首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The correction factor developed by Jann and Deppe (1990) to adjust the measured perforator values of particleboards and fibreboards measured at different moisture contents in the range of 3% ≤ u ≤ 9% to the level at 6.5% moisture content has two main deficiencies. It takes no account of the influence of the molar ratio of the resins used, which seems to remarkably impact the relationship between the moisture content and the measured perforator values. Moreover, research work revealed that the correction factor to be applied also depends on the moisture content, at which the perforator value was measured, according to an inversely proportional correlation. Besides, the correction factor according to Jann–Deppe leaves this fact unconsidered.  相似文献   

2.
The influence of moisture content (MC) on the dynamic modulus of elasticity of structural lumber was investigated using transverse vibration testing methods. The flexural rigidity (EI) of a transversely vibrating beam was calculated as the modulus of elasticity (E) multiplied by the moment of inertia (I). The increase in E of lumber due to reduction in moisture content was computed by assuming that the flexural rigidity remains constant with changes in moisture content. Reductions in I due to shrinkage were compensated by the increases in E which led to a proposal for a species-dependent MC adjustment model for modulus of elasticity. The model was validated using 38 mm × 89 mm × 4,290 mm western Canadian Spruce–Pine–Fir dimension lumber evaluated in the “as-received” and “dry” conditions. Results obtained from the species-dependent model agreed closely with those from the E adjustment equation for dimension lumber given in ASTM D 1990. The results show that the ASTM moisture adjustment procedures can be used to adjust dynamic E values for changes in moisture content also.  相似文献   

3.
在孟加拉诺阿卡利地区及相临裸地,对海岸植被(12年和17年生无瓣海桑Sonneratia apetala)进行探索性研究,以便了解海岸造林对土壤特性的影响.在三种不同地带(内陆、中部、海边),在12年生和17年生无瓣海桑林下,土壤深度分别为0-10,10-30和30-40cm,土壤湿度、土壤粒度、有机质、C含量、总N、pH、有效P、K、Na、Ca和Mg含量明显高(p≤0.05,p≤0.01,p≤0.001)于其相临裸地的数据,土壤含盐量明显(p≤0.01)低于其相临裸地的数据.在内陆CharAlim植被,土壤表面的土壤湿度,土壤粒度,有机质,C含量、总N、pH、土壤含盐量、有效P、K、Na、Ca和Mg含量分别为:31.09%、2.24 g·cm-3、2.41%、4.14%、0.58%、7.07、O.09 dS·cm-1、28.06 mg·L-1、O.50 mg·L-1、11.5 mg·L-1、3.30 mg·L-1和2.7 mmol·kg-1;而在相邻的Char Rehania贫瘠地区的相同土壤深度,其相关值分别为:16.69%、1.25g·cm-3、O.43%、0.74%、O.25%、6.57、0.13 dS·cm-1、13.07mg·L-1、O.30mg·L-1、1.4 mg·L-1、O.30 mmol·kG-1和0.50 nag·L-1.然而,在小内陆到海边的植被中,土壤湿度、土壤密度、有机质、C含量、总N、pH、有效P、K、Na和Ca含量逐渐降低,而土壤含盐量、Na和Mg含量却逐渐增加.虽然,在植被与相临裸地的不同土壤深度中土壤质地不同,植被地中砂土份额明显(p≤0.01)低于相临裸地,而粉砂土份额则明显(p≤0.001)高于裸地.在本研究中,所有参数的评价也在为其他地区相关研究得到应用.  相似文献   

4.
A study of self-binding fibreboards pressed without synthetic adhesives is reported. The board samples of steam-exploded grey alder (Alnus incana L. Moench) particles hot-pressed under a variety of conditions including moisture content of the mat (8% and 12%), temperature (150°C, 160°C, 170°C and 180°C) and pressure (0.2–8 MPa) are tested for moisture resistance (standard EN 1087-1), decay resistance (standard ENV 12038) and reaction to fire performance (ISO 5660-1). The moisture resistance of the board samples is found to satisfy the standard EN 622-5 for load-bearing medium-density fibreboard for use in humid conditions (MDF.HLS). The tested board samples are not fully resistant to the Trametes versicolor fungus, the mass loss of specimens exceeding 3%. Reaction to fire performance of the self-binding board evaluated by the average rate of heat emission being by about 25% higher is worse compared with MDF.HLS.  相似文献   

5.
Matched sample boards from 20 quarter-sawn boards of Victorian Ash (Eucalyptus regnans F. Muell and E. delegatensis R.T. Baker) were dried using three different levels of ramped pre-drying schedules to investigate the effects of moisture gradients on collapse recovery and internal checking. Prior to reconditioning, most wet cores were found in highly collapsed boards with low density. Reducing the gradients in these boards is crucial for recovering collapse and closing internal checks. If time allows the boards to be equilibrated prior to steam reconditioning, a target mean moisture content of ≤20% with a moisture gradient of close to 5% (core to surface moisture content) is likely to recover slightly more collapse than targeting a mean moisture content close to ≥15%. However, if time or kiln restraints limit equilibration it is likely to be better to target a percentage moisture content of closer to 15% in order to ensure that the core to surface moisture gradients are below 8–10%. The slight reduction in collapse recovery with this second approach is less important than the possibility that collapse and internal checks in the centre of boards with wet cores will not be closed. Care needs to be used with this latter approach not to over-dry some boards, since moisture contents below 15% will progressively reduce collapse recovery. For boards within these moisture content guidelines, the application of heat, rather than moisture pick-up, appears to be the most important component of the steaming reconditioning process. Hence, steaming only needs to be undertaken for long enough to heat the core of the board close to the target temperature of 100°C. A simple method for estimating this heat-up time for different thicknesses and species was demonstrated based on a key dimensionless group for heat transfer, the heat-transfer Fourier number.  相似文献   

6.
The knowledge of the convective heat and mass transfer coefficients is required for the characterization of the boundary conditions of the heat and mass transfer equations of a wood drying model based on water potential. A new experimental method for the determination of the convective mass transfer coefficient is presented. This method is based on the measurement of the moisture content, and indirectly the water potential, at the surface of a wood specimen at different drying times. Drying experiments were performed on red pine (Pinus resinosa Ait.) sapwood from nearly saturated to dry conditions at 56 °C, 52% relative humidity and air velocities of 1.0, 2.5 and 5.0 m s−1. The results show that the convective mass transfer coefficient is constant until the wood surface moisture content reaches about 80% and then decreases more or less gradually as the moisture content decreases further. The convective mass transfer coefficient increases with air velocity. A regression analysis shows that there is no significant improvement in considering the water potential gradient near the wood surface when the difference in water potential between the surface and the surrounding air (ψs − ψ∞) is used to determine the convective mass flux at the surface. Also, ψs − ψ∞ is more appropriate than the water vapour pressure difference (pvs − pv∞) as the responsible driving force of the moisture flux leaving the wood surface. The convective heat transfer coefficient was determined during the same experiments. A plateau is observed at high values of moisture content corresponding to the constant drying rate period. Received 27 February 1998  相似文献   

7.
Further study is needed on loblolly pine (Pinus taeda L.) growth in a systematic array of plantation designs or stocking rates commonly used in temperate forestry and agroforestry practices. Our objective was to determine loblolly pine growth responses and agroforestry implications of 13 plantation designs (i.e., stocking rates in trees ha−1 [TPH]) at mid-rotation (14 years old). Survival, diameter at 1.3 m above soil surface (dbh), height, basal area (BA), and volume (V) were measured in unthinned plantations ranging from 490 to 2,300 TPH. Stocking rate was positively correlated with BA (r ≥ 0.67) and V (r ≥ 0.55) and negatively correlated with survival (r ≤ −0.83) and dbh (r ≤ −0.83). Plantations with ≥2,000 TPH had closed canopies and excessively high BA and V at mid-rotation. The 4- and 5-row plantations (≥12 m alley spacing) had small dbh (≤17.5 cm). Single-row plantations with ≥3.6 m within row spacing and ≤700 TPH, and the 3-row multiple-row plantations (1,200 TPH), had acceptable BA (29.4–33.2 m2 ha−1) and V (127–136 m3 ha−1). Basal area was ≥30 m2 ha−1 in most plantations indicating thinning is needed to optimize individual tree growth. Besides timber, an array of design-dependent agroforestry and forestry products should drive the selection of any one of these plantation designs: pine straw or biomass production at ≥1,800 TPH, and alley cropping or silvopasture in single-row (≤1,000 TPH) and multiple-row plantations (<1,400 TPH).  相似文献   

8.
Extractives can affect the vibrational properties tanδ (damping coefficient) and E′/ρ (specific Young’s modulus), but this is highly dependent on species, compounds, and cellular locations. This paper investigates such effects for African Padauk (Pterocarpus soyauxii Taub.), a tropical hardwood with high extractives content and a preferred material for xylophones. Five groups of 26 heartwood specimens with large, yet comparable, ranges in vibrational properties were extracted in different solvents. Changes in vibrational properties were set against yields of extracts and evaluation of their cellular location. Methanol (ME) reached most of the compounds (13%), located about half in lumen and half in cell-wall. Water solubility was extremely low. tanδ and E′/ρ were very strongly related (R 2 ≥ 0.93), but native wood had abnormally low values of tanδ, while extraction shifted this relation towards higher tanδ values. ME extracted heartwood became in agreement with the average of many species, and close to sapwood. Extractions increased tanδ as much as 60%, irrespective of minute moisture changes or initial properties. Apparent E′/ρ was barely changed (+2% to −4%) but, after correcting the mass contribution of extracts, it was in fact slightly reduced (down to −10% for high E′/ρ), and increasingly so for specimens with low initial values of E′/ρ.  相似文献   

9.
The physical properties of soil on two hill slopes of 35% and 55% in orange orchard cultivated by the Mro tribe of Chittagong Hill Tracts （CHTs） were evaluated and compared with those of bushy hill forests. Soil samples were collected from three different depths （0-5 cm, 5-15 cm and 15-30 cm）, digging three profiles in each land use for determining moisture content, organic matter content and particle density. Maximum water holding capacity, field capacity, dry and moist bulk density and porosity were determined only for the surface soils. Moisture content at all the soil depths was significantly higher （p≤ 0.05） in orange orchard than in forest on both the slopes. Orange orchard contained lower mean soil organic matter than forest on 55% slope, while it contained higher values on 35% slope compared to forest. The highest value of the above two properties was found at surface soil in both the land uses on both the slopes, decreasing with the increase of soil depth. On both the slopes maximum water holding capacity and porosity of surface soil and particle density at all soil depths were lower in orange orchard compared to those in forest. Field capacity values of surface soil did not show consistency in trend for the differences between the two land uses on both the slopes. Bulk density value of moist and dry surface soil was higher in orange orchard than in forest on both the hill slopes.  相似文献   

10.
Prunus cerasoides has poor regeneration in its natural habitat due to its hard seed coat. The information about maturity time of fruit/seed is scanty and studies on seed germination enhancement lacking. In the present study, the main focus has been given to the physical fruits/seeds attributes which are reliable indicators of maturity and seed pretreatments which help to enhance germination. The fruits/seeds were collected from low and high elevational sites covering the altitudinal range of the species during the period of fruit/seed development (last week of February–mid April) for two consecutive years (2003–2004). The change in fruit colour from dark green to pale red or red was a useful indicator of seed maturity in P. cerasoides. Fruit moisture content between 46.57 ± 0.45% and 56.81 ± 1.14% and seed moisture content between 29.8 ± 1.35% and 34.13 ± 1.50% coincided with maximum germination (41.11 ± 13.96% and 59.99 ± 12.05%) across sites. The removal of seed coat (mechanical treatment) enhanced the germination of seeds to 70 ± 0% at low elevation and 100 ± 0% at high elevation in comparison of control or untreated seeds (germination range between 5.83 ± 0.83% and 31.68 ± 25.02%). Positive correlation existed between seed size and germination (r = 0.280; P < 0.01).  相似文献   

11.
GOSLING  P. G. 《Forestry》1991,64(1):51-59
Beechnuts are not easy to store or long-lived in storage. Theyexhibit storage characteristics between those of ‘orthodox’and ‘recalcitrant’ seeds. Superimposed upon thisis a pronounced dormancy which is overcome by between 4 and20 weeks of moist chilling. Pretreatment periods of 15–20weeks occupy most of the time between collection in Octoberand sowing the following April; hence short-term storage isa combination of moist storage and pretreatment. Most evidencepoints to the best short-term storage/pretreatment conditionsbeing 0–5°C at 28–32 per cent moisture content(m.c.) (fresh weight basis). Long-term storage requires a periodat low moisture content and the best conditions appear to be10 per cent m.c. and –10°C or below. The relative merits of overcoming seed dormancy before or afterdry storage are reviewed. There is no consensus about whichsequence is best but the former procedure is likely to be preferredby nursery managers because it allows storage of non-dormantnuts which can be withdrawn from store and sown immediately. Received 13 February 1990.  相似文献   

12.
Sap flow measurements, from July to August 2004, were coupled with micrometeorological, soil moisture, and soil temperature measurements to analyze forest water dynamics in irrigated and undisturbed (control) larch (Larix cajanderi) forest plots in eastern Siberia. Plots were irrigated with 120 mm (20 mm day−1) of water from 17 to 22 July. Sap flow measurements of ten trees at each plot were scaled up to daily stand canopy transpiration (E c ). Canopy transpiration at the irrigation and control plots was similar before irrigation. Forest evapotranspiration (E a ) was obtained from Ohta et al. (Agric For Meteorol 148:1941–1953, 2008) while E a in the irrigation plot was estimated based on the E c_irrig/E c_cont ratio. Rainfall during July–August was 63.4 mm but, after including water from thawing soil layers, the actual water input was 109.9 and 218.5 mm in the control and irrigation plots, respectively. Despite this large difference, a corresponding difference in E c (and E a ) was not observed [42.6 (61.5) mm and 46.4 (71.8) mm in control and irrigation plots, respectively]. Daily canopy conductance (g c ) increased as long as moisture was well supplied in the upper soil layers and evaporative demand was high. Soil moisture and rainfall contribution to E a was 36.9 and 24.6 mm in the control plot and 34.5 and 37.3 mm in the irrigation plot, respectively. Water supply from soil thawing layers in the control plot and high runoff (105.6 mm) rates in the irrigation plot accounted for the similarity in water dynamics. Under increased precipitation, the forest used less soil water stored from previous growing seasons.  相似文献   

13.
Soil moisture content from 0 to 2 m depth was monitored under 2–6 year old radiata pine (Pinus radiata) with three understoreys of bare ground, lucerne (Medicago sativa) and ryegrass/clover (Lolium perenne/Trifolium spp.) and under adjacent open-grown lucerne and ryegrass/clover pastures. By the fifth year soil moisture depletion/recharge pattern under the trees alone was similar to that under open pasture and under trees with pasture understoreys. Maximum plant available moisture storage was 207–223 mm in the top meter of this Templeton silt loam soil but only 69–104 mm at 1–2 m depth where coarse textures often predominated. Lucerne reduced soil moisture content (SMC) to lower levels during drier summers and extracted more water from 1 to 2 m depth than ryegrass/clover. Evapotranspiration (ET) during early summer when soil moisture was high was close to the Penman potential evapotranspiration (E p ), but the difference increased when SMC in the top meter dropped below 200 mm. The silvopasture treatments had higher ET in winter than pasture alone but this was still less than E p . Soil moisture deficits (SMD) at the end of each summer were sufficiently large to require slightly higher than normal winter rainfall and ET < E p to recharge the soil to field capacity before the next summer. The soil moisture results, taken together with root and growth data, suggest that trees and understorey pastures are complementary in the first three or four growing seasons but this balance subsequently declines in favor of the pine trees. Management options, to extend the period that understorey pastures are productive, include reducing tree stockings, more vigorous pruning, using competitive understoreys and changing from pines to deciduous trees. Research on new silvopastoral combinations is suggested.  相似文献   

14.
Thidiazuron (TDZ) induced somatic embryogenesis from immature zygotic embryos in Cinnamomum pauciflorum Nees while 2,4-dichlorophenoxyacetic acid (2,4-D), 6-benzylaminopurine (BA) or picloram only induced callus and/or adventitious buds. The highest induction frequency for somatic embryogenesis was achieved with MS medium (Murashige and Skoog in Physiol Plant 15:473–497 1962) supplemented with 2.5 μM TDZ using torpedo-shaped embryos (3–5 mm in length) as explants. In addition, induction medium was supplemented with 0.8 g l−1 casein, 0.4 g l−1 glutamine, and 10 g l−1 sucrose. Somatic embryos (SEs) initiated from root tips or hypocotyls without callus formation. SEs were maintained and multiplied via secondary somatic embryogenesis. Embryo maintenance medium was similar to induction medium except that TDZ was reduced to 0.5 μM. Secondary embryogenesis was enhanced by supplementation of 5 g l−1 activated charcoal in the culture. The best medium for embryo maturation was MS medium containing 30 g l−1 sucrose and 5 g l−1 Phytagel without plant growth regulators. A typical mature SE consisted of two large cotyledons and a short embryo proper. Approximately 82% of selected mature SEs were able to germinate and 63% could convert into plantlets on germination medium that was composed of half strength MS medium salts, 10 g l−1 sucrose, 3 g l−1 Phytagel, and 5 g l−1 activated charcoal.  相似文献   

15.
Bambusa nutans Wall., is an evergreen, perennial, and multipurpose bamboo having strong culms, which are largely used for construction, scaffolding, craft purposes, pulp, and paper industry. Multiple shoots from nodal segments (3–4 cm) of young branches of mature culms were established in Murashige and Skoog (1962) (MS) medium supplemented with various concentrations of 6-benzylaminopurine (BAP) (1.0–6.0 mg l−1) or in combination with α-naphthaleneacetic acid (NAA) (0.5–1.0 mg l−1) or kinetin (Kn) (1.0–2.0 mg l−1). February–March and December were found to be the best seasons for culture establishments. Maximum shoots were achieved on MS medium fortified with BAP (2.0 mg l−1). Embryogenic callus (slightly greenish compact, globular, and slow growing) was initiated from the base of severed sprouted buds in 2–3 subsequent subcultures on MS medium supplemented with 2,4-dichlorophenoxy acetic acid (2,4-D) (5.0 mg l−1) under dark incubations. Maturation and germination of well-organized somatic embryos was achieved on MS medium containing BAP and 2,4-D (1.0 mg l−1 each) with 20.0 mg l−1 ascorbic acid. Full-strength MS medium supplemented with 2% glucose favored further development of proliferated somatic embryos into plantlets. Genetic variations of field-established B. nutans plants regenerated through tissue cultures were assessed by amplified fragment length polymorphism (AFLP) analysis using 6 primer combinations. Four hundred and seven scorable fragments were amplified, of which 402 (98.8%) have recorded conservation at various morphogenetic stages leading to plantlets regeneration, therefore, revealed a high level of genetic stability.  相似文献   

16.
Wood products are considered to contribute to the mitigation of carbon dioxide emissions. A critical gap in the life cycle of wood products is to transfer the raw timber from the forest to the processing wood industry and, thus, the primary wood products. Therefore, often rough estimates are used for this step to obtain total forestry carbon balances. The objectives of this study were (1) to examine the fate of timber harvested in Thuringian state forests (central Germany), representing a large, intensively managed forested region, and (2) to quantify carbon stocks and the lifetime of primary wood products made from this timber. The analyses were based on the amount and assortments of actually sold timber, and production parameters of the companies that bought and processed this timber. In addition, for coniferous stands of a selected Thuringian forest district, we calculated potential effects of management, as expressed by different thinning regimes on wood products and their lifetimes. Total annual timber sale of soft- and hardwoods from Thuringian state forests (195,000 ha) increased from about 136,893 t C (~0.7 t C ha−1 year−1) in 1996 to 280,194 t C (~1.4 t C ha−1 year−1) in 2005. About 47% of annual total timber harvest went into short-lived wood products with a mean residence time (MRT) < 25 years. Thirty-one per cent of the total harvest went into wood products with an MRT of 25–43 years, and only 22% was used as construction wood and glued wood, products with the longest MRT (50 years). The average MRT of carbon in harvested wood products was 20 years. Thinning from above throughout the rotation of spruce forests would lead to an average MRT in harvested wood products of about 23 years, thinning from below of about 18 years. A comparison of our calculations with estimates that resulted from the products module of the CO2FIX model (Nabuurs et al. 2001) demonstrates the influence of regional differences in forest management and wood processing industry on the lifetime of harvested wood products. To our knowledge, the present study provides for the first time real carbon inputs of a defined forest management unit to the wood product sector by linking data on raw timber production, timber sales and wood processing. With this new approach and using this data, it should be possible to substantially improve the net-carbon balance of the entire forestry sector.  相似文献   

17.
A dramatic decline in forest cover in eastern Africa along with a growing population means that timber and poles for building and fuelwood are in short supply. To overcome this shortage, the region is increasingly turning to eucalyptus. But eucalyptus raises environmental concerns of its own. Fears that it will deplete water supply, affect wildlife and reduce associated crop yields have caused many countries in the region to discourage farmers from planting this exotic. This paper is part of a series of investigations on the growth and water use efficiency of faster growing eucalyptus hybrids, which was introduced from South Africa to Kenya. The hypothesis is that the new hybrids are more efficient in using water and more suitable for the semi-arid tropics than existing eucalyptus and two popular agroforestry species. Gas exchange characteristics of juvenile Eucalyptus grandis (W. Hill ex Maiden), two eucalyptus hybrids (E. grandis × Eucalyptus camaldulensis Dehnh.), Grevillea robusta (A. Cunn) and Cordia africana (Lam) was studied under field and pot conditions using an infrared gas analyzer was used to measure photosynthetic active radiation (PAR), net photosynthetic rate (A), stomatal conductance (g s) and transpiration rate (E) at CO2 concentrations of 360 μmol mol−1 and ambient humidity and temperature. A, E and g s varied between species, being highest in eucalyptus hybrid GC 15 (24.6 μmol m−2 s−1) compared to eucalyptus hybrid GC 584 (21.0 μmol m−2 s−1), E. grandis (19.2 μmol m−2 s−1), C. africana (17.7 μmol m−2 s−1) and G. robusta (11.1 μmol m−2 s−1). C. africana exhibited high E values (7.0 mmol m−2 s−1) at optimal soil moisture contents than G. robusta (3.9 mmol m−2 s−1) and eucalyptus (5.3 mmol m−2 s−1) in field experiment and G. robusta (3.2 mmol m−2 s−1) and eucalyptus (4.2 mmol m−2 s−1) in pot-grown trees. At very low soil moisture content, extremely small g s values were recorded in GC 15 and E. grandis (8 mmol m−2 s−1) and G. robusta (14 mmol m−2 s−1) compared to GC 584 (46.9 mmol m−2 s−1) and C. africana (90.0 mmol m−2 s−1) indicating strong stomatal control by the species. Instantaneous water use efficiency ranged between 3 and 5 μmol mmol−1 and generally decreased with decline in soil moisture in pot-grown trees but increased with declining soil moisture in field-grown trees.  相似文献   

18.
This study examined the origin of the moisture dependency of the longitudinal Youngs modulus of wood (E L ) in relation to the microfibril angle (MFA) of the S2 layer of the secondary wall. Microtomed early wood specimen of sugi (Cryptomeria japonica D.Don) were used for the experiment. The following was revealed:
1.  E L tends to decrease as the moisture content increases in the region below the fiber saturation point (FSP).
2.  The percentage reduction of E L from the oven-dried state to the FSP is almost constant regardless of the MFA.
Subsequently, the relationship between E L and the moisture content was simulated theoretically using the simplified wood fiber model proposed in our previous paper (Part 1, 2002). The simulation considered the two hypotheses proposed in Part 1 for the origin of the moisture content dependency of E L . The first is a traditional theory that the reduction of E L is caused mainly by the moisture dependency of the lignin-hemicellulose matrix. The second assumes that an intermediate domain exists between the rigid crystal and the compliant disordered amorphous regions in wood cellulose microfibril (CMF). It is assumed that such a domain fluctuates between the rigid crystal-like and the compliant amorphous-like states at which the elastic modulus is of the same order as the lignin-hemicellulose matrix in accordance with the moisture sorption.When the first hypothesis is adopted for the simulation, the percentage reduction of E L from the oven-dried state to the FSP should increase as MFA increases; this was contradicted by the experimental results (2). On the other hand, when the second hypothesis is applied to the simulation, the experimentally obtained results (1) and (2) are simulated reasonably. This suggests that the moisture dependency of E L is controlled by the second hypothesis.  相似文献   

19.
Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed by authors for rheological parameter identification.  相似文献   

20.
In order to analyze the effect of temperature gradient on moisture movement during highly intensive drying, such as microwave-vacuum drying, the profile of the temperature and moisture content in sealed wood whose opposite faces were subjected to temperature gradient for a short time was measured. The ratio of the moisture content (MC) gradient to the temperature gradient (dM/dT) was calculated and the factors influencing moisture movement under nonisothermal conditions were discussed. The results indicate that moisture moved in wood from the warm surface to the cold one even if opposite faces of the sealed wood assembly were exposed continuously to different but constant temperatures for a short period. The moisture content on the cold surface was higher than that on the warm surface. The moisture content gradient opposite to the temperature gradient was established, and the dM/dT was below 0.9%/°C. The temperature in the sample and the distance from the hot surface of the sample was strongly linearly correlated. With an increase in temperature, initial moisture content and experimental time, the dM/dT was significantly increased. __________ Translated from Journal of Beijing Forestry University, 2005, 27(2): 96–100 [译自: 北京林业大学学报, 2005, 27(2): 96–100]  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号