In this study, several theoretical models to numerically estimate shear properties of orthotropic materials are introduced. These approaches are based on the combination of Hankinson’s empirically derived formula with other empirical and analytical calculations. Next to shear moduli, which are estimated from the elastic moduli and Poisson’s ratios, shear strengths are also estimated from the in-axis strengths. The models are validated by mechanical tests on walnut wood (Juglans regia L.), for which a sufficient data set can be found in literature. The Arcan test is used to estimate the shear moduli, while the shear block test is used to estimate the shear strengths. The results show that the model, which is based on a combined use of Hankinson’s formula and tensor rotation, gives the best estimation of shear moduli as evaluated by the minimum differences to the experimentally obtained results. For the shear strengths, a combination of Hankinson’s formula and Norris’ failure criterion shows the best agreement in comparison to the experimental data. The theoretical calculations may be used for a time efficient estimation of shear modulus and strength in comparison to the very time-consuming experimental estimation.

The present study examines the three-dimensional hygric and mechanical behavior of oak wood. The moisture equilibrium state, characterized by the sorption isotherms, was obtained from measurements taken during adsorption and desorption cycles. Sorption behavior was analyzed with the Dent theory and compared considering the sorption direction (adsorption/desorption cycle). Sorption parameters were provided for possible numerical applications in hygric material models. The corresponding swelling and shrinkage behavior was examined and characterized by the moisture expansion parameters for all anatomical directions. Orthotropic mechanical material behavior was characterized by determining the elastic engineering (Young's moduli, shear moduli, and Poisson's ratios) and the bending, compressive and compressive shear strength material parameters. Influence of moisture content (MC) on the mechanical material properties was studied using Young's moduli, Poisson's ratios, and the investigated strength parameters. A significant difference between the sorption behavior in adsorption and desorption, known as the hysteresis effect, could be proved. Furthermore, swelling and shrinkage behavior did not show any dependency on the adsorption/desorption cycle. The results confirm the significant influence of MC on the Young's moduli and the strength properties, however, did not validate an influence on the Poisson's ratios.

Engineered wood products such as glulam or cross-laminated timber are widely established in the construction industry. Their structural behaviour and reliability clearly bases on the adhesive bonding. In order to understand and improve the performance of glued wood members a finite element modelling of standard single lap shear samples was carried out. A three-dimensional model of a longitudinal tensile-shear specimen with quasi-centric load application was developed. The main influences of wood and adhesive parameters on structural performance were identified. Therefore, variations of the elasticity, the annual ring angle, fibre angle, and the interface zone and their effect on the occurring stresses in the adhesive bond line were investigated numerically. The adhesive bond line is most significantly sensitive to the Young´s modulus of the adhesive itself. A variation of the fibre angle of the glued members in the standard test is an essential criterion and to be considered when preparing lap shear specimens. A model with representation of early- and latewood gives a more detailed insight into wooden adhesive joints.

Hardwood is tried to be increasingly established in building industry in order to realise a larger material output and to take at once into account the structural advantage of the improved mechanical properties of hardwood in comparison to softwood. For the partially required calculations therefore of stress and strain in boards and glued-laminated timbers (for example prognosis of potential delamination and aging of glued joints), it is essential to know the directional specific values of wood in the directions longitudinal, radial and tangential. This concerns the elastic specific values (YOUNG's modulus, shear modulus, POISSON's ratio), the specific values of strength (tension, compression, bending, shear) and also these of mass and heat flux (diffusion coefficient, thermal conductivity). Additionally, it is necessary to know the dependencies from moisture and duration for all specific values. Selected parameters for European beech wood will be presented concerning the dependencies of YOUNG's modulus, shear modulus, POISSON's ratio, thermal conductivity and diffusion coefficient from direction and moisture. The results summarize investigations on this topic realised at the Wood Physics Group of ETH Zurich.

Wood as the most important natural and renewable building material plays an important role in the construction sector. Nevertheless, its hygroscopic character basically affects all related mechanical properties leading to degradation of material stiffness and strength over the service life. Accordingly, to attain reliable design of the timber structures, the influence of moisture evolution and the role of time- and moisture-dependent behaviors have to be taken into account. For this purpose, in the current study a 3D orthotropic elasto-plastic, visco-elastic, mechano-sorptive constitutive model for wood, with all material constants being defined as a function of moisture content, is presented. The corresponding numerical integration approach, with additive decomposition of the total strain is developed and implemented within the framework of the finite element method (FEM). Moreover to preserve a quadratic rate of asymptotic convergence the consistent tangent operator for the whole model is derived. Functionality and capability of the presented material model are evaluated by performing several numerical verification simulations of wood components under different combinations of mechanical loading and moisture variation. Additionally, the flexibility and universality of the introduced model to predict the mechanical behavior of different species are demonstrated by the analysis of a hybrid wood element. Furthermore, the proposed numerical approach is validated by comparisons of computational evaluations with experimental results.

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 wood's 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 Young's moduli and the Poisson's 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 Young's moduli and the Poisson's 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. Poisson's 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.

A radial core from a Norway spruce (Picea abies (L.) Karst.) estimated to be about 107 years old was cut from a board and was analyzed for density and microfibril angle (MFA). Furthermore, cell geometry, wall thickness and lignin distribution were analyzed on three selected growth rings in detail. Intra-ring differences in the density profiles are also true for cell wall thicknesses as well as radial and tangential lumen diameters. A higher MFA was found for earlywood with a slow decrease toward the latewood region. The lignin was found to remain rather constant throughout the growth rings, which suggests a constant chemical composition of the cell wall material within the growth ring. From the recorded datasets on a cellular level, it can be concluded that the main adaptation regarding structure–property relationships toward the optimization of water transport and mechanical stability is mainly achieved at the cell level.

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.

Wood, due to its biological origin, has the capacity to interact with water. Sorption/desorption of moisture is accompanied with swelling/shrinkage and softening/hardening of its stiffness. The correct prediction of the behavior of wood components undergoing environmental loading requires that the moisture behavior and mechanical behavior of wood are considered in a coupled manner. We propose a comprehensive framework using a fully coupled poromechanical approach, where its multiscale implementation provides the capacity to take into account, directly, the exact geometry of the wood cellular structure, using computational homogenization. A hierarchical model is used to take into account the subcellular composite-like organization of the material. Such advanced modeling requires high-resolution experimental data for the appropriate determination of inputs and for its validation. High-resolution x-ray tomography, digital image correlation, and neutron imaging are presented as valuable methods to provide the required information.

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.

The moisture-dependent stress-strain behaviour of European beech wood (Fagus sylvatica L.) has been determined under compression, analytically described for the radial and tangential direction using a modified Ramberg-Osgood approach and realized numerically by a multi-surface plasticity model. The conspicuous decrease of the stress-strain characteristic perpendicular to the grain with increasing moisture content is similar to the behaviour of the presented ultimate strength values in the longitudinal direction. Based on input parameters previously quantified by means of an inverse finite element analysis, the simulation of a crosswise laminated two-layer arrangement during moistening was carried out. A distinct development of plastified regions can be observed due to the resulting swelling stresses.

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.

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.

Wood bondings have been used in timber construction and many other applications for a long time. Using layered arrangements, the homogenisation of the anisotropic physical and mechanical properties of solid wood reduce the influence of moisture and improve the mechanical behaviour of structural elements. However, new weak points can be created and effects such as debonding and delamination can occur. These failure characteristics depend on mechanical and hygroscopic loading as well as the composition of the structural components. Hence, delamination can be reduced by adjusting the properties of the appropriate adherent or by changing the structural assembly of the specific elements. Using finite element analysis, the influence of the adherent properties during the testing procedure and an example of cross-laminated wood panels on the adhesive load is investigated. The results show significant differences in the glue line stress distribution for varied growth ring angle, longitudinal Young’s modulus, diffusion as well as swelling and shrinkage properties.

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 degrees 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 degrees. The lowest ThCond shows spruce at an angle of approximately 60 degrees 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.