The process of absorption of water in a piece of solid wood, as well as the following stage of desorption is studied, when the water content is beyond the fiber saturation point. A model based on a numerical method with finite differences is built and successfully tested. This model takes into account a 3-dimensional transport of water controlled by diffusion, with three principal axes of diffusion and three various principal diffusivities. The model is able to predict the kinetics of absorption or desorption when the three principal diffusivities are known, as well as the operational conditions.Symbols C (i, j, k) Concentration at the position of the coordinates i, j, k and at the time t - CN (i, j, k) Concentration at the same position, at the time t + t - C_s Moisture content at the surface - D_L Longitudinal diffusivity - D_T Tangential diffusivity - D_R Radial diffusivity - L Increment of space along the longitudinal axis - T Increment of space along the tangential axis - R Increment of space along the radial axis - t Increment of time - EMC or C_eq Moisture concentration at equilibrium - K Factor of evaporation (cm/s) - L, T, R Dimensions of the board along the longitudinal, tangential, radial directions, respectively - M_L Dimensionless number for the longitudinal axis - M_T Dimensionless number for the tangential axis - M_R Dimensionless number for the radial axis - Th Thickness of a sheet (in Eq. 15)     

Medical CAT-scanning: X-ray absorption coefficients,CT-numbers and their relation to wood density

Summary This paper describes how X-ray absorption coefficients and CT-number in medical CAT-scanning can be calculated for dry and wet wood. A comparison with earlier recorded data for dry wood showed that the deviation between calculated and measured CT-numbers was not significant. Linear regression showed that wood density could be measured with an accuracy of ±4 kg/m³. Wood having the same green density but containing different amounts of water have different absorption coefficients and CT-numbers. A linear relationship between CT-numbers and density of wood containing water was developed. Wood density could be measured with an accuracy of ±13.4 kg/m³.     

Using near-infrared hyperspectral images on subalpine fir board. Part 2: Density and basic specific gravity estimation

Abstract

Wood density (ρ_MC) and basic specific gravity (BSG) are important properties in several forest products manufacturing processes. In this study, near-infrared hyperspectral images were tested to produce two-dimensional (2D) ρ_MC and BSG images of subalpine fir (Abies lasiocarpa Hook) board. A total of 107 cubic samples with the size of 4 cm were prepared from 14 boards. All samples were dried to various moisture contents (MCs) during several steps until being completely dried. The resulting MCs ranged from 1% to 137% (dry basis). After the last drying step, the samples were soaked in water to determine BSG. Hyperspectral images and weight measurements were acquired over each sample at each drying step. ρ_MC was also estimated at each MC level. Partial least squares (PLS) models were developed for estimating both ρ_MC and BSG from the near-infrared hyperspectral imaging (NIR-HSI) system absorbance spectra acquired over all the samples during each drying step. The ρ_MC model provides a reasonable accuracy with the validation data-set (R² = 0.81, RMSE = 39 kg/m³, and RPD = 2.3). For BSG, only models built with samples having MC of less than 12% are significant. The calibration data-set provides similar accuracy as the ρ_MC model (RMSE = 0.004, R² = 0.82, and RPD = 2.28), but the accuracy is lower with the validation data-set (RMSE = 0.007, R² = 0.53, and RPD = 1.39). Our data-set has BSG values varying only from 0.326 to 0.374, and further work is needed to apply these methods to a data-set that includes a more extended range of BSG variations for improving estimation accuracy.     

Shrinkage differential as a measure for drying stress determination

Summary A new approach for continuous and non-destructive drying strain measurement was formulated. It was based on the hypothesis that the effect of drying stress on shrinkage decreases with a decrease in board width. Tests performed at 30°C on tangentially sawn E. grandis specimens provided sufficient evidence to support the hypothesis and to merit its further investigation.     

Density measurements in Pinus sylvestris sawlogs combining X-ray and three-dimensional scanning

Abstract

Wood density is an important quality variable, closely related to the mechanical properties of the wood. Precise wood density measurements in the log sorting would enable density sorting of logs for products such as strength-graded wood and finger-jointed wood. Density sorting of logs would also give more homogeneous drying properties and thus improve the quality of the final products. By compensating the radiographs from an X-ray log scanner for the varying path lengths using outer shape data from a three-dimensional (3D) scanner, it is possible to make precise estimates of both green and dry density. Measurements on simulated industrial data were compared with densities measured in computed tomographic (CT) images for 560 Scots pine (Pinus sylvestris L.) logs. It was found that green sapwood density could be measured with predictability R ²=0.65 and root mean square error (RMSE) of 25 kg m^?3. Green and dry heartwood densities were measured with similar precision: R ²=0.79 and RMSE=32 kg m^?3 for green density and R ²=0.83 and RMSE=32 kg m^?3 for dry density.     

HTST hydrolysis of wood: Modelling of the kinetics and projections on reactor configuration

Summary We present experimental data on hydrolysis of wood in high temperature short residence time (HTST) and low acid concentration conditions. Effects of temperature, acid concentration, particle size and liquid/solid ratio are discussed. A kinetic model is proposed which accounts for effects of temperature and acid concentration. This kinetic model is used to predict performance of a twin-screw extruder as a hydrolyser which consists of ideal mixed flow or plug flow reactor units in series.Symbols A Acid concentration in liquid phase - A Acid concentration in solid phase - A₀ Initial mass of sulphuric acid, g - C Cellulose content of solid phase, % - d Diameter of wood particles, m - E₁ Activation energy of cellulose hydrolysis, cal. mol^-1 - E₂ Activation energy of glucose degradation, cal. mol^-1 - F Objective function, refers to Eq. (5) - G Glucose yield - G_e Glucose yield at equilibrium - G_{i, exp} Experimental glucose yield (Eq. (5)) - G_{i, th} Calculated glucose yield (Eq. (5)) - G_max Maximum glucose yield - k^* Parameter defined by Eq. (9) - k₁ Rate constant of cellulose hydrolysis, s^-1 - k₂ Rate constant of glucose degradation, s^-1 - k ₁ ^* Apparent rate constant of cellulose hydrolysis, s^-1 - k ₂ ^* Apparent rate constant of glucose degradation, s^-1 - k₁₀ Pre-exponential factor of constant k₁, s^-1 - k₂₀ Pre-exponential factor of constant k₂, s^-1 - K Parameter defined in Table 3 - m Constant - m_g Mass of glucose produced, g - M₀ Initial mass of wood, g - M Mass of saturated steam delivered, g - M Mass of saturated steam delivered after 120 s of reaction time, g - m₀ Initial mass of water, g - n Constant - N Number of reactor units - q_i Volume flow rate in reactor units, m³ · s^-1 - r_g Conversion rate of glucose, s^-1 - R Ideal gas constant, 1.987 cal · mol^-1 K^-1 - t Reaction time, s - T Temperature, K - V_i Volume of reactor units, m³ - W Water content of wood sample, % - X, X Parameters defined in Table 3 - Y, Z Parameters defined in Table 3 - Constant defined in Eq. (4), s^-1 - v Number of experimental points (Eq. (5)) - _i Residence time in plug flow unit, s - _i Residence time in mixed flow unit, s