Creep Of Wood Research Articles

Accelerated relaxation behavior during water desorption in the mechano-sorptive creep of wood: modeling and analysis based on the free volume concept and Kohlausch–Williams–Watts function

Abstract The accelerated creep behavior during water desorption of wood was modeled on the Kohlausch–Williams–Watts (KWW) function and free volume concept. Assuming that the water desorption rate dmc/dln t $\text{dmc/dln\,t}$ is involved in the creation of a new free volume, the net creep compliance during desorption J du(t) was obtained as ln J du ( t ) = β c ln t − β c ( ln τ c 0 + k f dmc / d ln t ) + ln J c ∞ $\mathrm{ln} {J}_{\text{du}}\left(t\right)={\beta }_{\text{c}} \mathrm{ln} t-{\beta }_{\text{c}}\left(\mathrm{ln} {\tau }_{\text{c}0}+{k}_{\text{f}}\,\text{dmc}/\text{d}\mathrm{ln} t\right)+\mathrm{ln} {J}_{\text{c}\infty }$ where β c, τ c0 and J c∞ are the stretching parameter, characteristic time of the relaxation, and long-time asymptote compliance at a constant moisture content (mc) equal to the initial mc of the accelerated creep, respectively, and k f is the coefficient of the water desorption rate. These parameters were determined by simulation using experimental data and literature results and were found to be reasonable. The simulation results agreed well with the experimental ones. This verifies the validity of the model and its derived equations.

Mathematical model of the stress-strain state of a rod with an inhomogeneous layered structure

The stress-strain state (SSS) of a rod with an inhomogeneous layered structure is considered. On the basis of a brief review and analysis of the current state of research of rod systems, the relevance of the study of the SSS of layered-heterogeneous wooden structures is substantiated, taking into account the presence of different resistance of layers to tension and compression. On this basis, the authors solve the problem of determining the SSS of layered-heterogeneous wooden rods in creep conditions, where factors such as humidity and temperature, as well as the difference in the resistance of wood layers to stretching and compression are taken into account. When solving the problem, the mechanical-sorption creep of wood is also taken into account.

Analysis of Factors Affecting Creep of Wood–Plastic Composites

Wood–plastic composite (WPC) materials are mainly used as flooring in buildings or as structural load-bearing plates, and will undergo creep deformation during use, resulting in structural failure and safety problems. Therefore, this work adopted the orthogonal test method to carry out creep tests on wood–plastic composites. We used the range method and variance analysis method to process the creep data and analyze the influence of the load, temperature, and relative humidity on the creep strain in specimens of wood–plastic composites. The results showed that the creep strain of the WPC specimens changed significantly with a change in the load stress, while a change in relative humidity had no significant effect on the creep strain. When the relative humidity was increased from 55% to 65%, the creep strain increased by 0.03%, but when the temperature was increased from 30 °C to 35 °C, there was no significant difference in the creep strain. However, when the temperature was increased from 30 °C to 40 °C and from 35 °C to 40 °C, a significant difference in the creep strain of the WPC specimens was observed.

A basic orthotropic viscoelastic model for composite and wood materials considering available experimental data and time-dependent Poisson’s ratios

Long-term deformation in creep is of significant engineering importance. For anisotropic materials, such as wood, composites and reinforced concrete, creep testing in several axial directions including shear is necessary to obtain a creep model which is able to predict deformation in the basic orthotropic case. Such a full set of experimental data is generally not available, and simplifying assumptions are typically made to conceive a useful 3D model. These assumptions should preferably be made based on the material behaviour and sound engineering arguments. This problem appears to be addressed in many different ways and sometimes the assumptions are not well justified. In the present study, we examine 3D creep of wood and composite materials. Particular emphasis is made on explaining the choices made in developing the model, considering practicality, incomplete material data and the specific behaviour of wood and composites. An orthotropic linear viscoelastic model is implemented as a material model in a commercial FE software. The constitutive equations are derived in the 1D case using a hereditary approach, then later generalized to the 3D formulation. Guidelines are shown how to implement it into the FE software to predict creep of components and structures. Although the model itself is conventional, the effect of considering time-dependent Poisson’s ratios is investigated here, as well an optimization approach when inserting inevitably asymmetric experimental creep data into the model. As far as the authors know, creep of wooden materials have not been defined using this approach before. The model of interest is calibrated against experimental data. Examples using experimental results from solid wood data and a unidirectional fiber composite are demonstrated. The results show that the model is able to capture the orthotropic behaviour adequately. Orthotropy requires symmetry of the creep compliance matrix, which typically is not the case experimentally. It is shown that in rendering the matrix symmetric, one needs to decide which direction is more important. It is also shown that the frequently employed assumption of constant Poisson’s ratios should be made with caution.

Short-term and long-term longitudinal load tests of wooden rods

The purpose of the experimental studies was to verify the reliability of the method for solving the stability problem of centrally compressed rods, taking into account the creep of wood. To achieve this goal, it is necessary to have a diagram of deformations of real wood and experimental data on the behavior of rods of the same wood under continuous load. Experimental studies include short-term axial compression tests of small specimens with recording of load-deformation diagrams, as well as short-term and long-term tests of cores of different flexibility for central compression, the description of which is presented in this article. The authors of the paper obtained the values of critical stresses under longitudinal bending for wooden rods with different lengths and flexibility. These results have a high convergence with the values obtained as a result of theoretical calculations.

ПОЛЗУЧЕСТЬ И ОБРАТНАЯ ПОЛЗУЧЕСТЬ НАТУРАЛЬНОЙ И ПЛАСТИФИЦИРОВАННОЙ ДРЕВЕСИНЫ БЕРЕЗЫ ПРИ ПРЕССОВАНИИ ПОПЕРЕК ВОЛОКОН

Pressed (modified) wood is a technological process of drying, impregnation and pressing (R 54577-2011 State Standard). The density of pressed wood, depending on the degree of pressing, ranges from 750 to 850 kg/m3. Strength, hardness toughness of this wood is several times greater than of natural wood. Coal oil, TCL (thermocatalytic cracking liquid), shale oil are most often used as wood antiseptics. They are plasticizers and change the nature of deformations during pressing.Experiments with natural and were conducted to obtain comparative data. Pressed wood contained 10% of TCL oily antiseptic by weight of absolute dry wood. Birch wood was used as the starting material. In this case, an antiseptic agent in an amount of 8-10% by weight of dry wood is evenly distributed over the entire cross section of the specimen. Direct and reverse creep was studied to assess time changes in deformation composition of wood, pressed across fibers. Measurements of direct and reverse creep of wood were carried out on a lever installation. The obtained modes can be used to optimize the technology of obtaining plasticized wood, because plasticization of wood with TCL oil reduces toughness 3-5 times (in the first phase of deformation), in the second phase - 1.1-1.5 times, in the third phase - 2.5-3 times. Transverse strain coefficient increases 1.2-1.3 times. Using creep curves of plasticized wood, it was found that oil impregnation gives 4-5 times greater reduction in the proportion of elastic deformations and, accordingly, increases the residual ones.

Time-temperature-stress equivalence in compressive creep response of Chinese fir at high-temperature range

Predicting long-term viscoelasticity properties of wood is important for designing dimensions for structure-used wood and optimizing thermo-mechanical treatment technique. In this study, compressive creep of Chinese fir wood was tested at a series of temperature (140–220 °C) and stress (0.03–0.15 MPa) conditions. The time-stress superposition principle (TSSP) and time-temperature-stress superposition principle (TTSSP) were applied to predict long-term creep behavior. Changes of chemical components and anatomical features of wood cell wall after creep test were investigated by Raman spectroscopy and scanning electron microscopy. The results showed that quasi-linear change of strain was acquired when temperature at or below 180 °C. TSSP and TTSSP were feasible to predict compressive creep at an up-threshold temperature of 180 °C. When temperature was 200 and 220 °C, obvious non-linear strain was obtained, and TSSP and TTSSP failed to construct master curves. Much significant degradations of carbohydrates was found at 200 °C, which could explain why TSSP failed to construct master curves from a chemical point of view. Crack and rupture on the cell walls and among the cell corners were observed when temperature at or above 200 °C and stress at or above 0.09 MPa. From an anatomical point of view, the destruction of wood cell structures could explain why TSSP failed to construct master curves. The finding in this studies broads the knowledge of the viscoelasticity of wood, and is helpful for further investigations on the thermo-mechanical manufacturing techniques.

Wood moisture accounting in creep equations

Wood as a structural material differs from other materials used in construction and industry, high variability of its properties. This is due, among other things, to the action of various variable factors – moisture content, temperature, inclination of fibers and others. The influence of wood moisture is especially strong – as a result of moistening the strength of wood considerably decreases in comparison with durability in absolutely dry condition. In addition, wood is a significantly nonlinear structural material. Non-linearity is manifested both in short-term and prolonged loading. Research of wood in time can be divided into two directions: longterm durability of wood and creep of wood. Modern models of wood creep are based on linear theories. Analysis of experimental data of various researchers on wood creep, has shown that the area of linear creep is observed at rather low levels of loading. At medium and high levels of experimental loading the existing models of linear wood creep theory do not correspond to experimental data. All this points to the actuality of the development of methods of calculating wooden structures for creep (linear and nonlinear), taking into account the wood moisture.

Swelling restraint of thermally modified ash wood perpendicular to the grain

The paper reports on creep of ash wood (Fraxinus excelsior L.) thermally modified at 180 and 200 °C, and subsequently subjected to compression in tangential and radial directions and simultaneously wetted, from the moisture content (MC) of 6% to above the fibre saturation point (FSP). The compressing load made 0.00, 0.25, 0.50 and 0.75 of impact stress at the proportional limit (Rc). The compression stress needed to restrain the swelling of wood, the so-called swelling pressure, was indirectly determined from isochrones of mechano-sorptive creep. The most important finding was that thermal modification reduces the strain of ash wood subjected to compression perpendicular to the grain to a degree proportional to the mass loss. The compression stress needed to restrain the swelling of thermally modified wood is ca. 10 and 20% smaller in the tangential and radial directions, respectively. This effect leads to a reduction in the anisotropy of swelling pressure of thermally modified wood perpendicular to the grain. Moreover, although upon thermal modification the mass loss of wood takes place, at the MC of 6% it shows practically the same modulus of elasticity (MOE) and Rc as the unmodified wood. After wetting to MC higher than the FSP, the thermally modified wood at 200 °C shows significantly higher MOE and Rc than the wood modified at 180 °C and untreated wood. Reduction of wood hygroscopicity, an inevitable effect of thermal modification, also reduces the range of changes in mechanical properties of wood caused by the increase in its MC to the FSP.

Coupled two-dimensional modeling of viscoelastic creep of wood

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.

THE CALCULATION OF WOODEN CONSTRUCTIONS TAKING INTO ACCOUNT THE CREEP OF WOOD ON THE EXAMPLE OF A STATICALLY INDETERMINATE LENTICULAR BLOCKED TRUSS

Objectives The aim of the study is to refine the calculation of wooden constructions, in particular, to use variable elastic modulus for the calculation of the second group of the limiting state in order to predict the deformations more accurately. Methods The study is carried out using the method of creep consideration based on the use of either variable elastic modulus or the “modulus of total deformations” for the calculations. These moduli, besides the elastic, account for residual deformations, while the fraction of the latter increases with increasing stress levels in the wooden elements. Results The calculation of statically indeterminate spatial timber-metallic lenticular block-truss loaded with a uniformly distributed load is carried out. At the first stage, the construction was calculated using the elastic modulus of all wooden elements E = 10000 MPa in accordance with the set of rules (SP 64.13330.2011 (updated version of SNiP II-25-80). At the second stage, the elastic modulus was replaced by variable, i.e., matched to the level of stresses in the elements by means of interpolation. The obtained deflection values are analysed and compared to the construction limiting value. The study was conducted without taking into account the flexibility of node connections and defects of the wood, which can also have a significant effect on the deflection value. Conclusion The use of a variable elastic modulus for calculations significantly influences the magnitude of deformations (in our case, deflections are increased by 30%). The study confirms the need to take into account the creep of wood and refine the calculations of wooden structures. Such approximating dependence at different moisture levels of wood will allow the calculation of wooden structures to be performed at a higher theoretical level.

Creep behavior of wood under cyclic moisture changes: interaction between load effect and moisture effect

Load and moisture content (MC) changes are the essential conditions for the mechano-sorptive (MS) creep of wood. An experiment was carried out on poplar to comprehend the mechano-sorptive creep from our point view. To restore the truth of MS creep behavior especially in the first humidifying stage, three well-matched sets of specimens were loaded in third-point bending under different humidity cycles. For each set, the applied load varied from 15 to 35 % of the short-term breaking load. It was found that the wood specimens exhibited a partial recovery during all the adsorption phase and deflection increase during all the desorption phase when low load level was applied. This phenomenon was very different from that a considerable creep at first adsorption observed by large amounts of researchers, which can be ascribed to the pseudo-creep due to the difference in the normal longitudinal swelling and shrinkage of wood. The results also indicated that an amplified load effect existed within the creep under cyclic moisture changes, which usually resulted in a fast increasing rate of viscoelastic creep to veil pseudo-recovery in the first humidifying stage.

Experimental and analytical study of the mechanical behavior of heterogeneous glulam–UHPFRC beams assembled by bonding: Short- and long-term investigations

In wood-based structural design, in addition to the criteria of strengthening and stiffness, the criterion of deformability due to the creep of wood, especially in horizontal members, is a factor that often limits the design. New developments have further improved the structural performance, such as the strengthening of timber (glulam) beams by bonding composite material combined with an ultra-high performance fiber reinforced concrete (UHPFRC) that is internally reinforced with or without carbon fiber reinforced polymer (CFRP) bars. Glulam, UHPFRC and CFRP may be an interesting composite mix for responding to the problem of the creep behavior of composite structures made of different materials with different rheological properties. This paper describes an experimental and analytical investigation on the short-term and long-term (creep) performances of glulam–UHPFRC–CFRP beams. The experimental investigations of creep behavior were conducted in indoor and outdoor environments under constant loading for approximately 1year. The measured results are compared to the results from an analytical model. This model was developed to predict the creep response of the glulam–UHPFRC–CFRP beams based on the creep characteristics of the individual components. The results indicate that heterogeneous glulam–UHPFRC beams improve both strength and stiffness and can also effectively reduce the creep deflection of wooden beams.

Mechano-sorptive creep in pulp fibres and paper

The factors affecting mechano-sorptive creep of wood and paper have been investigated for a long time. It has also been argued that single wood fibres do not exhibit mechano-sorptive creep and that the reasons for the accelerated creep under moisture cycling conditions instead are related to the bonds between the fibres. In order to examine the relevance of this argument, measurements on single pulp fibres of different composition were performed in tension, and the mechano-sorptive creep was compared to that of papers made from the same source of pulp fibres. All fibres tested were found to exhibit an increased creep rate during moisture cycling as compared to constant humidity conditions. Thus, pulp fibres show mechano-sorptive creep and in this sense behave similar to solid wood or paper products made thereof. A linear relation between the creep strain rate during cyclic humidity and the creep strain rate at a constant humidity was also noted for both fibres and paper. This relation was not affected by changes in hemicelluloses content or composition, neither for fibres nor for papers made of these fibres. However, in all cases, papers showed a much higher mechano-sorptive creep than the corresponding fibres they were made of.

Application of time–temperature–stress superposition on creep of wood–plastic composites

Time–temperature–stress superposition principle (TTSSP) was widely applied in studies of viscoelastic properties of materials. It involves shifting curves at various conditions to construct master curves. To extend the application of this principle, a temperature–stress hybrid shift factor and a modified Williams–Landel–Ferry (WLF) equation that incorporated variables of stress and temperature for the shift factor fitting were studied. A wood–plastic composite (WPC) was selected as the test subject to conduct a series of short-term creep tests. The results indicate that the WPC were rheologically simple materials and merely a horizontal shift was needed for the time–temperature superposition, whereas vertical shifting would be needed for time–stress superposition. The shift factor was independent of the stress for horizontal shifts in time–temperature superposition. In addition, the temperature- and stress-shift factors used to construct master curves were well fitted with the WLF equation. Furthermore, the parameters of the modified WLF equation were also successfully calibrated. The application of this method and equation can be extended to curve shifting that involves the effects of both temperature and stress simultaneously.

Influence of hygromechanical history on the longitudinal mechanosorptive creep of wood

Abstract An experiment was performed on Norway spruce to simplify our understanding of mechanosorptive creep. Two well-matched sets of specimens were subjected to the same humidity history but loaded in four-point bending at different times. One set was loaded dry, and thus experienced a considerable creep at first adsorption (the so-called ++ effect). The other set was then loaded wet and only exhibited, together with the dry-loaded set, the usual pattern of creep increase during drying (- effect) followed by a partial recovery during remoistening (+ effect). Both sets converged to the same response after a few cycles. The results confirm that a single type of mechanosorption, combined with strain-dependent hygroexpansion and humidity-dependent viscoelascity, is sufficient to account for all observed features of longitudinal creep of wood at ambient temperature.

Influence of Rheological Behaviour on Load-Carrying Capacity of Timber-Concrete Composite Beams Under Long Term Loading

Wood and concrete are materials with different time dependent behaviour and both have different reactions to thermal and humidity changes of environment. In timber-concrete composite beams it results in generation of inelastic strains and redistribution of stress in cross-section.To determine load carrying capacity of composite wood-concrete element is possible according to Additions B of EN 1995-1-1, Design of Timber Structures. But this calculation technique does not consider the influence of rheological properties of composite materials as shrinkage, eventually bulking of materials, the influence of composite on creep coefficient and the influence of environmental changes. In this paper, the analytical calculation model of long term behavior of the timber-concrete composite beams is presented, which considers the most significant rheological phenomena such as: viscous-elastic creep of concrete and wood, mechano-sorptive creep of wood, creep of shear connection, concrete shrinkage and strains due to thermal and relative humidity changes of environment. This model is applicable for simple beams with linear material properties and allows determining the final deflection in the middle span and stressing distribution in the middle cross- section of the composite beams affected by long term loading. Applying the presented calculation model and results of experimental tests, the analysis of rheological behavior influence on long term load carrying capacity of timber-concrete beams is described as well.

Comparação de dois modelos de escamoteadores sobre o desempenho dos leitões

The objective of this study was to evaluate the effect of creep models for heating and piglet performance. Model 1 (MOD 1), consisted of eight creep of masonry, electrical resistance heating built into the floor and lighting with fluorescent lamps of 7 W. Model 2 (MOD 2), consisting of eight creep of wood and heating by 60 W incandescent lamp The data of temperature and humidity were recorded from birth to 26 days of life and litter, the average litter weight was obtained on the second day and on day 21 of life. The statistical model for repeated measures using the MIXED of SAS (2002-2003) was used. We tested the effects of model, week and interaction. Creep MOD 1 got an average of 20.75% of the temperature data within the thermal comfort during the experimental period in relation to MOD 2. Only the first two weeks the MOD was better with 26.04%, while the MOD 1 was 3.90%. MOD 1 litters had higher final weight of 13.80% compared to litter the MOD 2. We conclude that the MOD 1 achieved the best results related to the environment and average weight gain of litters.

Comportamento suíno influenciado por dois modelos de maternidade

Neste estudo foram avaliados dois modelos de maternidade. O modelo um (MOD 1) era formado por duas salas similares, cada sala composta de oito celas parideiras individuais. O escamoteador era de alvenaria com tampa de madeira e aquecimento no piso, por resistência elétrica. A sala do modelo dois (MOD 2), continha 20 celas parideiras individuais, dentre as quais foram avaliadas apenas 16. Os escamoteadores eram de madeira com aquecimento por lâmpada incandescente de 60 W. Foram avaliadas as condições de ambiente das salas e do interior dos escamoteadores. O comportamento dos leitões foi avaliado no ambiente das salas e nos escamoteadores. O ambiente mais favorável para os leitões foi no MOD 1 com o maior percentual de valores de temperatura e umidade dentro da zona de conforto. A temperatura do piso do escamoteador do MOD 2 obteve os melhores resultados. O comportamento mais observado nos leitões foi o de permanecerem no interior do escamoteador e deitados aglomerado. A presente área de estudo carece de mais pesquisas que levem em consideração variáveis que possam afetar a temperatura no interior dos escamoteadores, como o material utilizado na construção dos abrigos e a troca de calor entre os leitões.

Micromechanics of creep and relaxation of wood. A review COST Action E35 2004–2008: Wood machining – micromechanics and fracture

Abstract Wood, like all polymeric materials, shows viscoelastic behaviour. The time dependent behaviour of wood depends on material anisotropy, temperature, moisture and stresses. To predict the behaviour of wood, numerous mathematical models have been developed largely relying on experimental results. In this paper, time dependent viscoelastic behaviour of wood is reviewed under constant and cyclic climatic conditions, separately. More emphasis is given on results obtained in recent years on the behaviour of thin wood tissues, single fibres, thermo-viscoelasticity of wood, influence of hemicelluloses and the modelling of the effect of transient moisture at the molecular level on the mechanical response.