Walter Sonderegger

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.

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.

On-line monitoring of hygroscopicity and dimensional changes of wood during thermal modification by means of neutron imaging methods

Abstract European beech (Fagus sylvatica L.) and Norway spruce [Picea abies (L.) Karst.] have been thermally modified in a combined air-steam atmosphere between 70°C and 150°C and pressures up to 4 bar, and the changes of dimensions and moisture contents (MC) of the samples were studied in-situ by means of neutron imaging (NI). This non-invasive testing method offers unique opportunities. NI is highly sensitive for hydrogen and thus well suited for monitoring the MC changes, although some metals (e.g., Al), indispensable for sample environments under high temperature and pressure, are practically transparent to neutrons. The results show that the modification induced changes of MC and dimensions both in radial and tangential direction can well be determined by NI. Dimensional changes from water saturation to oven-dry state, the sorption isotherms, and the differential swelling were observed. Additionally, the sorption behaviour at 20°C was investigated after thermal modification and colour measurements were carried out before and after the thermal treatment.

An investigation of the influence of selected factors on the properties of spruce wood

Thirty Norway spruce trees (Picea abies (L.) Karst.) from the forest district of the ETH Zurich were tested for bending MOR, static MOE of bending and dynamic MOE (calculated from eigenfrequency and sound velocity). The specimens were clear and were sampled from the whole of the stem. Their correlations to density, annual ring width, height in the tree, distribution over the stem diameter and the percentage of compression wood were statistically analysed. All three elasticity modules and the maximal stress can be very well predicted from a linear function of the sample density with a common gradient across the compression wood values but with different intercepts that decrease with increasing compression wood content. The other variables have highly significant impacts on the response variables too, however, this is largely irrelevant for the goodness of fit. Further, a clear increase of density, of MOE and of bending MOR was measured from pith to bark and similarly with decreasing annual ring width. Concerning the height of the stem, no distinct trend for the mechanical properties could be found.

Investigations on the physical and mechanical behaviour of sycamore maple (Acer pseudoplatanus L.)

Physical and mechanical properties of sycamore maple (Acer pseudoplatanus L.) were extensively investigated as basis for three-dimensional material modelling for structural simulations (e.g., with finite element method) based on this species. The physical properties of swelling, water absorption, water vapour resistance and thermal conductivity were tested and the mechanical properties of tensile, bending and compression strength and of Young’s modulus (static and dynamic) as well as of Poisson’s ratio, shear strength, shear modulus and fracture toughness were determined. The tests were carried out for most of the features depending on moisture content and also in all three anatomical main directions: longitudinal, radial and tangential.ZusammenfassungDie physikalischen und mechanischen Eigenschaften von Bergahorn (Acer pseudoplatanus L.) wurden eingehend untersucht, um eine Grundlage für eine dreidimensionale Materialmodellierung für Struktursimulationen (z.B. mit der Methode der finiten Elemente), basierend auf dieser Holzart, zu schaffen. Bei den physikalischen Eigenschaften wurden das Quellen, die Wasseraufnahme, der Wasserdampfdiffusionswiderstand und die Wärmeleitfähigkeit geprüft sowie bei den mechanischen Eigenschaften die Zug-, Biege- und Druckfestigkeit, der Elastizitätsmodul (statisch und dynamisch), die Poissonzahl, die Scherfestigkeit, der Schubmodul und die Bruchzähigkeit bestimmt. Die Untersuchungen wurden in den meisten Fällen feuchteabhängig durchgeführt und ebenso in allen drei anatomischen Hauptrichtungen: längs, radial und tangential.

Effects of aging on wood: a literature review

Knowledge of wood aging and the property changes of aged wood compared with recent wood are crucial for conservation of wooden cultural heritage objects and historic buildings constructed of wood and also for the reuse of old construction wood. Therefore, a thorough literature review is presented about the different aspects of wood aging to provide a database for further investigations. One focus lies on the different kinds of aging: natural aging under aerobic and anaerobic storage conditions in contrast to accelerated aging under heat treatment. Further, influencing factors like wood treatment and long-term loading on the aging process are discussed. Property changes of naturally aged wood that has been stored under aerobic conditions are also researched. The resulting chemical, physical, and mechanical changes are thus discussed as well as any changes in color.

Combined bound water and water vapour diffusion of Norway spruce and European beech in and between the principal anatomical directions

Abstract The combined bound water and water vapour diffusion of wood is of great interest in the field of building physics. Due to swelling stresses, the steady-state-determined diffusion coefficient clearly differs from the unsteady-state-determined diffusion coefficient. In this study, both diffusion coefficients and the water vapour resistance factor of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.) were investigated for the principal anatomical directions (radial, tangential and longitudinal) and in 15° steps between these directions. The values were determined with the cup method as the basic principle. The unsteady-state-determined diffusion coefficient is, independent of the direction, about half that of the steady-state-determined diffusion coefficient. Both diffusion coefficients are about two to three times higher for spruce than for beech. They are up to 12 times higher in the longitudinal direction than perpendicular to the grain for spruce, and up to 15 times higher for beech. With increasing moisture content, the diffusion coefficients exponentially increase. The water vapour resistance factor shows converse values to the diffusion coefficients.

Stiffness moduli of various softwood and hardwood species determined with ultrasound

Abstract Comprehensive data sets of the elastic constants are available for only a few European wood species. The goal of this study was to provide stiffnesses for the principal directions and planes for further selected European softwoods and hardwoods, to extend the currently existing data sets. For this purpose, a dynamic technique was chosen: the moduli were estimated on the basis of sound velocity and density measurements. The collected data may be helpful for specialists in the relevant research and practical fields, particularly when stresses and strains of structural elements have to be calculated using modelling approaches.