Creep of wood under a large span of loads in constant and varying environments

Extensive bending creep experiments were made to measure the viscoelattic and mechano-sorptive properties of pine wood. It is observed that under constant, similar to ordinary indoor environment, creep is linearly related to the loads when the loads are below 30% of the ultimate strength, and the relation becomes nonlinear when the loads are higher. Wood of small elastic modulus creeps more than that of large elastic modulus under the same load. The mechano-sorptive effect is stronger for heavily loaded wood than that for lightly loaded. It is stronger for wood with lower elastic modulus and vice versa. The mechano-sorptive effect is negligibly small at low moisture content and it gets very large at high moisture content. It depends therefore not only on the variation range and rate of the moisture content, but also on its magnitude value.     

The mechanism and development of creep during drying of red oak

The goal of this research was to determine the magnitude of mechano-sorptive and visco-elastic creep early in drying and to investigate the relationship of creep to moisture change and load level. All of the measurements were done in the tangential direction perpendicular to the grain using red oak (Quercus sp.) Mechano-sorptive creep was measured by two different methods. The first experiments were designed to measure mechanosorptive creep under actual drying conditions. The second group of experiments were designed to measure the effect of load as well as moisture change on the development of mechano-sorptive creep. Visco-elastic creep was measured by conditioning small wafers to a specific EMC value, loading the wafers and measuring the amount of creep or elongation which occurred over time. Results indicate that mechano-sorptive creep is the major strain early in drying. The magnitude of the creep was a linear function of moisture change from the green condition and increases with decreasing moisture content. Visco-clastic creep component was small and a function of the applied load.     

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 (R_c). 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.     

Zur dynamischen Torisonselastizität von Holz

The changes in torsional elasticity of untreated and fire retardant impregnated beech and spruce specimens under cyclic temperature load were determined in torsional vibration tests. A temperature of 60°C already caused a permanent 25% reduction in torsional modulus of elasticity which remained unaffected by further temperature rises and by repeated heating and cooling cycles up to 200°C. Above 200°C oxidation presumably occurred with wood strength deteriorating visibly. Also, previously water saturated specimens showed greater losses in torsional elasticity than air-dried or dried ones. Fire retardant treatment with acid salt mixtures reduced torsional modulus of elasticity by 35% already at room temperature and subsequently led to a rupture of specimens at 200°C. Apart from fluctuations in absolute values both wood species reacted similarly to temperature load and protective treatment.     

Deformation properties of Finnish spruce and pine wood in tangential and radial directions in association to high temperature drying¶Part IV. Modelling

Picea abies ) and Scots pine (Pinus sylvestris) wood under conditions relevant in the high temperature drying process. The programme included tension experiments on perpendicular specimens at temperatures 95 °C–125 °C. Based on the experiments, a constitutive model to describe the behaviour was developed for use in numerical simulation of drying stresses and is reported in this paper. The model takes into account all the needed strain terms: hygroexpansion (shrinkage, swelling), hygrothermal, elastic, viscoelastic and mechano-sorptive strains. Elastic and viscoelastic strains are modelled jointly as generalised Kelvin material, using the time–temperature–moisture-content superposition principle. Mechano-sorptive creep is modelled with a series of modified `mechano-sorptive Kelvin elements', in which a part of the strain is irrecoverable unless the direction of the acting stress changes.

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Verformungseigenschaften von finnischem Fichten- und Kiefernholz in tangentialer und radialer Richtung unter Bedingungen der Hochtemperaturtrocknung. Teil IV

     

Mechanical performance and dimensional stability of nano-silver impregnated densified spruce wood

The aim of this study was to investigate the effect of nano-silver treatment on some physical and mechanical properties of compressed low density wood species. Wood specimens were prepared from spruce (Picea abies), impregnated with water or nano-silver solution by empty cell process and compressed through radial direction in a hot press. The results showed that by nano-silver treatment, the spring back, bending strength (modulus of rupture) and impact load resistance were improved significantly. The best results for spring-back (0.04%) were seen in the nano-silver impregnated specimens that were compressed at 150°C for 4 hours. The modulus of rupture (MOR), modulus of elasticity (MOE) and impact load resistance in nano-silver impregnated densified specimens were gained for 53%, 41.2% and 175.7%, respectively (in comparison with controls). The maximum amounts of impact load resistance belonged to the nano-silver impregnated specimens which were compressed at press conditions of 150°C for 4 hours, showing the high ability of these specimens against high impact loads such as earthquake loads. An upcoming research (consisting of durability tests) will be done for evaluating the suitability of nano-silver impregnated densified spruce wood for exterior uses.     

Dynamic MOE of 55 species using small wood beams

Nondestructive impact-induced resonance vibrations and spectral analysis were used to determine the dynamic modulus of elasticity (MOE) of specimens from 45 hardwood and 10 softwood species. Relationships were obtained between air-dry specimens measuring 20×20×300 mm and small wood beams measuring 20×2×150 mm (nominal) cut from them. Dynamic longitudinal elastic modulus (E_L) and wave velocity (V_L) in the fiber direction were determined from the larger specimens. These were compared with the dynamic flexural elastic modulus (E_CB-cantilever mode) and the dynamic longitudinal elastic modulus (E_LB) with the corresponding wave velocity (V_BL) measured from the small beams. E_CB and E_LB were highly related to E_L. V_LB was similarly related to V_L. The methods developed for small wood beams allow rapid evaluation of MOE from the outer wood of standing trees or they can be used for studies involving detailed localized evaluation of elastic wood properties.     

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.     

Experimental verification of an orthotropic finite element model for numerical simulations of ultrasonic testing of wood poles

Ultrasonic testing is a non-destructive and non-invasive method which has been used for internal condition assessment of wood poles in electric transmission and distribution lines. The reliability of this method of evaluation relies on a good understanding of propagation of ultrasonic waves in wood. However, a full-waveform analysis in ultrasonic testing is rarely performed in practice because of difficulties in establishing realistic values for the elastic parameters, modeling the material damping and characterizing the dynamic response of an ultrasonic transmitter. In this paper, a calibrated orthotropic finite element model for numerical simulations of ultrasonic testing of a sound red pine pole is presented. In the calibrated model, the dynamic modulus of elasticity in the radial and tangential directions, Poisson’s ratio and damping ratios are estimated from ultrasonic testing; whereas the dynamic modulus of elasticity in the longitudinal direction is estimated from transverse-vibration testing. The measured response of an ultrasonic transmitter to a one-cycle sinusoidal pulse of 50 kHz is used as dynamic excitation and introduced in the numerical model as displacement-time history. Results of the first arrival of compression waves and the frequency response magnitude computed at three receiver locations are in good agreement with the obtained ones from ultrasonic testing. The calibrated orthotropic finite element model will be used for a better understanding of propagation of surface and compression waves in ultrasonic testing for the detection of early stages of decay in wood poles.     

Deformation properties of Finnish spruce and pine wood in tangential and radial directions in association to high temperature drying. Part III. Experimental results under drying conditions (mechano-sorptive creep)

Picea abies ) and Scots pine (Pinus sylvestris) wood under conditions relevant in the high temperature drying process. This paper reports the experimental results obtained for creep in tension under drying conditions at temperatures 95–125 °C. The results are compared to other researchers' measurements of mechano-sorptive creep at conventional drying temperatures (up to 80 °C). Based on this comparison the effect of temperature on the perpendicular to grain mechano-sorptive creep compliance is quantified.

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Determination of shearing stiffness parameters to design multi-layer spruce beams using welding-through wood dowels

This paper presents a methodology to characterize the secant shearing stiffness of welding-through wood dowels used in multi-layer spruce beams. Welding-through wood dowel’s stiffness behaviour as well as isostatic multi-layer beams’ behaviour were investigated experimentally. These elements are made by a specifically designed machine to guarantee repeatability. A model of finite elements was developed to reproduce the bending behaviour of the beams tested from non-linear connection behaviour. This model is then used to study the influence of parameters such as: layer number, spacing of dowels, load type (concentrated or distributed), strength class and beam span. After a sensibility analysis, a parameterized regression model allows to obtain k _ser and k _u fastener secant stiffness in compliance with EN 1995-1 standard to design multi-layer beams in an elastic analytical model way. This model is based on the work by Kreuzinger (1999).     

原木节子及内部孔洞缺陷的数学模型描述及模拟研究

为了探讨原木节子及内部缺陷对其力学性能的影响,本文借助应力波理论方法和原木内部节子及孔洞模型及理论分析,并以松木原木为试验材料,应用应力波法和频率法测出含不同节子及内部孔洞的原木波速和动态弹性模量。通过建立原木缺陷的数学模型说明节子及孔洞的影响机理和试验分析表明,木材节子及空洞的存在影响了应力波波速及弹性模量等力学性能水平,应力波等技术对原木内部缺陷检测具有较高的理论意义和应用价值。     

ANALYSIS OF THE ELASTIC MODULUS OF AGAR GEL BY INDENTATION

This study used the approach taken by Bourne to obtain the compressive and shear components for an agar gel. But in addition, the low strain regime for the same experiments was used to evaluate the gel's elastic modulus using the theory of elasticity. Compression testing, using lubricated teflon platens and a travelling microscope, was used to independently measure the gel's elastic modulus (52 kNm⁻²) and Poisson's ratio (0.32). The compressive and shear coefficients compared favourably with analyses performed by other researchers, and indicated that shear dominated in the failure of the gel under the probe. Indentation measurements of reaction force (P) and deformation (h) were analysed with the theoretical relationship for frictionless flat cylinder indentation. A good linear fit (r²= 0.991) was obtained for P/h versus indenter radius, indicating the validity of the analysis. The value of the elastic modulus derived from indentation was 79 kNm⁻², an overestimate of 52%. The larger value suggests that additional factors to those considered in the theoretical model influence the indentation process, such as the presence of pores in the gel, frictional forces during indentation, and elastic mismatch between indenter and gel. Similarity in results for the elastic modulus determined from indentation compared to compression, indicates that a simple testing method can yield a more comprehensive evaluation of food textural quality.     

Untersuchungen über die Variation des Last-Verschiebungsverhaltens in Nagelverbindungen

1088 single-nail tension tests were carried out on 109 spruce double-shear specimens in order to analyze the variation of the load-slip behaviour of non pre-drilled nailed connections. The following parameters of the load-slip behaviour were determined from each test: maximum load, elastic slip modulus, initial slip modulus and slip modulus according to ISO 6891, as well as the three parameters of a fitted function. Tests with nails penetrating a knot resulted in a 20% increase in maximum load and elastic slip modulus. The autocorrelation of the parameters of the load-slip behaviour as well as their mutual correlation do not depend on the nail spacing in the range of 73 mm to 657 mm.     

Mechanical properties of wood of two Mexican oaks: relationship to selected physical properties

The relationship between 27 mechanical parameters and density, shrinkage coefficients, moisture content, anisotropy ratio, and the fiber saturation point (FSP) in small specimens of Quercus crassifolia Humb. & Bonpl. and Q. laurina Humb. & Bonpl. wood, from Oaxaca, Mexico, is described. Wood from both oaks displayed low dimensional stability, while mechanical testing results revealed a strong material in most forms of stress, as well as a high impact-resistance wood. Wood mechanical properties in dry and green state were positively correlated with density, shrinkage coefficients, and FSP. Density was generally the single best predictor for mechanical strength, although the modulus of elasticity was a better predictor of the modulus of rupture in the bending tests. Univariate models to predict mechanical properties using the best regressor of the physical properties gave R² values from 0.104 (p?=?0.015) to 0.494 (p?<?0.001), whereas multivariate regression significantly increased the predictive power of the models, with R² values from 0.190 (p?=?0.004) to 0.646 (p?<?0.001).     

An analytical model for embedment stiffness of a dowel in timber under cyclic load

The fundamental part of a dowel-type connection is the embedment of a steel dowel into the timber that surrounds it, and the stiffness of the timber in embedment is represented by the foundation modulus. A standard experimental method for identifying the foundation modulus under static load is modified to assess the secant stiffness exhibited under one-sided cyclic load. It is shown that the steady-state secant stiffness is significantly higher than the static stiffness under initial loading, and that, if the amplitude of the cyclic component of the load is sufficiently small, a simple analytical elastic model predicts the foundation modulus well. The analytical model is based on a complex stress function for the timber in embedment and the frictional interaction between the dowel and the timber. The foundation modulus calculated in this way can be used to predict the stiffness of complete connections for analysis of vibration in frames modelled with semi-rigid joints. Although the application of the model is limited to vibration about a non-zero mean load, with no load-sign reversal, this form of vibration encompasses various important types of in-service vibration of structures, such as that induced by turbulent wind or footfall.     

The mechanical properties of densified VTC wood relevant for structural composites

The mechanical properties of densified wood relevant for structural composites were studied. Low density hybrid poplar (Populus deltoides × Populus trichocarpa) was densified using the viscoelastic thermal compression (VTC) process to three different degrees of densification (63, 98, and 132%). The modulus of rupture (MOR) and the modulus of elasticity (MOE) of the control (undensified) wood and of the VTC wood were determined. The bonding performance of the control and VTC wood, using two phenol-formaldehyde (PF) adhesives, was studied. Four different 3-layer composites were also prepared from undensified and VTC wood, and tested in four-point bending. The results showed that the bending properties of the VTC wood (MOR and MOE) were significantly improved due to the increased density. The bonding performance of VTC wood with PF adhesives was comparable with or better than in the case of the control wood. Increased density of the face layers in the 3-layer VTC composites was advantageous for their mechanical performance.     

基于丝状单元的木塑复合板材制备及包装应用性能研究

研究以高密度聚乙烯塑料(HDPE)丝状单元代替传统胶黏剂制得的新型木塑复合材料的性能及其在包装领域的应用特性.以桉木丝为原料,采用热压成型的方式制备木塑复合板材,研究了板材密度、偶联剂和HDPE含量对板材性能的影响,通过ANSYS Workbench软件建立包装箱有限元模型,分析箱体在静态弯曲条件下受力情况和承载特性....     

Numerical prediction of checking during timber drying and a new mechano-sorptive creep model

The dependence of the amount of checking on shrinkage induced stress level during timber drying is considered. This relationship is investigated by comparing theoretically calculated maximal stress with observed amount of checking in a set of drying tests. Different ways of modelling wood creep behaviour are included. It is found that a correlation exists in all the cases studied, except when creep is described with the traditional equation for mechano-sorptive creep. It is shown that this exception is due to a special mathematical property of the equation. A new mechano-sorptive creep model is thus proposed and briefly investigated. Using this model, a method for predicting the amount of checking in timber drying is developed.     

Changes in dimensional stability and mechanical properties of Eucalyptus pellita by melamine–urea–formaldehyde resin impregnation and heat treatment

This study investigated the influence of heat treatment on the dimensional stability and mechanical properties of untreated and low molecular weight melamine–urea–formaldehyde (MUF) resin treated Eucalyptus pellita wood. Wood samples were heat treated under vacuum atmosphere in laboratory conditions at temperatures between 160 and 240 °C for 2–10 h. The results showed that anti-shrink efficiency and anti-swelling efficiency of MUF resin-impregnated heat-treated eucalypt wood were improved by up to 47 and 49 % at 240 °C for 10 h separately, which were greater than those of heat-treated wood. In relation to mechanical properties, the modulus of elasticity and modulus of rupture decreased with increasing temperature and time, but the reduction of properties appeared to be smaller for MUF resin-impregnated heat-treated wood. Therefore, heat treatment combined with resin treatment of eucalypt wood shows potential to improve the wood quality of solid wood products.