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1.
Summary Non-linearity of softwood in axial mechano-sorptive creep during moisture cycling can be characterised as departure from linear behaviour. Linear behaviour is shown by experimental measurement to be a gradual approach to a creep limit with exponentially decreasing increments of compliance, Ji, per moisture cycle, when tested under a constant stress. On the other hand, when the stress is progressively increased by a small increment after each moisture cycle, the compliance increments will progressively increase, having a value of , where n is the cycle number. By subtracting one compliance increment from the succeeding one, the value of Ji can be obtained. Analysis of experimental results in bending tension and compression showed that the compression test pieces departed from linearity at total strains around 0.14% to 0.15%, the bending test pieces showed slight evidence of non-linearity at about the same strain, whilst the tensile pieces were approximately linear up to 0.18% strain.This project was supported by the U.K. Science and Engineering Research Council and by the European Commission  相似文献   

2.
This study examined the bending creep behavior of hot-pressed wood during cyclic moisture changes. Sugi (Cryptomerica japonica D. Don) specimens were pressed in the radial direction under six combinations of nominal compressive strain (33% and 50%) and press temperatures (140°C, 170°C, 200°C). Creep tests were conducted at 20°C with three cyclic relative humidity changes between 65% and 95% under 25% of short-breaking stress. The effect of moisture content (MC) change on elastic compliance and mechanosorptive (MS) compliance was investigated. The relation between MS compliance and thickness swelling was studied. The results indicated that total compliance increased over the history of cyclic moisture changes; and its behavior was closely related to the changes in MC and thickness swelling. The total compliance increased during adsorption and decreased during desorption. Elastic compliance increased linearly with MC and was dependent on press temperature and compression. With increasing MC change, MS compliance increased during adsorption and decreased during desorption. The first adsorption led to greater MS compliance than did the subsequent adsorption with the same amount of MC change. In general, the elastic parameterK E and the MS parameterK Mincreased with compression and decreased as the press temperature increased. The MS parameterK M was apparently greater than the elastic parameterK E. The MS parameterK M increased with swelling coefficient KSW of the hot-pressed specimen during adsorption and decreased with an increasing shrinkage coefficientK SH during desorption.  相似文献   

3.
Summary The validity of the following equation of water adsorption into wood substance which was derived in the previous report, is examined: d(W)/dt = k0(1 – exp(-k1/t))W(l – W), which can approximately be written as: d(W)/d(logt) = rW(l – W), where W is moisture content; t is time (t > 0); k0, k1 and r are constants. After measuring dimensional change with change in time under various relative humidities, the change of moisture content was indirectly calculated from the proportional relationship between dimensional change and moisture content. It was found that the theoretical equation satisfactorily agreed with the experimental results. These results lead to the conclusion that the equation was valid. Furthermore, the properties of the equation, whose constants are determined from experimental results, is discussed. The rate of water adsorption of wood shows interesting and systematical properties, especially near relative humidities corresponding to the fiber-saturation point.  相似文献   

4.
Populus species, characterized by fast growth and easy vegetative propagation, are widely used in agroforestry practices. The substantial water requirement of poplars make them interesting subjects for water balance studies. No information exists on soil moisture requirements for initial root and shoot growth of Populus cuttings. This study on leafless hardwood cuttings of Populus x euramericana (Dode) Guinier cv. Robusta examined the dynamics of water use during propagation, as influenced by two initial soil water potentials (–0.006 and –0.06 MPa). Differences in the initial water potential of the cuttings was achieved by three pretreatments i.e., fresh, soaked and dried. Initial shoot was –1.45, –0.10 and –2.10 MPa in fresh, soaked and dried cuttings, respectively. Soil moisture had a major effect on rooting. Water-stressed cuttings took a longer time to root and had fewer roots. Pre-soaking of cuttings stimulated rooting, particularly under the drier soil moisture conditions. Initially the water potential of cuttings decreased with time and with the formation of roots it stabilized in all the pretreatments. The reduction in water potential of cuttings after planting was related to an increase in resistance to water flow in the xylem.This revised version was published online in November 2005 with corrections to the Cover Date.  相似文献   

5.
Summary This paper studies the influence of the used creep function, and the duration of the experimental sustained loading from which the creep function was derived, in modelling the long-term buckling strength of timber columns. The sensitivity of this buckling strength to creep and initial deflection is numerically studied using an energy approach that takes into account the initial deflection. Various creep functions — power laws as well as exponential laws — derived from bending experiments of different durations are used for the numerical simulation. The crucial need for experimental results issued from very long load period experiments is identified. Such experiments may lead to a more credible creep analysis of timber structures.Symbols i radius of giration - s stress level - EL effective MOE - E0 instantaneous MOE - I inertia moment - K,K geometrical matrices - L length of beam or column - Lf effective length - Ncr axial force, buckling load - U elastic strain energy - W external forces work - t time variable - u moisture content (m.c.) - v total deflection - vc, ve, v0 creep, instantaneous elastic and initial deflections respectively Greek Letters (t) time function - , e, 0 total, instantaneous and initial strains respectively - twist rotation - (t) creep coefficient or fractional creep or relative creep - (x) shape function - load factor - longitudinal shortening - total potential energy - creep factor - relaxation time  相似文献   

6.
Summary An investigation into the bonding energy relationships for water in wood indicates that as the temperature increases at constant total moisture content, water moves from within the chemical structure to the adsorption surface. The analysis is evaluated for the wood Araucaria klinkii Lauterb and it is indicated that at 25 °C, less water is held in the chemical structure during adsorption than during desorption.Symbols A amplitude of liquid surface profile - A0 amplitude of solid surface profile - a mean radius of curvature of liquid surface (bubble radius), Å - a0 mean radius of curvature of solid surface, Å - ac a constant value of a, Å - F a function of temperature - f capisorption energy fraction - G a function of - g specific Gibbs free energy of saturated water vapour relative to unsaturated water vapour at the same temperature, J/kg - gc specific Gibbs free energy associated with capisorption, J/kg - gp specific Gibbs free energy associated with physisorption, J/kg - h change in specific enthalpy of liquid water as it is desorbed, J/kg - l latent heat of vaporisation of free water, J/kg - m wave number/m - ps pressure of water vapour at saturation, Pa - R specific gas constant for water vapour, J/kg K - r relative humidity - s change in specific entropy of liquid water as it is desorbed, J/kg K - T temperature, K - w dry basis moisture content - x ln ps/ps25 - y In r - z length coordinate, m - , , constant coefficients - change in mean height of liquid surface, Å - 0 a constant length, Å - constant - distance from solid to liquid vapour interface measured normal to solid surface, Å - 0 a constant value of , Å  相似文献   

7.
The effects of temperature on mechano-sorptive (MS) creep of delignified hinoki wood (Chamaecyparis obtusa Endl.) were investigated using longitudinal (L) and radial (R) specimens during adsorption and desorption over the temperature range of 20°–80°C. The results were compared with those of stepwise delignified specimens tested at a constant temperature of 20°C. It was found that the effects of temperature on the MS creep of delignified specimens are more remarkable than for untreated specimens. The tendencies of increasing MS creep with temperature, delignification, and their combination were observed. The increase in MS creep for L specimens was relatively small and almost equal in both adsorption and desorption processes, while for R specimens the MS creep was small in desorption, but significantly different in adsorption. In addition, good correlation was observed between the MS coefficient (K) and instantaneous compliance (J 0). The increase in MS creep occurs as a result of temperature increase or decrease in lignin content, or their interacting effects. However, in the case of desorption for R specimens, the increase of MS creep was unexpectedly small due to a remarkably increased J 0. Part of this report was presented at the 15th Annual Meeting of the Chubu Branch of the Japan Wood Research Society in Fukui, October 2005  相似文献   

8.
Cortes  Pilar  Espelta  Josep Maria  Savé  Robert  Biel  Carme 《New Forests》2004,28(1):79-88
The use of an enriched CO2 atmosphere in tree nurseries has been envisaged as a promising technique to increase productivity and to obtain seedlings with a higher root/shoot ratio, an essential trait to respond to water stress in Mediterranean-type ecosystems. In that framework, we have analyzed the effects of three levels of atmospheric CO2 concentration (350, 500 and 700ppm) on the germination rate, growth and morphology of seedlings of two Mediterranean oaks used in reforestation programs: the evergreen Quercus ilex L. and the deciduous Quercus cerrioides Wilk. et Costa. CO2 enrichment increased the germination rate of Q. cerrioides (from 70±7 to 81±3%) while it decreased that of Q. ilex (from 71±10 to 41±12%). Seedlings of both species increased approximately 60% their total biomass in response to CO2 enrichment but at two different CO2 concentrations: 500ppm for Q. cerrioides and 700ppm for Q. ilex. This increase in seedlings biomass was entirely due to an augmentation of root biomass. Considering germination and biomass partitioning, an enriched CO2 atmosphere might not be appropriate for growing Mediterranean evergreen oaks, such as Q. ilex, since it reduces acorn germination and the only gains in root biomass occur at a high concentration (700ppm). On the other hand, a moderate CO2 enrichment (500ppm) appears as a promising nursery technique to stimulate the germination, growth and root/shoot ratio of deciduous oaks, such as Q. cerrioides.  相似文献   

9.
Summary A model that predicts heat and moisture transfer through wood in the hygroscopic range and which is based on the principles of irreversible thermodynamics, was evaluated with unsteady-state nonisothermal moisture desorption experimental data. The model predicted the phenomenon of thermal diffusion during the initial stages of desorption and results in a very good simulation of the desorption curve and the center's temperature change with time.Symbols Cp specific heat of air (= 0.24 cal/g K @ 70 °C) - CT specific heat of wood, cal/g K - D transverse diffusion coefficient, cm2/s - Eb activation energy, cal/mol - Eo heat of vaporization, cal/mol - EL differential heat of sorption, cal/mol - G specific gravity of wood - H relative humidity, % - hT convective heat transfer coefficient, cal/cm2 s K - hc convective mass transfer coefficient based on the concentration of moisture in wood, cm/s - hv convective mass transfer coefficient based on the concentration of moisture in the air in equilibrium with the wood surface, cm/s - KM coefficient for diffusion due to moisture gradient, g/cm s % - KT transverse thermal conductivity coefficient, cal/cm K s - M moisture content, % - Po saturated vapor pressure, atm - R universal gas constant, cal/mol K (= 82.056 cm3 atm/mol K) - t time, s - T temperature, K - x distance, cm Greek Letters evaporation or condensation criterion - wood density, g/cm3 - W water density (=1), g/cm3 - a air density, g/cm3 Department of Wood Science and Forest Products Virginia Polytechnic Institute and State University Blackburg, Virginia 24061-0503  相似文献   

10.
利用热扩散技术(TDP)分别监测新疆杨冠基部、杆基部处液流密度的变化,并结合同步气象观测,分析液流密度与大气蒸发潜力(ET0)的关系,结果表明:典型晴天日,新疆杨树冠、杆基部的液流密度日变化格局总体上与大气蒸发潜力相一致,但前者与ET0的相关性更强,可用Hill函数式得到较好的拟合,而后者与ET0间呈斜率不等的线性关系;在午间,冠基部液流密度是杆基部的3倍以上,在清晨,冠基部液流比杆基部液流提前平均约1 h启动。冠、杆基部边材液流间的"净"量因季节而异,在生长旺盛的7、8月份略有"亏损",而在6、9月份略有"盈余",新疆杨单株日吸收水分量与冠层日失水量并不完全一致。6—9各月冠基部液流日平均通量一般呈晴天>云天>阴天的规律,这与太阳辐射及大气蒸发潜力的变化格局相一致,而基部液流有时会呈现出云天>晴天的趋势。  相似文献   

11.
The objective of this research was to evaluate the mechanosorptive deflection of shear creep of nail-plate-jointed laminated-veneer lumber. The joint was composed of steel gusset and nails, 40 mm in length and 2.8 mm in diameter (Zn40). The specimens were loaded parallel (PA) and perpendicular (PE) to the grain. Shearing loads applied were 50 and 100kgf, and the load levels were 11% and 22% (PA) and 7% and 14% (PE) of the maximum strength obtained from static testing. The creep test specimens were loaded for 1500h. A few general conclusions could be drawn from this study: The mechanosorptive deflection (ms) is defined as ms = t — (c + sh) — o, where t, is total deflection, c is pure creep, sh is shrinkage-swelling behavior, and o is the initial deflection. Changes in relative humidity may cause more severe creep deflection than constant humidity, especially during the drying process. The mechanosorptive deflection is greater at the lower load level than that at the higher load level. The mechanosorptive effects seem to be somewhat more resistant in the parallel direction than in the perpendicular direction.  相似文献   

12.
Summary Plastic stress arising in wood during drying is calculated according to the theoretical model developed earlier. The mechanism of stress reversal and the type of resudual stress corresponding to different values of material constants are shown. The results are in qualitative agreement with experimental evidence.List of symbols A coefficient of swelling below the fibre saturation point - C concentration of moisture in wood; weight of moisture per weight of dry wood - C 0 uniform concentration of moisture in wood at the beginning of drying - C 1 equilibrium concentration of moisture at the boundary during drying - C =C-C 1 - non-dimensional concentration - D diffusivity - D 0 first term in the expansion of diffusivity as function of concentration: D=D 0(1+D 1 C+...) - D 1 secondterm in the expansion (see D 0) - E Young's modulus - e ij deviator of tensor of strain: - e ij P deviator of plastic strain: - e ij E deviator of elastic strain - F fibre saturation point (concentration at which the function (c) changes slope) - F =F-C 1 - g(x,t) function which assumes the value 1 in the elastic zone and 0 in the plastic zone - k von Mises' yield stress - L half width of the sample - M total moisture content - P plastic power - S ij deviator of stress - S kk =S 11+S 22+S 33 - S ij E =2 e ij - T ij tensor of stress - T kk =T 11+T 22+T 33 - T non-zero component of stress in a beam or plate - non-dimensional stress - actual stress rate in an elastic zone, fictitious stress rate in a plastic zone - t time - t increment of time - x y z spatial coordinates - X increment of spatial coordinate - Y - Y 0, Y 1 terms in the expansion of Y(C): Y(C)=Y 0(1+Y 1 C+...) - non-dimensional Y - , (c) coefficient of osmotic expansion (dependent on concentration) - ij tensor of strain - kk =11+22+33 - =yy=zz non-zero component of strain in the case of a plate or beam - modified strain - elastic constants of an isotropic body - non-dimensional spatial coordinate - Poisson's ratio - non-dimensional time  相似文献   

13.
A new relaxation property is discussed on the basis of creep behavior of wet wood specimens pretreated with heating at various temperatures followed by quenching. The treated samples showed more marked relaxation than that of an untreated sample. The relationship between relaxation time and heating history was represented by an equation ln() = –( f – k 1)T + [ln( g) + k 2], where ln() is the logarithmic relaxation time of wet samples after quenching, T is the difference between the heating temperature and the glass transition temperature (T g), ln( g) is the logarithmic relaxation time at T g, is a constant, f is the coefficient of thermal bulk expansion, and k 1 and k 2 are constants. It was concluded from the analysis of experimental results that the change in the relaxation property caused by heating and the following quenching is due to the temporary free volume created by freezing of molecular chain motion of wood components, most probably lignin, during quenching.This work was presented at the 52nd Annual Meeting of Japan Wood Research Society, Gifu, April 2002  相似文献   

14.
Summary During timber drying, surface differential shrinkage within a board causes a high surface tensile stress and associated instantaneous strain. Acoustic Emission (AE) is generated when localised irreversible stress release events act to reduce the strain energy imparted to the material. A nonlinear one-dimensional drying model was used to calculate through-thickness moisture, stress and strain profiles during drying. The surface stress and instantaneous strain were used to calculate the strain energy at the surface. AE was measured during drying and the relationship between strain energy and the cumulative count (total ringdown counts) was investigated. The cumulative count is related to the unrecoverable strain energy rather than the elastic or recoverable strain energy. The cumulative count is not a useful measure of the propensity for surface checking. However the peak AE rate values are closely related to the surface instantaneous strain.Symbols E Young's Modulus (GPa) - instantaneous strain - m mechano-sorptive effect - c creep strain - u unconfined shrinkage - n net shrinkage - o set (residual) strain - pl proportional limit strain - Win strain energy (kJ/m3) - Uu unrecoverable strain energy (kJ/m3) - Ur recoverable strain energy (kJ/m3) - W in * nondimensional strain energy - U u * nondimensional unrecoverable strain energy - U r * nondimensional recoverable strain energy - stress (MPa) The authors are pleased to acknowledge the assistance of Emeritus Professor A. R. Oliver, University of Tasmania, the Australian Furniture Research and Development Institute and the Tasmanian Timber Promotion Board.  相似文献   

15.
The effect of plantation spacings (types A-E) on the bending strength and dynamic modulus of elasticity of 41-year-old Taiwan-grown cedar (Cryptomeria japonica D. Don) was investigated. The results indicate that the highest values for the static bending modulus of elasticity (MOE), modulus of rupture (MOR), and dynamic modulus of elasticity (ED, E Dt) occurred in trees obtained from those most densely planted (type A); there was a significant difference between type A and the other four spacing types (B, C, D, and E), but there were no significant differences among those four types. Interrelations among MOE, MOR, E D, and E Dt could be represented by positive linear regression formulas, which revealed highly significant differences. The relations among the square value of stress-wave transmission velocity (Vt 2 and Vt 2) and MOE, MOR, E Dl, and E Dt, respectively, could be represented by positive linear regression formulas. The differences were highly significant.Part of this report was presented at the International Wood Engineering Conference '96, New Orleans, LA, USA, October 1996  相似文献   

16.
Summary Commercial preparations based onBacillus thuringiensis var.thuringiensis (Serotype H1) may have a deliterious effect on populations of spider mites(Tetranychus urticae). This effect could be related to the content of -exotoxin in that preparations.  相似文献   

17.
Summary The diffusion equation for water in wood is expanded in terms of temperature and moisture gradient on the assumption that the driving force for the diffusion of water in wood is the partial pressure of water vapour. An analytic expression is then developed for the activation energy of diffusion in terms of enthalpy and entropy changes associated with the sorption process. The expression is compared with another published curve and some similarity was observed.Symbols C water concentration, kg/m3 - D diffusion coefficient for water vapour in wood with vapour pressure as the driving potential, kg/ms Pa - Dc diffusion coefficient for water vapour in wood with water concentration as the driving potential, m2/s - Dc a constant value of Dc, m2/s - E activation energy of diffusion, J/kg - F flow density, kg/m2 s - f h/l - h specific enthalpy, J/kg - L l/R T - l latent heat of vapourization of free water, J/kg - ls latent heat of vapourization of sorbed water, J/kg - p partial pressure of water vapour, Pa - ps pressure of water vapour at saturation, Pa - R specifc gas constant for water, J/kg K - r relative humidity - s specific entropy, J/kg K - w dry basis moisture content - x length coordinate, m - a constant temperature equal to 6,800 K - -/ln r - w density of wood (dry mass/moisture volume) at a given moisture content, kg/m3 - s/R - L style as 2 lines above - free water relative to sorbed water The author is grateful to the Editorial Board in relation to the use of (4)  相似文献   

18.
Summary The possibility of using microtomed longitudinal sections of wood for the study of longitudinal shrinkage has been investigated using hoop pine (Araucaria cunninghamii Ait.). Cutting of the sections was shown to distort them and to affect their subsequent shrinkage. However, by increasing the inclination angle of the microtome knife, sections could be cut without change in longitudinal dimensions. The longitudinal shrinkage behaviour of such sections more than 80 m thick was little different from that of thicker sawn specimens.Measurements of longitudinal shrinkage using this technique showed that very thin sections (40 m) had greater longitudinal shrinkage than sawn wood. Partial delignification with sodium chlorite also increased longitudinal shrinkage. The longitudinal shrinkage was reproducible over successive cycles of moisture content change but not reversible, the length of a specimen at a given moisture content being less during desorption than at the same moisture content during adsorption. It is considered likely that the longitudinal shrinkage of wood is markedly affected by stresses which develop within the cell wall as the wood dries.
Zusammenfassung Die Möglichkeit, Mikrotom-Längsschnitte zur Prüfung der Längsschwindung zu verwenden, wurde an Hoop Pine (Araucaria cunninghamii Ait.) geprüft. Dabei zeigte sich, daß die Längsschnitte beim Schneiden verschoben werden, was die nachfolgende Schwindung beeinflußt. Nachdem der Schnittwinkel des Mikrotommessers vergrößert worden war, konnten die Schnitte ohne Veränderung der Längsabmessungen abgetrennt werden. Das Längsschwindungs-Verhalten solcher Schnitte von über 80 m Dicke unterschied sich nur wenig von demjenigen dickerer, gesägter Proben.Messungen der Längsschwindung mit Hilfe des beschriebenen Verfahrens zeigten, daß sehr dünne Schnitte (40 m) eine größere Längsschwindung aufweisen als gesägte Schnitte. Die teilweise Delignifizierung mit Natrium Chlorit erhöhte ebenfalls die Längsschwindung. Die Längsschwindung blieb während aufeinanderfolgender zyklischer Feuchtigkeitsänderungen reproduzierbar, war aber nicht umkehrbar. Die Länge einer Probe war, bei gleichem Feuchtigkeitsgehalt während der Desorption geringer als während der Adsorption. Es darf als wahrscheinlich angesehen werden, daß die Längsschwindung des Holzes deutlich durch Spannungen beeinflußt wird, die sich beim Trocknen des Holzes innerhalb der Zellwand entwickeln.
  相似文献   

19.
Summary It is commonly assumed that specific enthalpy is uniform throughout water sorbed on wood. It is suggested here that this is not the case and that as a result the isosteric heat and the differential heat of wetting are two distinct functions. An analysis is developed which enables the distribution of specific enthalpy within the adsorbed water to be approximated. The results are presented with reference to klinki pine.Symbols a parameter, Eq. (14) - h specific enthalpy of sorbed water, J/kg - h average specific enthalpy of sorbed water, J/kg - h isosteric heat, J/kg - h1 integral heat of wetting, J/kg - k a constant - l latent heat of vaporization of free water, J/kg - Ps pressure of water vapour at saturation, Pa - q differential heat of wetting, J/kg - R specific gas constant for water, J/kg K - r relative humidity - T temperature, K - enthalpy function defined in Eq. (10), J/kg - moisture content - p prevailing moisture content The author is grateful to Dr. A. N. Stokes for a substantial simplification of the original derivation of Eq. (13)  相似文献   

20.
This study investigated the behavior of piezoelectric, dielectric, and elastic constants and the crystallinity in wood cellulose by repeated measurements (n = 42) between 100°C and 220°C. There was an insignificant change in the piezoelectric constant during repeated measurements in this temperature range. On the other hand, thermal decomposition of the amorphous region contributed to the decreasing trend of dielectric and elastic constants, although only a small increase in the elastic constant was found at the time of the initial measurements. The increase in the repeated measurements in this temperature range resulted in an increase in the piezoelectric loss modulus constantd 25, which is closely related to energy loss. Thed 25 peak shifted to a higher temperature with increasing measurements, which might be due not to the increase in rigidity of the wood specimen but to the increase in total peak area, which was observed during the later measurements. At the same time, variations of piezoelectric loss modulusd 25 ande 25 at advanced stages of the measurements suggested damage and structural changes in the wood.  相似文献   

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