Daniel Keunecke

Determination of Young’s and shear moduli of common yew and Norway spruce by means of ultrasonic waves

Despite the exceptional position of yew among the gymnosperms concerning its elastomechanical properties, no reference values for its elastic constants apart from the longitudinal Young’s modulus have been available from literature so far. Hence, this study’s objective was to determine the Young’s moduli EL, ER and ET and the shear moduli GLR, GLT and GRT of yew wood. For that purpose, we measured the ultrasound velocities of longitudinal and transversal waves applied to small cubic specimens and derived the elastic constants from the results. The tests were carried out at varying wood moisture contents and were applied to spruce specimens as well in order to put the results into perspective. Results indicate that EL is in the same order of magnitude for both species, which means that a high-density wood species like yew does not inevitably have to have a high longitudinal Young’s modulus. For the transverse Young’s moduli of yew, however, we obtained 1.5–2 times, for the shear moduli even 3–6 times higher values compared to spruce. The variation of moisture content primarily revealed differences between both species concerning the shear modulus of the RT plane. We concluded that anatomical features such as the microfibril angle, the high ray percentage and presumably the large amount of extractives must fulfil important functions for the extraordinary elastomechanical behaviour of yew wood which still has to be investigated in subsequent micromechanical studies.

Three-dimensional elastic behaviour of common yew and Norway spruce

In view of its high density, yew wood has a remarkably low longitudinal Young’s modulus, which makes it unique among coniferous woods. However, the elastic response of yew related to other load directions is largely unknown. Therefore, our goal was to comprehensively characterise the three-dimensional elastic behaviour of yew wood. To achieve this, we performed tensile tests on dog-bone-shaped yew specimens and determined the three Young’s moduli and six Poisson’s ratios using a universal testing machine and a digital image correlation technique. All tests were also applied to spruce as reference species. After including the shear moduli determined in a prior study by our group, all elastic engineering parameters of yew and spruce were ascertained. Based on these values, the three-dimensional elastic behaviour was describable with deformation bodies and polar diagrams. Evaluating these illustrations revealed that yew had a lower stiffness only in the longitudinal direction. In all other three-dimensional directions, spruce was clearly more compliant than yew. Particularly, in the radial–tangential plane, both species varied largely in their degree of anisotropic elasticity. All mentioned differences between yew and spruce originate at the microstructural level.

Measurement of standard and off-axis elastic moduli and Poisson's ratios of spruce and yew wood in the transverse plane

It is well known that in the radial–tangential plane of softwoods, the elastic modulus in the principal directions is clearly higher than the off-axis elastic moduli, which decrease to a minimum at a growth ring angle α of about 45°. However, this angular dependency was experimentally proven by only a few early publications. The aims of this study were (1) to analyze this relationship with up-to-date equipment in compression tests on miniature softwood specimens with varying growth ring angles and (2) to compare the experimental results with those calculated by a tensor transformation to assess whether it is admissible to treat the investigated wood species as orthotropic materials. Two softwoods with distinctly different anatomic structures (Norway spruce and common yew) were chosen, and further properties such as Poisson’s ratios were determined. The results confirm the above-mentioned angle-dependent tendency for spruce elasticity, but also show that it is not valid for softwoods in general since the behavior of yew was completely different. The tissue textures of both species, particularly density and density distribution, were discussed as possible reason for these observed differences. The determined Poisson’s ratios for principal and off-axis load directions may be useful for modeling of material behavior.

Fracture characterisation of yew (Taxus baccata L.) and spruce (Picea abies [L.] Karst.) in the radial-tangential and tangential-radial crack propagation system by a micro wedge splitting test

Abstract Common yew (Taxus baccata L.) and Norway spruce (Picea abies [L.] Karst.) are gymnosperm species that differ in their microscopic structure and mechanical characteristics. Compared to spruce, the density of yew wood is high, but the modulus of elasticity is low when loaded parallel to the grain. Information about the transverse load direction is largely lacking. Therefore, the goal of this study was to assess the elastic and fracture mechanical behaviour of both wood species in the radial-tangential plane (crack opening mode I). For this purpose, micro wedge splitting tests were performed. Characteristic elastic and fracture parameters (initial slope, critical load, specific fracture energy) were determined. After the tests, the fracture surfaces were evaluated using microscopic methods. The results reveal clear differences between the species regarding microscopic fracture phenomena and prove that yew wood was significantly stiffer than spruce wood. We suggest that the density and the cell geometry are predominantly responsible for both elasticity and failure behaviour in the transverse direction.

Comparison of two optical methods for contactless, full field and highly sensitive in-plane deformation measurements using the example of plywood

In the present paper, the suitability of Electronic Speckle Pattern Interferometry (ESPI) and Digital Image Correlation (DIC) for the measurement of two-dimensional strain distribution on mechanically stressed wood specimens is evaluated. Particular attention is dedicated to the basics of the individual techniques in order to discuss potential advantages and disadvantages. The results of a model experiment with plywood show that the results delivered by both methods are very similar and of high quality. ESPI provides reasonably fast experimental set-up and data acquisition, and fast, straightforward post-processing. Compared to ESPI, DIC is a more versatile method demanding skilled sample preparation, and post-processing may be time consuming.

Synchrotron-based tomographic microscopy (SbTM) of wood

Abstract To understand better the structure-property relationships of wood in situ, nondestructive synchrotron-based tomographic microscopy (SbTM) with subcellular resolution is useful. In this context, an in situ testing device was developed to determine the cellular response of wood to mechanical loading. Different rotationally symmetric specimens were tested to synchronize the failure areas to the given scanning areas. Norway spruce samples were uniaxially compressed in the longitudinal direction and scanned in situ at several increasing relative forces ending up in the plastic deformation regime. A sufficiently high quality in situ tomography was demonstrated. The reconstructed data allowed the observation of the load-dependent development of failure regions: cracks and buckling on the microstructure were clearly visible. Future investigations with SbTM on different wood species, loading directions, and different moisture contents are promising in terms of the micromechanical behavior of wood.

Microstructural properties of common yew and Norway spruce determined with SilviScan.

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, therefore, 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 microfibril 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.

Combination of X-ray and digital image correlation for the analysis of moisture-induced strain in wood: opportunities and challenges

In this present study, the moisture-induced deformation behaviour of a spruce sample was analysed one- and two-dimensionally with high resolution on the radial-tangential surface. For this purpose, an artificial speckle pattern was applied to the surface which was then recorded by a CCD camera during the deformation. The generated TIF images were analysed with a strain mapping software (VIC 2D) that computed the two-dimensional strain field from the surface deformation.Selected options to evaluate two-dimensional data generated with X-ray imaging and digital image correlation are presented. Combining and correlating these techniques enables detailed analysis of structure-function relationships during swelling (and shrinkage) processes in wood. However, several issues still have to be solved to enhance effectiveness and user-friendliness of such investigations, as elucidated in detail in this paper.ZusammenfassungIn dieser Arbeit wurde das feuchte-induzierte Deformationsverhalten einer Fichtenholzprobe ein- und zweidimensional mit hoher Auflösung auf der radial-tangentialen Probenoberfläche analysiert. Dazu wurde ein künstliches Specklemuster auf die Probenoberfläche aufgebracht, welches dann mit einer CCD-Kamera während der Deformation gefilmt wurde. Die generierten TIF-Dateien wurden mit einer Bildkorrelationssoftware (VIC 2D) ausgewertet, welche die zweidimensionalen Dehnungen auf der Probenoberfläche berechnete.Es werden ausgewählte Möglichkeiten zur Auswertung zweidimensionaler Daten vorgestellt, die durch Röntgendurchstrahlung und digitale Bildkorrelation generiert wurden. Einerseits ermöglicht die Kombination und Korrelation dieser Techniken eine detaillierte Analyse von Struktur- Eigenschafts-Beziehungen während der Quell- (und Schwind-) Prozesse in Holz. Andererseits müssen noch einige Probleme gelöst werden, um die Effektivität und die Benutzerfreundlichkeit derartiger Untersuchungsmethoden zu verbessern, was in diesem Artikel im Detail beleuchtet wird.

Fracture tolerance of reaction wood (yew and spruce wood in the TR crack propagation system).

The fracture properties of spruce and yew were studied by in-situ loading in an environmental scanning microscope (ESEM). Loading was performed with a micro-wedge splitting device in the TR-crack propagation direction. The emphasis was laid on investigating the main mechanisms responsible for a fracture tolerant behavior with a focus on the reaction wood. The fracture mechanical results were correlated with the features of the surface structure observed by the ESEM technique, which allows loading and observation in a humid environment. Some important differences between the reaction wood and normal wood were found for both investigated wood species (spruce and yew), including the formation of cracks before loading (ascribed to residual stresses) and the change of fracture mode during crack propagation in the reaction wood. The higher crack propagation resistance was attributed mainly to the different cell (i.e. fiber) geometries (shape, cell wall thickness) and fiber angle to the load axis of the reaction wood, as basic structural features are responsible for more pronounced crack deflection and branching, thus leading to crack growth retardation. Fiber bridging was recognized as another crack growth retarding mechanism, which is effective in both wood species and especially pronounced in yew wood.

Micromechanical properties of common yew (Taxus baccata) and Norway spruce (Picea abies) transition wood fibers subjected to longitudinal tension

The longitudinal modulus of elasticity of common yew is astonishingly low in light of its high raw density. At least this was found for specimens examined at the solid wood level and at the tissue level. However, to reveal if this low axial stiffness is also present at the cellular level, tensile tests were performed on individual yew fibers and on spruce fibers for reference. The results revealed a low stiffness and a high strain to fracture for yew when compared with spruce. This compliant behavior was ascribed to a relatively high microfibril angle of yew measured by X-ray scattering. It can be concluded that the high compliance of yew observed at higher hierarchical levels is obviously controlled by a structural feature present at the cell wall level. In future studies, the biomechanical function of this compliant behavior for the living yew tree would be of particular interest.