Stefan Hering

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.

Determining moisture-dependent elastic characteristics of beech wood by means of ultrasonic waves

The present study investigates the influence of moisture content on the elastic characteristics of beech wood (Fagus sylvatica L.) by means of ultrasonic waves. A set of elastic engineering parameters (i.e. three Young’s moduli, three shear moduli and six Poisson’s ratios) is determined at four specific moisture contents. The results reveal the significant influence of the moisture content on the elastic behaviour of beech wood. With the exception of some Poisson’s ratios, the engineering parameters decrease with increasing moisture content, indicating a decline in stiffness at higher moisture contents. At the same time, wood anisotropy, displayed by the two-dimensional representation of the velocity surface, remains almost unchanged. The results prove that the ultrasonic technique is suitable for determining the elastic moduli. However, non-diagonal terms of the stiffness matrix must be considered when calculating the Young’s moduli. This is shown experimentally by comparing the ultrasonic Young’s moduli calculated without, and allowing for, the non-diagonal terms. While the ultrasonic technique is found to be reliable to measure the elastic moduli, based on the measured values, its eligibility to measure the Poisson’s ratios remains uncertain.

Moisture-dependent elastic and strength anisotropy of European beech wood in tension

While the general mechanical behaviour of wood is known, its moisture-dependent elastic and strength anisotropy remains little studied. Given the anisotropic and hygroscopic nature of wood, a characterisation of wood mechanical behaviour will require knowledge of its moisture-dependent properties in relation to the three principal axes of anisotropy. The present study examines the influence of the moisture content (MC) on the elastic and strength anisotropy of beech wood (Fagus sylvatica L.). Selected elastic and strength parameters, including the anisotropic Young’s moduli, Poisson’s ratios, yield and ultimate stress values and the fracture toughness in the TR, TL, RT and RL directions, are determined in uniaxial tension and compact tension tests at different moisture conditions. A distinct moisture dependency is shown for the elastic and strength behaviour of beech wood. With the exception of some Poisson’s ratios, all investigated elastic and strength parameters are shown to decrease with increasing MC. The two- and three-dimensional representation of the compliance matrix, and the two-dimensional visualisation of a yield surface, provides a valuable overview on the moisture-dependent elastic and strength anisotropy of beech wood.

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.

Moisture-dependent orthotropic tension-compression asymmetry of wood

Abstract The influence of moisture content (MC) on the tension-compression (Te-Co) asymmetry of beech wood has been examined. The elastic and strength parameters, including Te and Co Young’s moduli, Poisson’s ratios, and ultimate and yield stress values, were determined and compared in terms of different MCs for all orthotropic directions. The results reveal a distinctive Te-Co strength asymmetry with a moisture dependency that is visualized clearly by the Te to Co yield stress ratio. The Te-Co asymmetry is further shown by the inequality of the elastic properties, known as the “bimodular behavior”. The latter is proven for the Young’s moduli values in the radial and tangential directions and for individual Poisson’s ratios. Although the bimodularity of the Young’s moduli is significant at low MC levels, there is no evidence of moisture dependency on the Te-Co asymmetry of the Poisson’s ratios.

Non-destructive determination and quantification of diffusion processes in wood by means of neutron imaging.

Abstract Diffusion processes in samples of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] Karst.) were determined and quantified by means of neutron imaging (NI). The experiments were carried out at the neutron imaging facility NEUTRA at the Paul Scherrer Institute in Villigen (Switzerland) using a thermal neutron spectrum. NI is a non-destructive and non-invasive testing method with a very high sensitivity for hydrogen and thus water. Within the scope of this study, diffusion processes in the longitudinal direction were ascertained for solid wood samples exposed to a differentiating climate (dry side/wet side). With NI it was possible to determine the local distribution and consequently the total amount of water absorbed by the samples. The calculated values scarcely differ from those ascertained by weighing (≤3%). The method yields profiles of the water content over the whole sample, thus allowing the local and temporal resolution of diffusion processes within the sample in the main transport direction (longitudinal). On the basis of these profiles, it was possible to calculate the diffusion coefficients along the fibre direction according to Ficks second law.

Thermal behaviour of Norway spruce and European beech in and between the principal anatomical directions

Abstract Thermal conductivity (ThCond), thermal diffusivity and heat capacity of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.) have been determined for all principal directions – radial (R), tangential (T) and longitudinal (L) – depending on the moisture content (MC) and ThCond was additionally measured in 15° steps between these directions. The ThCond was determined in a guarded hot plate apparatus. For determining thermal diffusivity and heat capacity, the same apparatus was supplemented with thermocouples and the temperature evolution was evaluated numerically by a partial differential equation. The results show expectedly that ThCond increases with increasing MC, whereby the highest increment was observed in T and the lowest in L direction. ThCond is higher for beech than for spruce in all anatomical directions and the conductivity for both species is more than twice as high in L direction than perpendicular to grain. The highest ThCond is found for beech at a grain angle of approximately 15°. The lowest ThCond shows spruce at an angle of approximately 60° between T and R direction. Thermal diffusivity is similar for both species and decreases with increasing MC. Its differences with regard to the anatomical directions correlate with those of the ThCond values. Heat capacity is lower for beech than for spruce and shows a clear increase with increasing MC.

Quantitative determination of bound water diffusion in multilayer boards by means of neutron imaging.

Diffusion processes into multilayered samples of Norway spruce (Picea abies [L.] Karst.) exposed to a differentiating climate (dry side/wet side) were determined and quantified by means of neutron imaging (NI). The experiments were carried out at the neutron imaging facility NEUTRA at the Paul Scherrer Institute (PSI) in Villigen (Switzerland).With NI the influence of different adhesives (polyvinyl acetate (PVAc), urea formaldehyde resin (UF), epoxy resin (EP), one-component polyurethane (1C PUR)) on the diffusion process could be determined by varying the layer number and the thickness of adhesive joints of the samples. Thereby, neutron transmission images were used to measure time dependent water profiles in the diffusion direction. Using Fick’s second law, diffusion coefficients for radial and tangential water transport in spruce wood and in the adhesive joints were calculated depending on moisture content (MC). It was found that the diffusion coefficients of the adhesives (1C PUR, EP at high MC) were up to three orders of magnitude lower than those of spruce wood. PVAc and UF had a smaller barrier effect compared to wood, which in contrast to 1C PUR and EP, clearly depends on the MC.ZusammenfassungEs wurden Diffusionsprozesse an mehrlagigen Proben von Fichte (Picea abies [L.] Karst.), welche einem Differenzklima (trocken/feucht) ausgesetzt waren, mittels Neutronenradiographie untersucht und quantifiziert. Die Experimente wurden an der Radiographiestrahllinie NEUTRA am Paul Scherrer Institut (PSI) in Villigen (Schweiz) durchgeführt.Mittels Neutronenradiographie konnte der Einfluss verschiedener Klebstoffe (Polyvinylacetat (PVAc), Harnstoffharz (UF), Epoxidharz (EP) und Einkomponenten-Polyurethan (1K-PUR)) auf den Diffusionsprozess bestimmt werden, indem die Anzahl und die Dicke der Klebfugen variiert wurden. Dabei wurden Neutronen-Transmissionsbilder verwendet, womit zeitabhängige Profile in Diffusionsrichtung gemessen werden konnten. Anhand des zweiten Fick’schen Gesetzes konnten die Diffusionskoeffizienten für die Klebstoffe sowie für Fichte in radialer und tangentialer Richtung in Abhängigkeit der Feuchte berechnet werden. Dabei wiesen die Klebstoffe (1K-PUR, EP bei hohen Feuchten) bis zu drei Zehnerpotenzen niedrigere Diffusionskoeffizienten als Fichtenholz auf. Bei PVAc und UF war die Sperrwirkung gegenüber dem Holz geringer und es zeigte sich im Gegensatz zu 1K-PUR und EP eine deutliche Abhängigkeit von der Holzfeuchte.

Moisture-dependent, viscoelastic creep of European beech wood in longitudinal direction

In the present study, the pure viscoelastic behaviour of European beech wood is analysed in the longitudinal direction at three different moisture contents. The moisture-dependent creep compliance is identified using a four-point bending test device. The viscoelastic behaviour is ascertained to be linear with moisture content and quantified by means of a Kelvin-Voigt model approach.ZusammenfassungIn dieser Arbeit wird das viskoelastische Verhalten von Rotbuchenholz in longitudinaler Richtung bei drei verschiedenen Holzfeuchten untersucht. Mit Hilfe eines Vierpunktbiegeprüfstandes wurde die feuchteabhängige Kriechnachgiebigkeit bestimmt. Ein linearer Zusammenhang zwischen der Holzfeuchte und der Kriechnachgiebigkeit wird nachgewiesen und anhand eines verallgemeinerten Kelvin-Voigt Ansatzes quantifiziert.

Viscoelastic characterization of wood: Time dependence of the orthotropic compliance in tension and compression

This study is concerned with the viscoelastic behavior of wood. The time dependency of the orthotropic compliance for beech wood is investigated by performing tensile (Te) and compressive (Co) creep experiments in all woods orthotropic directions. Time evolution of the creep strain in the axial and lateral directions is recorded using the digital image correlation technique, to determine the time dependent Youngs moduli and the Poissons ratios needed for the calculation of the diagonal and nondiagonal elements of the viscoelastic compliance matrix. The results of this study demonstrate the viscoelastic character of wood, revealing the significant time influence on the mechanical behavior. The unequal time dependency of the Youngs moduli and the Poissons ratios obtained for the individual directions highlights the orthotropic nature of the viscoelastic compliance. Differences between the time dependent behavior for the compliance determined in Te and Co further indicate that the viscoelastic behavior of wood depends on the loading modality. Supported by the unequal evolution of the Te and Co creep strain, the results suggest that the time dependent stress–strain relationship of wood is essentially different in Te and Co. Poissons ratio values, which are shown to increase with time in Te and decrease in Co, demonstrated this fact. The substantially different time dependency of the nondiagonal elements of the compliance matrix further emphasizes the complexity of the viscoelastic character of wood. Visualized by the time evolution of the corresponding nondiagonal elements ratio, differences between the Te and Co viscoelastic behavior become particularly visible in the time dependency of the orthotropic compliance asymmetry.