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Microstructural Properties of Common Yew and Norway Spruce Determined with Silviscan

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Microstructural Properties of Common Yew and Norway Spruce Determined with Silviscan IAWA Journal, Vol. 30 (2), 2009: 165–178 MICROSTRUCTURAL PROPERTIES OF COMMON YEW AND NORWAY SPRUCE DETERMINED WITH SILVISCAN Daniel Keunecke1,*, Robert Evans2 and Peter Niemz1 SUMMARY Yew wood holds a special position within the softwoods with regard to its exceptional elasto-mechanical behaviour. Despite a relatively high density, it is highly elastic in the longitudinal direction (the modulus of elasticity is low and the stretch to break high). In the radial-tangential plane, its elastic anisotropy is clearly less pronounced compared to other softwoods such as spruce. Knowledge of the anatomical organisation of yew wood is an indispensable precondition for the correct interpretation of this conspicuous mechanical behaviour. The aim of this study, there- fore, was to interpret the difference in elasto-mechanical behaviour of yew and spruce (as a reference) through their relative microstructures as measured by SilviScan, a technology based on X-ray densitometry, X-ray diffractometry and optical microscopy. This system is able to measure a variety of structural features in a wood sample. The results reveal that the elasto-mechanical response of yew is primarily due to large microfi- bril angles and a more homogeneous cross-sectional tissue composition (regarding tracheid dimensions and density distribution) compared to spruce. With respect to structure-property relationships, it was concluded that yew wood combines properties of normal and compression wood and therefore takes an intermediate position between them. Key words: Taxus baccata, Picea abies, SilviScan, microstructure, micro- fibril angle, modulus of elasticity, density, anisotropy. INTRODUCTION For centuries, common yew (Taxus baccata L.) has been well known for its extraordi- nary longitudinal elasticity and toughness. Among other things, its wood was used for certain weapons (longbows, lances, crossbows) requiring these properties, particularly the low modulus of elasticity (MOE) and at the same time a high elastic strain parallel to the grain. The few available literature references (Sekhar & Sharma 1959; Jakubczyk 1966; Wagenführ 2000; Märki et al. 2005) also indicate a high elasticity (with MOE between 6.2 and 12 GPa). Although there are other elastic softwood species, none of them has an air-dry density (= density at 12% wood moisture content) as high as that 1) Institute for Building Materials (Wood Physics), ETH Zurich, 8093 Zurich, Switzerland. 2) CSIRO Materials Science and Engineering, Clayton, Victoria 3168, Australia. *) Corresponding author [E-mail: [email protected]]. Associate Editor: Lloyd Donaldson Downloaded from Brill.com09/27/2021 08:13:19PM via free access 166 IAWA Journal, Vol. 30 (2), 2009 of yew (620–720 kg m-3). Thus, yew holds an exceptional position, especially since there is usually a strong species-spanning positive interrelation between density and axial MOE (for example, see data compilation in Sell 1997). This unusual combination (high density, low MOE) prompted us to investigate yew elasticity in more detail. In previous studies by our group (Keunecke et al. 2008a, b; Keunecke & Niemz 2008), we determined the axial MOE of adult yew and (as a refer- ence) spruce heartwood at standard climatic conditions (20 °C, 65% relative humidity (RH)) for three different hierarchical levels (solid wood; tissue with a thickness of 220 µm in the tangential direction and a width of 3.5 mm in the radial direction; tracheids). Furthermore, we calculated the three-dimensional elastic behaviour of both species at the solid wood level (Keunecke et al. 2008b). The two crucial findings of these studies were: 1) Across all studied hierarchical levels, the axial MOE of yew was clearly lower than that of the 30% less dense spruce (Table 1). In other words: The lower MOE was exhibited not only by specimens subjected to cell-cell interactions but also by in- dividual tracheids for which pure cell wall mechanics applied. Therefore, the axial stiffness of yew is obviously controlled by a feature that is present even at the cel- lular level. 2) Especially in the radial-tangential (RT) plane, the anisotropy of the elastic behavi- our calculated for uniaxial tensile load was clearly less pronounced for yew than for spruce. This means in detail: In the case of spruce wood, even small deviations from the principle load axes (R, T) effect a considerable increase of compliance at the same stress level. The maximum compliance (a combination of the compli- ances in the radial and tangential directions) is reached at an angle of about 45°, central between both axes. Yew behaves differently: the deformation is clearly less anisotropic; it is largest along the principle axes and only slightly decreases to a minimum near 45°. Table 1. Mean axial stiffness of yew and spruce determined at three hierarchical levels at 20 °C and 65% relative humidity. Species Hierarchical level MOECW (GPa) MOECSA (GPa) Yew Tracheids1 13.9 (36.6%) — Tissue2 15.6 (26.9%) 7.0 (23.9%) Solid wood3 16.9 (11.4%) 10.5 (13.6%) Spruce Tracheids1 26.2 (28.3%) — Tissue2 29.4 (18.6%) 9.9 (21.5%) Solid wood3 27.2 (8.2%) 12.8 (9.2%) The data presented are mean values. Tracheid level: yew, number of specimens (n) = 18; spruce, n = 21. Tissue level: yew, n = 41; spruce, n = 40. Solid wood level: yew, n = 12; spruce, n = 10. The data for the tissue level are mean values for two specimen series per species. Figures in parentheses are coefficients of variation. MOECW = modulus of elasticity based on the cell wall area; MOECSA = modulus of elasticity based on the cross-sectional area including lumens. 1) Keunecke et al. (2008a); 2) Keunecke & Niemz (2008); 3) Keunecke et al. (2008b). Downloaded from Brill.com09/27/2021 08:13:19PM via free access Keunecke, Evans & Niemz — SilviScan studies of yew and Norway spruce 167 After evaluating supplementary measurements, microscopic images and literature sources, we assume the following reasons for these two findings: Finding 1: The low axial MOE of yew can be explained by the large microfibril angles (MFA) we measured with three different techniques (wide angle X-ray diffrac- tion (WAXD), small angle X-ray scattering (SAXS), and the ray/tracheid cross-field pit aperture method). All of these methods, however, have their weaknesses: a) the data were difficult to obtain as the preparation was very time-consuming; b) when applying WAXD and SAXS (beam aligned in the radial wood direction), the MFA of a large number of cells was averaged from part of a whole growth ring. Thus, the broad radial MFA variations were not taken into account; c) the pit aperture method is only reliable for estimating the MFA in latewood tracheids (Huang et al. 1997). Finding 2: The large differences between yew and spruce regarding their degree of anisotropic elastic behaviour in the RT plane can be explained by a smaller earlywood (EW)/latewood (LW) density gradient of yew compared to spruce. This conclusion, however, is based on only a few references (e.g., Wagenführ 2000) and on exemplary microscopic analyses. The goal of this present study, therefore, was to support and to quantify the previous results (so far standing on “shaky ground”), and to further improve our knowledge of the structure-function relationships of yew wood. We decided to run yew and spruce samples through the SilviScan microstructure analyser, a system with a high spatial resolution and able to measure much more rapidly than conventional methods. SilviScan was developed at CSIRO (Commonwealth Scientific and Industrial Research Organisa- tion) to assess wood structural features such as density, MFA, tracheid diameters and cell wall thickness by a combination of X-ray densitometry, X-ray diffractometry, and digital microscopy (Evans 1994, 1999; Evans et al. 1999, 2000). MateRIAL AND METHODS Material Two samples each of yew and spruce, the origins of which (natural forests) are shown in Table 2, were cut from tree disks taken from randomly selected trunks at breast height. This means a total of four samples were investigated. As we wanted to avoid examining special tissues such as compression wood, we chose tree disks without apparent eccentric increment. The yew samples were from different sites, as were the spruce samples. However, in addition, the spruce differed greatly in the width of growth rings (Table 2). The spruce sample with narrow growth rings is also termed “spruce (n)” in the following, and the sample with wide growth rings “spruce (w)”. Since the yew samples attracted our main attention, the differences between the spruce samples are not elucidated in great detail. The samples were processed to radial sections of 2 mm thickness (tangential) and 7 mm height (longitudinal); their radial length varied between 90 and 260 mm, depending on stem diameter. They were cut out of the tree disks with a twin-blade circular saw from close to the pith (the first few rings were left out) to the sapwood; the sapwood consisted of less than ten growth rings in the yew samples; the spruce samples were free Downloaded from Brill.com09/27/2021 08:13:19PM via free access 168 IAWA Journal, Vol. 30 (2), 2009 Table 2. Origin, radial length, number of analysed growth rings, and mean growth ring width of the samples analysed with SilviScan. Yew no. 1 Yew no. 2 Spruce (n) Spruce (w) Regional origin Zurich, Arnsberg, Montane region Foothills of Switzerland Germany in Grisons the Alps, (alt. 1700 m), Switzerland Switzerland Radial length (mm) 150 90 160 260 Analysed growth rings 138 52 116 55 Mean growth ring width (mm) 1.1 1.7 1.4 4.7 The samples were 2 mm wide in the tangential and 7 mm high in the longitudinal direction; the radial length depended on the respective pith-to-sapwood distance of the samples. Spruce (n) = spruce with narrow growth rings; spruce (w) = spruce with wide growth rings.
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