Rheological behavior of wood under uniaxial compression along and perpendicular to the grain in constant environment was examined. Tests with constant deformation rate until failure and stress relaxation tests with constant deformation applied stepwise were carried out. The experimental results of stress relaxation showed nonlinear material behavior over time that got more prominent under high deformation levels. Considerable amount of stress relaxed during applying the deformation. Wood experienced greater stress relaxation along the grain than perpendicular to it. Three rheological models for orthotropic material were calibrated to the experimentally determined stress–time curves in longitudinal and transverse directions simultaneously. Small deformation levels assuming linear strains were accounted for in the models. Required elastic material parameters were determined from the tests with constant deformation rate. A model including the highest number of viscoelastic material parameters was the most successful in predicting stress relaxation of wood under stepwise deformation. Modeling indicated that wood behavior was very close to linear viscoelastic in relaxation under small deformation. The obtained material parameters made the model suitable for predicting rheological behavior of wood comprehensively, under sustained deformation or load in constant conditions
Three coupled two-dimensional viscoelastic creep models for orthotropic material are analyzed. The models of different complexity are mathematically formulated and implemented in a finite element software. Required viscoelastic material parameters are determined by calibration procedure, where numerical results are compared against experimentally obtained viscoelastic strains caused by tensile or shear loading. Finally, a comparison method is used to evaluate the accuracy of strain predictions of each particular model. The analysis shows that all the models are able to accurately predict viscoelastic creep simultaneously in two perpendicular directions for various periods of time and wood species. Calculated numerical values of the viscoelastic material parameters suitable for the three models and wood species, i.e., Douglas fir (Pseudotsuga menziesii), Norway spruce (Picea abies), Japanese cypress (Chamaecyparis obtusa), and European beech (Fagus sylvatica L.), under constant tensile loading are also given.
A three-dimensional (3D) rheological model for an orthotropic material subjected to sustained load or deformation under constant climate has been mathematically formulated. The elastic and viscoelastic compliance matrices are symmetric, where the mathematical derivation of the latter is shown. The model is linear and requires constant numerical values for the elastic and viscoelastic material parameters. The model’s ability to predict the natural time-dependent response in three material directions simultaneously is demonstrated on a Douglas fir (Pseudotsuga menziesii) specimen subjected to a constant uniaxial tensile load. The material extends in a longitudinal direction and contracts in the transverse directions with time. The required material parameters are taken from the literature when possible, otherwise they are assumed. Furthermore, the influence of misalignment between the directions of observation and wood material directions on induced time-dependent strains is analyzed. The analyses show that the misalignment has a large effect on the material behavior. In some cases, the specimen under constant uniaxial tension even extends in the perpendicular transverse direction with time. The obtained results clearly demonstrate the high importance of considering the alignment of material directions precisely in order to be able to interpret the time-dependent behavior of wood correctly.
Changes in relative humidity of the ambient air, RH (%), cause wetting and drying of wood material, which results in non-uniform moisture contents or moisture gradients, and consequently in moisture-induced stresses and strains in the glued-laminated timber (glulam) members. The aim of the present paper is to perform a hygromechanical analysis to predict the mechanical behavior of glulam specimens exposed to two RH regimes, causing wetting from 50% to 90% RH and drying from 90% to 50% RH, and compare the numerical to the experimental results. The aims are also to quantitatively analyze the influence of characteristic material parameters required in the multi-Fickian moisture transport model and the mechanical model on moisture-induced strains and stresses in glulam specimens and to determine the possibility of cracking of the material by analyzing the maximum tensile stresses perpendicular to the grain. Accurate numerical predictions of moisture contents and moisture-induced strains are obtained in the glulam specimens during wetting and drying as compared to the experimental results. The influence of a particular characteristic material parameter on moisture-induced strains and stresses is characterized as significant, but not crucial when a rough numerical estimation of the mechanical behavior of the glulam beam exposed to RH changes is required.
A hygro-mechanical (H-M) analysis of a wooden specimen sustaining a mechanical load while subjected to varying relative humidity was performed to predict the long-term rheological behavior of wood. The numerical analysis was based on the experimental results of total strains, monitored in two orthotropic material directions on oak wood specimens under constant uniaxial compression and with moisture content (MC) variation. For the moisture analysis, a multi-Fickian moisture transport model (MFMTM) was used to obtain temporal and spatial MC fields, which were the input data in the mechanical analysis. The presented mechanical model assumed a decomposition of the total strains into the elastic, viscoelastic and mechanosorptive strains and the strains due to shrinkage and swelling. The moisture and mechanical analyses required material parameters, which were taken from the literature or were empirically obtained by a fitting procedure. The performed H-M analysis gave accurate numerical predictions of the experimentally obtained total strains in two orthotropic directions simultaneously. Thus, the analysis developed has a high potential for predicting the long-term rheological behavior of timber structures, assuming that the material parameters are determined previously, based on specific, extensive, multidimensional experimental analyses.
The paper presents sensitivity analysis of coupled heat and moisture transfer model for timber exposed to fire. The objective of the analysis is to discover the non-influential model parameters and the model simplification accordingly. To achieve this, the standardized regression coefficient (SRC) method is introduced to determine the impact of specific permeability of dry timber K, bound water diffusion coefficient , vapour diffusion coefficient and heat of sorption on the two model outcomes, charring depth and total moisture content . The SRC method revealed that the least influential parameter is specific permeability of dry timber K. Therefore the model was adequately simplified by a more simple description of the energy equation, while preserving the accuracy of the results. Thus, the efficiency of the present coupled heat and moisture transfer model was increased.
Load-bearing capacity of slender dowel-type fasteners of round ring-shank nails and U-shaped connectors (U-conn:s) of square cross-sectional nail thread used as connections in Timber-Concrete Composite structures have been studied, experimentally and with two different theoretical models. Tensile strength and withdrawal capacity of fasteners and embedment strength of timber were tested and evaluated. The parameters were used for calculation of the plastic hinge location in the fastener on the timber side and the load-bearing capacity of the connections. For this, both the European Yield Model known from the Eurocode 5, as well as a model where the load-bearing capacity is determined from mechanical equilibrium of fasteners in deformed state were used. Double shear push-out tests were carried out to determine the load-carrying capacity of the connections. The calculated values are compared to the experimental results. The models showed similar results for the U-conn:s regarding both plastic hinge location and load-bearing capacity. For the nails, all models show good correlation with experimental values, however, the model derived from the deformed state predicted both the plastic hinge location and the load-carrying capacity more accurately.
We present a procedure for accurately calibrating a dynamic vapor sorption (DVS) instrument using single salts. The procedure accounts for and tailors distinct calibration tests according to the fundamental properties of each salt. Especially relevant properties influencing the calibration are the heat of solution, heat of condensation, and the kinetics connected to the salt phase transition, as these influence the microclimate surrounding the salts during calibration. All these issues were dealt with to obtain precise calibration results. The DVS instrument comprises two control modes to generate and measure the relative humidity (RH). Both control modes were separately examined and combined to overcome the shortcomings of each of the two control modes and thereby obtain the most accurate results. Repeated calibration testing with the single salts (LiCl, MgCl2, Mg(NO3)2, NaCl, and KNO3) enables five discrete sorption isotherm measurements within the range of 11%-93%RH. The equilibrium RH of the solution for LiCl, MgCl2, Mg(NO3)2, NaCl, and KNO3 was determined with a standard deviation of 0.06%-0.15% (0.45% for KNO3) RH. By comparing the measured calibration values with the well-known equilibrium RH of each salt solution, the presented method's results are both accurate with significant agreement and precise with small variation.
Strength parameters for fasteners determined in accordance with the methods prescribed for the European CE-marking leads quite different values for seemingly similar products from different manufactures. The results are hardly repeatable, to some extent due to difficulties in selecting representative timber samples for the testing. Beside this uncertainty, the declared values concerns only structural timber, so no values are available for e.g, LVL or plywood.
This paper gives:
A theory on the lateral load-carrying capacity of timber connections with slender fasteners is presented. The base of the theory is the coupled mechanical phenomena acting in the connection, while the wood and the slender fastener deform and yield prior to failure. The objective is to derive a sufficient description of actions and responses which have determining influence on the load-carrying capacity of timber connections with slender fasteners. Model assumptions are discussed and made, but simplifications are left out. Even so, simple mathematical equations describing the lateral capacity are derived from mechanical equilibrium of the deformed fastener. The herein proposed theory is verified against tests. The tests were designed to vary the influence of isolated mechanical phenomenon as much as possible. The theory shows a very high accuracy and precision when predicting the load-carrying capacity of the tested connections.