Selecting and/or processing wood according to its processing characteristics
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
In this study, three issues associated with the segregation of wood according to their processing characteristics have been addressed. In the first part of the study, the influence of inhomogeneity on the acoustic velocity measured by a resonance based acoustic tool "WoodSpec" and a transit-time based tool "Fakopp-2D" was investigated. Four laminated panels with varying degree of inhomogeneity were prepared and acoustic velocities were measured by both tools. The acoustic velocity measured by the Fakopp-2D tool was always higher than that measured by the WoodSpec tool. The difference in the two velocities was found to differ with the magnitude of inhomogeneity in the panel. 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 wood. In contrast the velocity measured by the Fakopp-2D tool was influenced by the amount of energy propagating at the dilatational speed. The progression of the wave-front in panels was investigated using Fakopp-2D tool by picking up the arrival time of the signal at off-axis probes. The wave-front profile was found to be determined by the stress-wave speed (stiffness) at different angles from the axis symmetry. In logs, the transit-time velocity was found to be sensitive to the localised stiffness of the wood lying in between the measurement probes with in the propagation path. A strong correlation was observed between the velocities measured by WoodSpec and the Fakopp-2D tool in young radiate pine and eucalyptus logs; however the acoustic velocity by Fakopp-2D was higher than the WoodSpec velocity. The results imply that Fakopp-2D could be used to rank young trees according to their stiffness. In the second part of the study, two trials were conducted to test the hypothesis that acoustics can be used to segregate pulp logs into categories which will require different amounts of energy during mechanical pulping and will produce pulps with different strengths. Pinus radiata logs of varying age, length, SED, LED, taper, and volume were measured for acoustic velocity, segregated into four different velocity groups and chipped separately. It was shown that acoustics could segregate logs into groups that perform very differently in terms of pulp and paper properties when refined to a given freeness or at a certain energy input. At a given target freeness there was a 20% difference in energy requirement between the lowest and highest velocitylogs. Similarly there was a 17% difference in tensile strength between the lowest and highest velocity logs for a given specific energy. In the third part of the study, investigations were made to explore the potential of acoustic velocity in ranking young Eucalyptus nitens trees according to growth stress level. Longitudinal growth strain was measured in 155 selected trees at the breast height and acoustic velocity was measured in the same trees using the Fakopp tool. Measurements in the first 34 trees showed some relationship between growth strain and acoustic velocity which eventually vanished as the measurements were progressively extended to all 155 trees. The results indicated the dangers of drawing possible inferences on the basis of small sample sizes. A large variation observed in growth strains along the tree height and on the two opposite sides in logs suggested that a single strain measurement is not sufficient enough to assess the mean tree strain level even in young and small diameter eucalypts trees and measurements on two opposite sides at a specific height is an approach for the screening purpose. The relationships between longitudinal growth strain and certain key wood properties were also investigated. Green density, green moisture content, basic density, radial shrinkage, outerwood and corewood densities, volumetric shrinkage and dynamic MoE at 12% me 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 while the corewood shrinkage was not related. 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 the shrinkage property variations within the stem. Fisher's LSD test indicated statistically significant lower volumetric shrinkage, lower outerwood MoE and less collapse in the wood from trees with the lowest growth strains as compared to those from the highest growth strains. The results suggest that Eucalyptus nitens trees with low strains could exhibit a lower degree of the drying defects like collapse and internal checking during processing The large variation in wood quality characteristics in plantation grown timbers makes some screening for wood quality necessary for the effective management of wood resources and for the allocation of logs according to "Fitness for Purpose" to capture greater value of forest product value chain.