FACTORS WHICH AFFECT STIFFNESS

Silvicultural practices are known to affect the stiffness of wood. The initial spacing of seedlings appears to influence stiffness. Lasserre et al. [2009] found that close planting of Pinus radiata seedlings (2500 stems/ha vs. 833 stems/ha) led to an increase in the resonance MoE of 81 %. Lasserre et al. [2004] found that the effect of seedling planting density (at the same stocking levels as above) was significantly greater than the effect due to clonal variations. Lasserre et al. [2008] compared the effect of seedling spacing across two different sites. They found that the effect of stocking density on MoE was significantly greater than the effect of site variation between clones. Carson et al. [2014] assessed the mid-rotation performance of Pinus radiata trees subjected to variation in stocking density and pruning in several sites across New Zealand. They found that: “increasing stand density is consistently associated with stiffer and denser outerwood." Lasserre et al. [2009] found that increasing the planting density of seedlings had other effects in addition to the expected increase in MoE. They observed a significantly reduced MFA, an increase in tracheid length, increase in latewood percentage and cell wall thickness, a decrease in growth-ring width and overall diameter. Density and fibre width were not significantly affected.

Other authors provided dissenting opinions on the effect of stocking density. Gra- bianowski et al. [2004] evaluated the effect of the variation in acoustic velocity of two stands of different stocking density (625 stems/ha vs. 100 stems/ha) of 27 year-old

Pinus radiata. They found no significant difference between the two. They also assessed two narrow contiguous strips planted perpendicular to the prevailing NW wind, with one strip upwind, and one downwind of a larger stand of trees providing shelter. They found that the downwind (sheltered) trees had a significantly higher mean stiffness.

1.3.1 Microfibril angle

One of the strongest determinants of wood stiffness is the microfibril angle [Barnett and Bonham 2004, Chauhan et al. 2006a, Evans and Ilic 2001]. The MFA is the mean angle between the tracheid cell axis and the orientation of the cellulose microfibrils in the S₂ layer of the cell wall. Cave [1968] showed that the cellulose microfibrils in the cell wall are highly anisotropic. This result suggests that the MFA is the predominant factor in the anisotropic stiffness of the cell wall. His work showed that the longitudinal and transverse Young’s moduli of the cell wall are approximately 35 and 10 GPa, respectively. Reiterer et al. [1999] strengthened this result by showing that the relationship between stiffness and MFA also applies to larger groups of fibres. It has also been observed [Cave 1969] that in order to maximise wood stiffness in the longitudinal direction, the MFA should be minimised.

Barnett and Bonham [2004] explained that the MFA enables a tree to alter its effective stiffness. The large MFA in juvenile wood allows a young tree to flex when required, i.e., allows a larger strain before failing. This is necessary, for example, to

prevent the tree breaking in a strong wind. Mature softwood trees also have the ability to produce higher MFA regions known as compression wood. These regions allow a tree to exhibit tension or compression on one side, which allows the stem to bend if required.

MFA is typically measured using X-ray diffractometry [Cave 1997, Chauhan et al. 2006a, Stuart and Evans 1995]. Originally this was conducted using photographic film, which was very slow to process. More recently, CCD-devices have rapidly sped up the process. Hayes and Pink [2012] demonstrated the feasibility of measuring the MFA of wood samples acoustically. They built a machine for measuring the ToF velocity of increment cores in a range of orientations. By applying a mechanical model of plant cell walls [Cave 1968] to measurements from the machine, the MFA of the core sample was estimated. Mason et al. [2017] measured the ToF velocity ofPinus radiata discs using a robotic scanning device. The MFA of the discs were then tested using X-ray diffractometry. The found a strong negative correlation between MFA and ToF velocity (r <−0.84).

1.3.2 Spiral grain

The tracheid cells which form the majority of xylem tissue in softwoods are, in most wood, not aligned with the axis of the tree stem. The angle formed between the tracheids and the stem is termed the grain angle. Due to the interlocking structure of tracheid cells, nearby tracheids have approximately constant GA, which forms a visible spiral in the grain around the stem. For this reason, this effect is often referred to asspiral grain. Spiral grain can be observed visually as a twisting in the grain of the tissue up the stem. This reduces the stiffness of the resulting timber and can lead to warping as the timber dries [Buchanan 2007]. Spiral grain can be visually inspected in timber but is more difficult to see in live trees because the grain angle typically changes as the tree grows outwards.

1.3.3 Reaction wood

Compression wood occurs in regions where a softwood stem experiences an excess of compressional force. It typically occurs on the lower side of a leaning stem, but can also be a result of asymmetrical branch growth or environmental stress such as wind. In a cut stem, compression wood is identified as a dull-coloured region of growth, where growth rings are accentuated, and there is less contrast between earlywood and latewood [Kollmann and Côté 1968]. In regions of compression wood the length of tracheid cells expand longitudinally, and cell walls thicken. This acts to correct stem lean [Chauhan et al. 2006b]. Compression wood is undesirable in timber as it has a high MFA, increased density, and lower stiffness and strength, when compared to normal wood [Butterfield 2006]. It also exhibits larger longitudinal shrinkage than regular wood when dried.

1.3 FACTORS WHICH AFFECT STIFFNESS 27

A different type of growth, known as tension wood, occurs in hardwoods. This eccentric growth occurs on the upper side of a leaning stem, where the hardwood fibres contract longitudinally to correct the lean. Tension wood typically has a higher stiffness and strength than normal wood, with a lower MFA and greater cellulose content [Butterfield 2006]. Collectively, compression and tension wood are known asreaction wood.

1.3.4 Moisture content

As described in Section 1.1.3, water is absorbed in wood in both the cell wall and the cell lumen. The MC at which the cell wall becomes saturated is known as the fibre saturation point. Mechanical properties tend to decline as water is absorbed up until fibre saturation [Bodig and Jayne 1982, Kretschmann 2010]. At MC above fibre saturation, water is absorbed into the cell lumen, and the mechanical properties are relatively constant.

Bodig and Jayne [1982] suggest that for MC values below fibre saturation, wood strength decreases exponentially as MC increases,

σu=Ae−BM, (1.16)

where σu is the wood strength, M is the moisture content, and A, B are constants fitted to the model. Bodig and Jayne [1982] also suggest that this model will apply to other mechanical properties (such as stiffness).

Kretschmann [2010] suggested a slightly different model

P =P12% P12% Pgreen ! 12−M Mp−12 , (1.17)

where P is an arbitrary mechanical property, P12% is the value of the property at 12% MC, Pgreen is the value of the property when the wood is green, M is the moisture content, andMp is the intersection moisure content9. Kretschmann [2010] states that

Mp is slightly less than the fibre saturation point; he provides values of Mp for several species. Kretschmann [2010] provides data suggesting that (1.17) is an appropriate model for strength and stiffness properties in tension, compression, and bending; both parallel and perpendicular to the grain.

Reducing MC tends to increase stiffness and decrease density. Therefore, according to (1.6), as wood is dried, acoustic velocity should undergo a significant increase. Several authors have found that MC correlates negatively with ToF velocity [Hasegawa et al. 2011, Llana et al. 2018, Mishiro 1995, Sakai et al. 1990, Sandoz 1993]. Carter et al.

9_{Defined by Kretschmann [2010] as the point at which mechanical properties begin to increase when}

[2007] found that air dry Pinus radiata wood had a 6% higher dynamic MoE than green wood. Based on these studies, it is not clear whether the observed increase in acoustic velocity with MC is reflective of the models described by (1.16) and (1.17).

1.3.5 Temperature

Kretschmann [2010] explains that mechanical properties of wood tend to decrease linearly with increasing temperature, provided that the temperature does not exceed approximately 150 °C. He describes this effect as a reversible temperature effect, i.e., if the temperature is returned to its previous value, the mechanical properties will also return. Kretschmann [2010] suggests that the relationship between temperature and mechanical properties are modelled by

∆P% = P−P21

P21 !

×100 =A+BT+CT2, (1.18)

where ∆P% is the percentage change in the mechanical property; P21 is the value of the property at 21 °C; A, B, and C, are constants for the model; and T is temperature in degrees Celsius. Kretschmann [2010] provides values for the model constants when the wood property being determined are MoE or MoR. If C is non-zero, the model includes a second-order term, though Kretschmann [2010] shows that in most cases C

is zero or very small.

In addition to reversible temperature effects, Kretschmann [2010] shows that sus- tained exposure to high temperature can induce a permanent degradation in mechanical properties. Typically this will occur if the wood has been heated to a high temperature (exceeding 100 °C) for a number of hours. Kretschmann [2010] explains that this effect is dependent on a number of factors including: moisture content, heating medium, temperature, period of exposure, and species of wood.

Chapter 2