John C. F. Walker

Kiln-Drying of Lumber

The Structure of Wood Wood-Water Relationship Evaporation and Humidification Wood Drying Kinetics Moisture Diffusion Multiple-Mechanism Models Lumber Quality Stress and Strain Behaviour Airflow and Convection Kiln Operation Pre-treatments of Green Lumber Less-Common Drying Methods Subject Index.

Acoustic measurements on standing trees, logs and green lumber

Acoustics provide opportunities for non-destructive evaluation of the mechanical properties of wood, especially stiffness. In this work, Fakopp 2D a time of flight (TOF) tool and WoodSpec a resonance-based system were used. In a scoping study on young Pinus radiata trees (aged 8–11) no systematic differences were found between the Fakopp acoustic velocity on opposite sides of young trees. These Fakopp values were then averaged and correlated with that from the equivalent log using WoodSpec. Finally stem and log values were correlated with acoustic velocity in outerwood and in corewood lumber. External measurements on the log (standing tree) correlated well with those for lumber cut adjacent to the bark and modestly for the corewood (R2 of 0.89 and 0.74, respectively). In a separate study, the acoustic velocities were measured on standing trees in three adjacent stands aged 8, 16 and 26 and the data were used to construct a “Russian Doll” model to demonstrate the enormous range in wood stiffness within and between trees. The stiffest 20% of the population was estimated to be 146, 87 and 76% stiffer than the poorest 20% in the corewood (age 8), intermediate (age 16) and outerwood zone (age 26) respectively.

Sorting of logs using acoustics

Abstract The practical implications and benefits of using acoustics for sorting pine logs at the skids is reported. The aim is to segregate logs according to their intrinsic stiffnesses and to determine the proportion of lumber making framing or structural grades. The proposition is that there are significant financial benefits to sawmillers if the poorest logs – those which yield low stiffness timber – can be identified and diverted to other end uses that do not require high stiffness.

Selecting trees for structural timber

Abstract Twenty-eight cubic metres of timber (i.e. a total of 2248 boards) cut from 108 Pinus radiata trees from two 25-year-old plantations from Canterbury and Nelson in the South Island of New Zealand were tested in tension. Within- and between-tree variations of stiffness, strength and density were examined. Comparisons between density and stiffness for selecting trees for structural timber indicated that stiffness is the better parameter for selecting superior trees within the natural population of a forest stand; and the quality and grade recovery of structural timber would be increased significantly if trees were to be selected on the basis of stiffness. These conclusions apply to both stands although regional differences are evident with the Nelson timber being somewhat stiffer.Zusammenfassung 28 m3 Schnittholz, entsprechend 2248 Brettern, wurden aus 108 Kiefernstämmen (P. radiata) geschnitten. Die Proben, die aus einer 28jährigen Plantage in Neuseeland stammen, wurden auf Zugfestigkeit geprüft. Die Variation von Steifigkeit, Festigkeit und Dichte innerhalb und zwischen den einzelnen Bäumen wurde untersucht. Beim Vergleich von Dichte und Steifigkeit als Parameter zur Auswahl von Schnittholz erwies sich die Steifigkeit als der bessere Parameter, um die besten Bäume eines Standorts auszuwählen. Qualität und Ausbeute an günstigen Güteklassen könnten signifikant ansteigen, wenn Bäume aufgrund ihrer Steifigkeit ausgewählt werden. Dies gilt für beide untersuchten Standorte, wenn auch regionale Unterschiede bestehen; denn die Proben aus Nelson waren durchweg etwas steifer.

Changes in chemistry, color, dimensional stability and fungal resistance of Pinus radiata D. Don wood with oil heat-treatment

Abstract Pinus radiata wood specimens were heat-treated at 160–210°C in linseed oil and the effects of treatment on chemical composition, color, dimensional stability, and fungal resistance were examined. The degradation of hemicelluloses was the most remarkable feature, which is the principal reason for alterations in wood properties. Removal or migration of extractives, oil uptake and the accumulation of oil on the wood surface were observed. The color of heat-treated wood became more uniform and darker, and its dimensional stability (i.e., anti-swelling efficiency) and fungal resistance were improved by up to 60% and 36%, respectively. The viscosity of the oil after treatment was elevated with the treatment temperature and was higher in comparison to heated oil without wood present.

Effects of moisture content and temperature on acoustic velocity and dynamic MOE of radiata pine sapwood boards

The effects of moisture content from 17 to 159% MC and temperatures from −71°C to +58°C on resonance-based acoustic velocity and dynamic modulus of elasticity (DMOE) were investigated using 36 boards of radiata pine sapwood. Acoustic velocity decreased with increasing moisture content and temperature, although effects differed noticeably below and above fibre saturation point (FSP). Below FSP, acoustic velocity decreased rapidly and linearly with increasing moisture; whereas above FSP changes in velocity were rather gradual and curvilinear with marked differences in velocity patterns between temperatures below and above freezing. Acoustic velocity decreased linearly with increasing temperature but there was an abrupt discontinuity at the freezing point for wood above FSP. Changes in moisture content and temperature affecting wood density and acoustic velocity led to changes in DMOE, although this relationship was not straightforward because changes of velocity with moisture were linear or curvilinear and depended on whether wood was frozen or unfrozen, whereas changes of density with moisture content were invariably linear. For practical work, it is important to estimate the relative effect of changing moisture content or temperature with respect to standard conditions on DMOE, thus general guidelines were devised to account for significant changes.

Genetic control of very early compression and opposite wood in Pinus radiata and its implications for selection

The long time frame for evaluating selection candidates is a major barrier to the deployment of genetic gain from tree breeding programs. This situation is compounded in wood quality studies by constraints on the number of sampled individuals when trees are older and larger. This paper documents the degree of genetic control and genetic association for wood quality traits in 18-month-old leaning Pinus radiata seedlings. Trees were separately assessed for basic density, green and dry acoustic velocity, and longitudinal and volumetric shrinkage in opposite and compression wood. Heritability estimates were low to moderate for both compression and opposite wood (ranging from 0.15 to 0.38). Estimated genetic correlations were very high in opposite wood, where green velocity displayed the highest correlations with both longitudinal (−0.89) and volumetric (−0.64) shrinkage, closely followed by dry velocity. These correlations were substantially lower for compression wood. The estimated correlations between compression and opposite wood characteristics were high for most traits except for longitudinal shrinkage. We suggest how these results could be used for very early screening for wood stiffness and dimensional stability. We propose that information on early genetic control of wood quality and the methodologies used to elicit it should be integrated in breeding and deployment programs.

The drying stress and check development on high-temperature kiln seasoning of sapwoodPinus radiata boards

This two-part set of papers presents a study of the stress and check development inPinus radiata boards under high-temperature kiln-seasoning conditions. In the first part, a one-dimensional stress model incorporating the various strain components is developed based on a physically realistic model of drying behaviour. The drying of sapwood is assumed to be characterised by two principal stages, in which the earlier one is dominated by an evaporative front receding into the board, whereas in the later one bound water and vapour diffusion prevail. As the drying proceeds, the tracheids (pits) under water tension may cavitate randomly. This leads to a progressive fall in relative liquid permeability with moisture content. The sapwood consistently loses liquid continuity at about 60% local moisture content, significantly higher than the fibre-saturation point. The moisture in the evaporative zone is driven by a small but adequate vapour pressure difference between irreducible saturation and the fibre-saturation point which corresponds to an equilibrium moisture content of 99%. The wood is assumed to be an elasto-viscoplastic material with its mechanical properties varying with both temperature and moisture content. Under high-temperature conditions, the wood may be capable of sustaining some extent of plastic strain after passing through the yield point. Checking is assumed to occur only after the ultimate strain is exceeded.ZusammenfassungDiese zweiteilige Arbeit beschreibt die Spannungs- und Rißentwicklung in Kiefernbrettern unter den Bedingungen der Hochtemperatur-Trocknung. Im ersten Teil wird ein Spannungsmodell entwickelt, das die verschiedenen Dehnungskomponenten beimhaltet und auf einer physikalisch realistischen Vorstellung des Trocknungsverhaltens beruht. Es wird angenommen, daß die Trocknung des Splintholzes in zwei Stufen erfolgt: die erste ist gekennzeichnet durch eine Verdampfungsfront, die sich langsam ins Innere des Holzes zurückzieht; in der zweiten Stufe sind gebundenes Wasser und Dampfdiffusion vorherrschend, Mit fortschreitender Trocknung können die Tracheiden unter der Zugspannung des Wassers zufallsverteilt kollabieren. Dies führt zu einer fortschreitenenden Verringerung der Durchlässigkeit mit fallender Feuchte. Das Flüssigkeitskontinuum im Splintholz wird unterbrochen ab etwa 60% Holzfeuchte, d.h. deutlich über dem Fasersättingungspunkt. Die Feuchtebewegung in der Verdampfungszone wird angetrieben durch einen geringen, jedoch ausreichenden Druckunterschied zwischen nicht unterschreitbarer Sättigung und Fasersättigungspunkt, entsprechend einer Gleichgewichtsfeuchte von 99%. Das Holz wird als ein elasto-viskoplastisches Material angesehen, dessen mechanische Eigenschaften sich sowohl mit der Temperatur als auch mit der Feuchte ändern. Unter Hochtemperatur-Bedingungen kann das Holz noch einige plastische Restspannungen enthalten. Risse sollten nur entstehen, wenn die äußerste tolerierbare Spannung überschritten wird.

Differences in acoustic velocity by resonance and transit- time methods in an anisotropic laminated wood medium

Abstract The influence of inhomogeneity on acoustic velocity measured by a resonance-based WoodSpec acoustic tool and a transit-time-based Fakopp-2D tool was investigated. Four laminated panels with different degrees of inhomogeneity were prepared and acoustic velocities were measured using both tools. Velocities measured by the Fakopp-2D tool were always higher than those measured by the WoodSpec tool in all panels. The difference in the two velocities was found to vary depending on the magnitude of inhomogeneity. It was demonstrated that the velocity measured by the resonance frequency obeys the law of mixtures and is controlled by the volume-weighted average stiffness of the material. In contrast, the velocity measured by the Fakopp-2D tool was the velocity of the pulse that first reached the detector travelling at the dilatational speed.

Relationships between longitudinal growth strain and some wood properties in Eucalyptus nitens

Summary The relationships between longitudinal growth strain and wood properties of Eucalyptus nitens were investigated. Sixty-three 10-y-old trees were selected for this study. Longitudinal growth strain, green density, green moisture content, basic density, radial shrinkage, outerwood and corewood densities, volumetric shrinkage and dynamic modulus of elasticity (MOE) at 12% moisture content and length-weighted fibre length were determined. Amongst all the studied wood properties, only shrinkage-related properties were found to have some association with the mean growth strain in trees. The mean growth strain was moderately but significantly related to the volumetric shrinkage of the outerwood, but not to the shrinkage of the corewood. However, the volumetric shrinkage differential (difference between outerwood and corewood shrinkage) was strongly related to the growth strain (r = 0.70), suggesting that the growth stress gradient might be related to variations in shrinkage properties within the stem. The wood of trees with the lowest growth strains had statistically significantly lower volumetric shrinkage, lower outerwood MOE and less collapse than wood of trees with the highest growth strains. The results suggest that E. nitens trees with low strains could exhibit a lower degree of drying defects such as collapse and checking during processing.