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 approac

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

ISSN:0018-3768ISSN:1436-736

Rheological Model for Wood

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.Comment: 37 pages, 13 figures, 10 table

Moisture-dependent orthotropic elasticity of beech wood

Elastic material properties are one of the most important material characteristics in mechanical modelling. Wood with distinctively different properties in the longitudinal, radial and tangential directions exhibits a strong moisture-dependent material characteristic in the elastic range. In order to characterise beech wood as an orthotropic material, all of the independent elastic properties were determined at different moisture conditions. These characteristic properties have never been determined before as a function of moisture content yet are vital to the field of wood modelling. All elastic parameters, except for some Poisson's ratios, show a decrease in stiffness with increasing moisture content. In comparison to available literature references at a moisture content of ω≈12%, the identified values were of the same order of magnitude. The determined material properties can be used to investigate the mechanical behaviour of beech wood structures including different moisture condition

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

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 M

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 woo

Moisture-dependent orthotropic tension-compression asymmetry of wood

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 ratio

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 leve

Finite element analysis of wood adhesive joints

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 wit