Variability in Mechanical and Drying Properties for Blackbutt Timber in New South Wales

Variability is a key issue in the processing of biological materials, in this case the drying of hardwood timber. This paper reports measurements of variability of mechanical properties and correlations with basic density and diffusion coefficients, which are relevant to the drying of blackbutt, Eucalyptus pilularisSm, from Northern New South Wales. Specifically, radial, circumferential and height variations are reported for two regrowth blackbutt logs. Both blackbutt logs consisted of more than 80% heartwood, thus diffusion was considered as the main transport mechanism of moisture for modelling purposes.The modulus of elasticity for boards taken from two blackbutt trees increased from their green state to their kiln-dried state, generally by a factor of around two. An analysis of variance of wood samples from different parts of the two blackbutt logs showed that the variability of basic density, diffusion coefficient and most mechanical properties of timber, are significant in the radial direction, at a confidence level of at least 75%.Therefore, density and the microfibril angle are probably the actual sources of the variations of mechanical properties, since both parameters vary in the radial direction. Furthermore, from pith to bark, the diffusion coefficient and shrinkage decreased, while the basic density increased. A potential reason for this is that timber with high diffusion coefficients means that more moisture is lost per unit time because there is less wood (i.e. low basic density) to resist moisture transport, and less wood material means that the timber experiences more shrinkage.Strong correlations between the diffusion coefficient and the modulus of elasticity were also suggested using principal components analysis. The principal component accounted for 70% of the total amount of variation between the basic density, diffusion coefficient and green modulus of elasticity (perpendicular to the grain). From this, there is a strong possibility that the variabilities of the basic density, diffusion coefficient and green modulus of elasticity are correlated with each other.It is possible that high modulus of elasticity (whether it is longitudinal or green transverse to the grain) meant low diffusion coefficients, because of high basic density – i.e. more wood per unit volume possibly increased the stiffness of the timber. Such correlations may then be utilized to predict important drying parameters, such as the diffusion coefficient of a particular board prior to kiln-drying, by simply measuring the basic density, or using acoustic methods to measure the longitudinal modulus of elasticity. This parameter (longitudinal modulus of elasticity) is strongly correlated with the diffusion coefficient and basic density, because the principal component accounted for 98% of the total amount of variation between these parameters.