Variation of the dynamic elastic modulus and wave velocity in the fibre direction with other properties during the drying of Eucalyptus regnans F. Muell

Dynamic elastic modulus (E_L) and wave velocity (V) were determined using resonance vibrations from initially green, 100 × 50 mm sample boards of Eucalyptus regnans F. Muell., and after several stages of drying to oven dry. E_L and V were determined from impact induced vibrations and spectral analysis. E_L and V from green wood were positively related to basic density and normal shrinkage, only V was negatively related to green density, and both E_L and V were negatively related to green moisture content and the number of internal checks after drying. The latter relationship has the potential to provide a simple method of segregating highly check prone material. No significant relationships were obtained with collapse. Outside the hygroscopic range, in low shrinkage material, E_L increased little or gradually, while in high shrinkage, collapse prone material, it increased more rapidly, but no clear breakpoint was evident. In the hygroscopic range, E_L increased rapidly in all samples. V increased curvilinearly throughout the entire moisture range, but no difference between collapse prone and non-collapse prone material was observed. Received 16 February 1998     

Effect of pre-drying schedule ramping on collapse recovery and internal checking with Victorian Ash eucalypts

Matched sample boards from 20 quarter-sawn boards of Victorian Ash (Eucalyptus regnans F. Muell and E. delegatensis R.T. Baker) were dried using three different levels of ramped pre-drying schedules to investigate the effects of moisture gradients on collapse recovery and internal checking. Prior to reconditioning, most wet cores were found in highly collapsed boards with low density. Reducing the gradients in these boards is crucial for recovering collapse and closing internal checks. If time allows the boards to be equilibrated prior to steam reconditioning, a target mean moisture content of ≤20% with a moisture gradient of close to 5% (core to surface moisture content) is likely to recover slightly more collapse than targeting a mean moisture content close to ≥15%. However, if time or kiln restraints limit equilibration it is likely to be better to target a percentage moisture content of closer to 15% in order to ensure that the core to surface moisture gradients are below 8–10%. The slight reduction in collapse recovery with this second approach is less important than the possibility that collapse and internal checks in the centre of boards with wet cores will not be closed. Care needs to be used with this latter approach not to over-dry some boards, since moisture contents below 15% will progressively reduce collapse recovery. For boards within these moisture content guidelines, the application of heat, rather than moisture pick-up, appears to be the most important component of the steaming reconditioning process. Hence, steaming only needs to be undertaken for long enough to heat the core of the board close to the target temperature of 100°C. A simple method for estimating this heat-up time for different thicknesses and species was demonstrated based on a key dimensionless group for heat transfer, the heat-transfer Fourier number.     

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     

Effect of mean moisture content on the steam reconditioning of collapsed <Emphasis Type="Italic">Eucalyptus regnans</Emphasis>

Sixteen quarter-sawn boards (100 × 40 mm²) of regrowth Eucalyptus regnans (Mountain Ash) were conditioned to various moisture contents to investigate the effect of mean moisture content on collapse recovery. The results support the recommendation that boards should be reconditioned at a mean moisture content of between 15 and 20%. It is likely that the actual amount of collapse recovery was nearly as good for moisture contents up to about 25%. The main disadvantage with reconditioning boards with a moisture content of between 20 and 25% was the additional normal shrinkage that occurs because of the early reduction or removal of drying stresses. The samples in this study were dried under mild conditions for long periods of time to minimise the presence of moisture gradients.     

Radial variation of collapse,volumetric shrinkage,moisture conent and density in Eucalyptus regnans F. Muell.

Summary Examination of segmented increment cores from three trees of Eucalyptus regnans demonstrated radial variability as follows: collapse increased with distance from the periphery to about 85% of the radius where it commenced a precipitous decline towards the pith; total volumetric shrinkage increased to about 45% of the radius, gradually declined to 85% radius, then abruptly fell as with collapse; moisture content increased to about 75% radius and abruptly declined at 85–90%; basic density declined with distance from the periphery to about 75% radius, then showed a slight increase towards the pith; per cent saturation remained relatively constant from the periphery to about 90% of the radius, then abruptly declined. It is argued that compression failures in the fibre walls of the brittle heart core were responsible for the precipitous decline in collapse, moisture content and per cent saturation near the pith. Examination of data after separation by density into latewood and earlywood increments demonstrated that latewood tends to be more highly water saturated than earlywood. Similar levels of collapse for material of different densities and correspondingly different saturation values indicated that level of saturation was a more important determinant of collapse susceptibility in latewood than in earlywood.     

The distribution and interrelationship of collapse,volumetric shrinkage,moisture content and density in trees of Eucalyptus regnans F. Muell.

Summary The relationships among collapse, volumetric shrinkage, moisture content and basic density and their distribution within the stem were examined for trees of E. regnans. It was found that collapse and volumetric shrinkage each were significantly related to moisture content (positively), basic density (negatively), and (positively) to the derived values P (per cent of theoretical saturation) and Q (per cent of cell cavity volume containing water). Notwithstanding the inverse association evident between moisture content and density in the living tree, moisture content was shown to be independently related to each of collapse and volumetric shrinkage.Within the stem, collapse and volumetric shrinkage were negatively correlated with height in the tree although the significance of this relationship was qualified by the adjustment for other variables. Samples containing sapwood displayed collapse values significantly lower than heartwood samples. This was reflected in the presence of a positive relationship between collapse and distance from the periphery when all material was considered, but a negative relationship when sapwood-containing samples were omitted. Basic density was positively correlated with height in the tree and negatively correlated with distance from the periphery. Moisture content, P and Q were negatively correlated with both height in the tree and distance from the periphery although the latter relationship for moisture content was dependent on adjustment for density.The highly significant relationship between volumetric shrinkage and collapse suggested that the former could confidently be used in assessing collapse severity. Moisture content was determined to be an independent indirect indicator of collapse whereas basic density, P, Q and green density could all be of indicatory value, especially the last in view of its ease of measurement.     

The effects of within-species and between-species variation in wood density on the photodegradation depth profiles of sugi (Cryptomeria japonica) and hinoki (Chamaecyparis obtusa)

Low density wood is more rapidly eroded than denser wood when exposed to the weather, possibly because it is more susceptible to photodegradation. Fourier transform infrared microscopy was used to examine: (1) the depth of photodegradation in earlywood and latewood of sugi (Japanese cedar) and earlywood of hinoki (Japanese cypress) exposed for up to 1500 h to artificial sunlight emitted by a xenon lamp (375 W/m² within the 300 to 700 nm spectral range); and (2) the relationship between the density of wood tissues and depth of photodegradation. The depth of photodegradation varied between species (sugi and hinoki) as well as within a growth ring (sugi earlywood and latewood), and there was an inversely proportional relationship between depth of photodegradation and wood density. These findings may explain why low density earlywood is more rapidly eroded than latewood during weathering, and more generally, why there is an inverse relationship between the density of wood species and their rate of erosion during artificial and natural weathering. Part of this work was presented at the 54th Annual Meeting of the Japan Wood Research Society, Sapporo, August 2004     

Genetic control of intra-ring wood density variation in hybrid larch (Larix gmelinii var. japonica × L. kaempferi) F1

Genetic parameters for various wood density traits were estimated in 29-year-old trees of 18 full-sib families of hybrid larch (Larix gmelinii var. japonica × Larix kaempferi) F₁. Intra-ring density variation (IDV) was also evaluated using a model that expresses the pattern curve from earlywood to latewood as a power function. A high IDV indicates an abrupt change in wood density from earlywood to latewood. The ring width and wood density traits of individual rings were determined by X-ray densitometry. Overall wood density (RD) was shown to increase with increasing ring number, ranging from 0.42–0.59 g/cm³, whereas IDV of individual rings decreased gradually from pith outwards. Estimates of individual tree narrow-sense heritability of RD and IDV were 0.66 and 0.67, respectively. IDV showed negative genetic and phenotypic correlations with RD (r _g = −0.99, r _p = −0.72). The predicted genetic gains in latewood proportion and IDV were higher than that of RD. These results suggest that the intra-ring density variation is under moderate genetic control equivalent to wood density. The trend of increasing wood density from earlywood to latewood was associated with changes in both tracheid diameter and cell wall thickness.     

Shrinkage and collapse in thin sections and blocks of Tasmanian mountain ash regrowth

Summary Total collapse values from the green condition to various equilibrium moisture contents (EMC) were obtained by comparing shrinkage before reconditioning in matched blocks and sections of the wood of Eucalyptus regnans. It was shown that while collapse to 17% EMC comprised the largest portion of total collapse, a significant component, apparently caused by drying stress, developed below 17% EMC. Between 5% EMC and 0% moisture content a collapse recovery of some 1% occurred, seemingly because of changes in drying temperature. After reconditioning, collapse recovery was only partial and a high level of residual collapse remained. It was shown that collapse recovery was highest near the sapwood-heartwood boundary, while residual collapse was highest near the heartwood centre. While residual collapse was weakly negatively correlated with specific gravity, this relationship was not significant after adjustment was made for change in shrinkage after reconditioning in sections. All types of collapse were positively correlated with R-ratios calculated for blocks. However, relationships were not as well defined when R-ratios for sections were employed, the exception being for collapse below 17% EMC which was highly negatively correlated with R. The difference between shrinkage before reconditioning and shrinkage after reconditioning in sections, while totalling near zero, was positively correlated with specific gravity. It was demonstrated that this quantity could not constitute collapse in the traditional sense of collapse of the cell lumens. A possible association with moisture content of the material was discussed.     

Non-destructive evaluation of checking in thermally modified timber

Non-destructive measurement techniques were used to study the characteristics of thermally modified timber (TMT), especially checking. Two non-destructive measurement techniques were evaluated: air-coupled ultrasound (ACU) and electrical impedance spectroscopy (EIS). In the studied TMT (N = 38), density and pith location in the cross-section were positively correlated with the number of checks in the cross-section. Several ultrasound signal features correlated with TMT characteristics, most strongly with number of checks. Severely checked samples and other samples were considered as two different classes of checking, and the potential of ultrasound for differentiating between the two classes was tested by using Bayesian classification method. The correct classification rate was 97 %. EIS parameters correlated with density and latewood content of TMT more strongly than the ACU parameters. Thus, for determination of checking and density or earlywood/latewood content, both EIS and ACU measurements are recommended.     

Shrinkage and collapse in thin sections and blocks of Tasmanian mountain ash regrowth

Summary Shrinkage and moisture content measurements for thin sections of Eucalyptus regnans have shown that although total volumetric shrinkage was positively correlated with specific gravity, the relationship did not conform to the general equation s = f (Stamm 1935, 1952) in that the straight regression line was significantly different from a line passing through the origin. This contrasted with the relationship between shrinkage from 17% EMC to 0% moisture content and specific gravity which, in principle, complied with the Stamm formula. The discrepancy arose because shrinkage from the green condition to 17% EMC was negatively related to specific gravity owing to the high variability of the measured fibre saturation point and its negative correlation with specific gravity. These observations were affirmed in a comparison of unit shrinkage-specific gravity regressions where relationships were positively significant but the regression constants were not.Shrinkage behaviour in blocks of E. regnans did not comply with the Stamm formula although the average fibre saturation point (41.8%) was not significantly different from that for sections (38.2%). The near similarity derived from a significantly greater unit shrinkage in blocks (0.49) than that in sections (0.30). It was suggested that the much greater external shrinkage in blocks contained a significant increment of cell collapse induced by drying stress below the fibre saturation point.The authors wish to thank Dr. Allan Miller for statistical assistance and Dr. Aleck Hunter for mathematical assessment     

Effect of brief presteaming on shrinkage,collapse and other wood-water relationships in Eucalyptus regnans F. Muell

Summary Brief presteaming of green, increment-core segments of mountain ash (E. regnans F. Muell) produced small but significant changes in a number of moisture-shrinkage parameters. Both total volumetric shrinkage and recoverable collapse were significantly greater in presteamed samples, the increase in collapse being largely attributable to an increase in moisture content after reconditioning (M_r). The change in shrinkage after reconditioning also was significantly related to changes in M_r. The change in M_r was interpreted as reflecting altered permeability in presteamed material and was shown to be negatively related to estimated residual collapse. Initial moisture content and per cent saturation showed a highly significant decrease after presteaming, while green volume showed a small but significant increase. Neither the estimated intersection point, unit shrinkage nor the R-ratio were significantly different after presteaming. However, the unit shrinkage-specific gravity relationship was changed by presteaming such that it more closely complied with Stamm's approximation.The author expresses his appreciation to Dr. Alan Miller for assistance with statistical analysis and to Mr. Chris McEvoy for technical assistance     

Formation of the density profile and its effects on the properties of particleboard

Summary Two types of particleboards bonded with an isocyanate resin, one with uniform vertical density profile (homo-profile), and the other with conventional U-shaped profile, were fabricated to various density levels using lauan (Shorea spp.) particles. The fundamental relationships between the density profile and the board properties were determined, and the results are summarized as follows: 1. In homo-profile boards, the moduli of rupture (MOR) and elasticity (MOE), internal bond (IB) strength, and screw withdrawal resistance (SWR), are highly correlated to the board mean density. 2. The bottom limit of the board density is estimated to be ca. 0.25 g/cm³, based on the correlation regressions between mechanical properties and mean density. 3. At equal mean density level, the MOR and MOE of the conventional particleboards are higher than the homo-profile boards, due to the higher density near the faces. However, the reverse is true for IB, owing to the presence of the low density core in the former. 4. The net impact of peak density on MOR and MOE is greater at higher mean density level while raising the core density results in more pronounced improvement in IB at lower density. 5. In addition to the compaction ratio, the dimensional stability of the board is also affected by the peak area and mat moisture content. Received 9 January 1997     

Tangential and radial shrinkage variation within trees in sugi (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis>) cultivars

The transverse shrinkage variation within trees was examined for five sugi cultivars. The within-tree trends of tangential shrinkage (α _T) were different by cultivar, whereas radial shrinkage (α _R) increased from pith to bark in most cultivars. The tangential/radial shrinkage ratio (α _T/α _R) decreased from pith to bark in most cultivars, because the radial variation of α _R was larger than that of α _T. The cultivars showed significant differences among cultivars in α _T, α _R, and α _T/α _R, but the difference among cultivars for α _T/α _R was smaller. The relationships between transverse shrinkage and microfi bril angle (MFA), basic density (BD), tree ring parameters, and modulus of elasticity were examined. The α _T and α _R showed positive relationships with BD, latewood percentage, latewood density, and modulus of elasticity, and negative relationships with MFA and ring width. The relationships with earlywood density were weak. Sugi exhibited variation in transverse shrinkage within stem and among cultivars, with the variation affected by MFA, density, and tree ring parameters. Part of this article was presented at the 56th Annual Meeting of the Japan Wood Research Society, Akita, Japan, August 2006, and the 57th Annual Meeting of the Japan Wood Research Society, Hiroshima, Japan, August 2007     

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     

Collapse-type shrinkage characteristics in plantation-grown eucalypts： I . Correlations of basic density and some structural indices with shrinkage and collapse properties

Collapse-type shrinkage is one of highly refractory drying defects in low-medium density plantation-grown eucalypt wood used as solid wood products. Basic density (BD), microfibril angle (MFA), double fibre cell wall thickness (DWT), proportion of ray parenchyma (RP), unit cell wall shrinkage, total shrinkage and residual collapse, which are associated with collapse-type shrinkage characteristics, were investigated by using simple regression method for three species of collapse-susceptible Eucalyptus urophyll, E. grandis and E.urophyllaxE.grandis, planted at Dong-Men Forest Farm in Guangxi autonomous region, China. The results indicated that : unit cell wall shrinkage had a extremely strong positive correlation with BD, moderately strong positive correlation with DWT, and a weakly or moderately negative correlation with RP and MFA; total shrinkage was positively correlated with BD, DWT and RP and negatively related to MFA, but not able to be predicted ideally by any examined factors alone owing to lower R^2 value (R^2≤0.5712); residual collapse was negatively correlated with BD and DWT, linearly positively correlated with MFA, and had strongly positive linear correlation with RP. It is concluded that BD can be used as single factor (R^2≥0.9412) to predicate unit cell wall shrinkage and RP is the relatively sound indicator for predicting residual collapse     

Predicting oven-dry density of Sugi (Cryptomeria japonica) using near infrared (NIR) spectroscopy and its effect on performance of wood moisture meter

We propose a non-destructive method to predict the oven-dry density of Sugi (Cryptomeria japonica D. Don) using near infrared (NIR) spectroscopy so as to calibrate a commercial moisture meter. A prediction model for oven-dry density was developed using NIR spectra obtained from Sugi samples with a known density. The density of air-dried Sugi boards was predicted with the developed model. Then, the moisture content (MC) of the boards was measured by a hand-held capacitance-type and an in-line microwave moisture meters. For each board, the moisture meters were calibrated by the predicted density. The predicted density was correlated with the measured one with an R ² of 0.81 and a standard error of prediction (SEP) of 15.3 kg/m³ within the measured density of 279.2–436.4 kg/m³, indicating that the developed model was applicable for predicting oven-dry density of Sugi. The MC readings of both moisture meters showed a good correlation with the oven-dry MC that ranged from 12.1 to 28.9 %. For both moisture meters, the density calibration with the NIR-predicted density gave a higher R ² and a lower SEP than with the conventional calibration with the mean density. These results demonstrate that the present density calibration using NIR spectroscopy could improve the performance of the moisture meters for the air-dried Sugi boards with varying densities.     

Nondestructive sampling of Eucalyptus globulus and E. nitens for wood properties. I. Basic density

A non-destructive sampling strategy for basic density, based on removing 12 mm bark-to-bark cores, was developed in E. globulus and E. nitens. Fifty trees of each species, aged 5 to 9 years, were sampled across a range of sites. Core samples were removed on both a north-south and an east-west axis from 6 fixed heights in the base of the tree (0.5 m, 0.7 m, …, 1.5 m). Whole-tree values were calculated from disc samples removed at eight percentage heights (0, 10, 20, …, 70%) and correlations between the cores and whole-tree values were used to determine the optimal sampling height. Core samples were found to be reliable predictors of whole-tree density, explaining between 84% and 89% of the variation between trees. Core sampling of E. globulus and E. nitens to estimate basic density of whole-trees and stands is feasible; cores from trees at all E. globulus sites gave high correlations with whole-tree values. For E. nitens, site differences were apparent, and it is recommended that a small destructive sampling program should be undertaken prior to commencing a major sampling program. Recommended optimal sampling heights are 1.1 m for E. globulus and 0.7 m for E. nitens. Core orientation was not important and density was not related to tree size. Six whole-tree samples or eight core samples are required for estimating the mean density of a stand at a specific site to an accuracy of ±20 kg m⁻³ with a 95% confidence interval. Received 17 September 1998     

Relationships of anatomical characteristics versus shrinkage and collapse properties in plantation-grown eucalypt wood from China

To explore the influence of the basic density on collapse-type shrinkage properties and to quantify the relationships of the main anatomical features with shrinkage and collapse properties, all above-mentioned parameters were determined and analyzed for three species of collapse-susceptible eucalypts, Eucalyptus urophylla, Eucalyptus grandis, and E. urophylla × E. grandis, planted in South China. The correlation coefficients were also determined and the corresponding regression equations were established with the anatomical parameters measured by using multiple linear regression. The results indicated that: (1) basic density was strongly positively linearly related to both unit tangential shrinkage (r = 0.970) and unit radial shrinkage (r = 0.959), weakly positively related to total shrinkage (r = 0.656 and 0.640 for tangential and radial, respectively), and weakly negatively related to residual collapse (r = 0.632 and 0.616 for tangential and radial, respectively). (2) The main factors affecting unit shrinkage were cell wall proportion (WP), microfibril angle (MFA), and double fiber cell wall thickness (DWT); factors playing an important role in total shrinkage were WP, ray parenchyma proportion (RP), and MFA, while RP had the highest effect on residual collapse (r = 0.949 and 0.860 for tangential and radial, respectively). (3) All corresponding regression models obtained were very suitable for the evaluation of relationships between the anatomical parameters and unit shrinkage, total shrinkage, and residual collapse, as measured using a moisture content of 28% as the fiber saturation point for all specimens.     

Turbulence in the wood system with small and short stresses

Small compressive stresses (500 N, 1000 N, 2000 N) of small duration were exerted on 2 × 2 × 2 cm³ wood specimen of Eucalyptus hybrid. Two relative humidity levels of 93% RH and 27% RH were maintained in dessicators. The per‐cent change in Equilibrium Moisture Content (EMC) was measured. It was found that the initial percent change of EMC was 2.62 times higher in low moisture content specimens when compared to the higher moisture content specimens. The EMC was found to decrease on applying compressive stress irrespective of the stress level. An explanation for this effect in terms of re-alignment of hydrogen bonds is presented in this paper. Received 8 December 1997