Influence of density and microstructure

Dense hardwoods such as greenheart (Guaiacum spp.) and oak (Quercus spp.) are favoured for their strong timber. The grade stresses and moduli of elasticity for the higher strength classes of softwoods tabulated in BS 5268-2 and EN 338 correspond to higher values of average density, compared to those of lower strength classes. On a simplistic level, it might be considered logical to assume that a doubling of density for any similar material will result in a corresponding increase in both stiffness and strength in the same way that two identical timber beams placed side by side will be twice as strong as a single beam. Where this notion falls down, however, is when the denser material is dissimilar; in the case of wood this could be due to differences in micro-structure or chemical composition.

In a review paper on microfibril angle (MFA, defined in Figure 2.11), Cave and Walker (1994) question arguments by others that density has a major effect on the yield and quality of timber, highlighting that changes in stiffness observed from pith to bark cannot be solely attributed to changes in density. Brazier (1986) also questioned the significance of density for the selection for wood quality in improvement programmes aiming to improve strength and stiffness.

He identified two features of greater influence, grain inclination and presence of juvenile wood. British-grown Sitka spruce, with its tendency to have a large juvenile core owing to fast growth, is known to tend to produce lower structural grades than imported softwoods with lower rates of growth. The proportion of juvenile wood within battens, and not density, would therefore seem to be (potentially) a reliable indicator of likely grade.

MFA

Figure 2.11: Simplified structure of wood cell wall, showing angular arrangement of the microfibrils in the S2 layer (after

Dinwoodie, 1981).

Brazier (1986) noted the influence of microfibril angle on stiffness, and was concerned that the strength reducing features he observed were all adversely affected by forest management policies which favoured enhanced rates of growth on short rotations. Brazier illustrated the effect of microfibril angle on stiffness with a linear fit graph (MFA v.

MOE). It is significant that in Brazier‟s graph MOE has not been corrected for sample density – implying that density was not significant for the batch tested. Given the significance of microfibril angle, and the fact that it cannot be measured in an industrial setting, the question then is how closely can its effect be estimated from related variables such as ring width, proximity to pith and percentage of juvenile wood.

Brazier (1991), commenting on the weaker material he observed near to the base of Sitka spruce trees, stated that it occurred in battens cut from both juvenile core and adult wood. Brazier suggested that the reason for the low performance of the near-butt wood might be due to a combination of low density, irregular grain associated with a buttressing effect from the roots, and possibly also “unusual” cell microstructure. In more recent work, Xu and Walker (2004) assembled stiffness profiles from pith to bark and from butt to the upper top logs for radiata pine (Pinus radiata), and identified a zone of high microfibril angle and low stiffness within the base of the trees. Like Brazier, they advocated segregating this material. Note that this was apparently the subject of a trial at PRL/BRE, however the work (on material from the Forest of Ae) was apparently not fully reported.

Cockaday (1992) determined the variation of bending stiffness, microfibril angle and dry density throughout four Sitka spruce trees. A large proportion of Cockaday‟s work, however, concerns the effect of sample size and orientation with respect to growth rings on his measurements; hence the testing of more homogeneous timbers such as the tropical hardwood ramin (Gonystylus spp.), together with laminated composites. Cockaday showed that bending stiffness was

strongly and inversely related to microfibril angle. He concluded that the relationships observed between MOE and density were either very poor or insignificant, and considered that extremes of density in his samples were associated with compression wood. His samples were carefully selected to be straight grained. The determination of rate of growth was also outside the scope of his work.

Cockaday found no single consistent model to represent the variability of MOE with all four trees studied. Both MOE and MFA were found to vary systematically across the radius of the tree but not axially, whilst density was not found to vary systematically. This suggests that Cockaday did not encounter any markedly low stiffness at the base of the trees studied, which contrasts with the findings of Brazier (who was his PhD supervisor). Since Cockaday‟s samples were clear of knots, there is no reason to suppose that for full-scale samples with knots there will be any improvement in any observed relationship between bulk density (i.e. including knots) and stiffness.

Pendini (1992 and undated) reported, from studies into the variation of MFA in Sitka spruce grown in Denmark, that MFA decreased very rapidly from growth ring number 3 until numbers 9 to 12 where it stabilised. This was found to be more apparent at breast height (1.3 m) than at other sample heights (25%, 50% and 65% of tree height).

Within a growth ring it was found that MFA decreased with increasing height in the stem. The fastest growing trees were found to have the highest MFA in both juvenile and mature wood, although it was also observed that the narrow growth rings, formed in some trees when they are suppressed, tend to have tracheids with a high MFA. Overall, the data suggested that fast growth will lower the quality of juvenile wood in Sitka spruce but did not support the theory that the number of juvenile growth rings will increase with increasing growth rate. From the graphical data presented of trends in MFA from pith to bark, greatervalues of MFA were observed at 1.3 m than higher in the stem.

Treacy et al. (2001), in a report on the mechanical and physical wood properties of Sitka spruce grown in Ireland, detail inverse relations between both MFA and MOE, and between MFA and MOR; although in the case of the latter MFA accounted for only 34% of the variation.