共查询到20条相似文献,搜索用时 62 毫秒
1.
J. Ilic 《Wood Science and Technology》1999,33(5):425-437
Summary Assessments of internal checking and the physical properties of 124 trees of Eucalyptus regnans F. Muell. have shown that for material dried under relatively mild predryer conditions (30 °C, 65% RH) internal checking
was highly positively correlated with each of collapse, moisture content and normal shrinkage, and weakly negatively correlated
with total external shrinkage. Collapse alone explained 47% of the variation in internal checking. Incidence of internal checking
in sample boards could be estimated with moderate success by each of the following properties measured on board ends: collapse,
the number of internal checks and initial moisture content. Material with high mean basic density above 530 kg/m3 was associated with low levels of internal checking and collapse. However, the maximum naturally occurring density of E. regnans was not high enough to obviate collapse and internal checking. It was observed that growth rings in 100 × 50 mm backsawn
boards in which the earlywood air-dry density was below 450 kg/m3 showed internal checking. The size and number of internal checks increased with a decrease in earlywood density. It was shown
that drying E. regnans below temperatures of 24–30 °C does not eliminate collapse, thus raising doubt about the validity of a temperature threshold
concept in that range.
Received 17 September 1997 相似文献
2.
Influence of wood planing on the second-order effects of moisture sorption in sugar maple 总被引:1,自引:1,他引:0
Summary Two types of machines, a conventional planer, and a fixed-knife pressure-bar planer were used to prepare matched specimens
of sugar maple wood. After adsorption and desorption, both experiments at 21 °C, the EMC, swelling in all principal directions
as well as compliance coefficient in radial compression were measured. Two specimen sizes were used for these expe‐riments.
For a given equilibrium moisture content, tangential and radial dimensions were greater after desorption than after adsorption,
as previously described. When equilibrium was reached by gaining moisture, the wood was stiffer in radial compression compared
to when the equilibrium was reached after losing moisture. The magnitude of this phenomenon, second-order effects of moisture
sorption, was slightly affected by the type of planing. These effects on swelling were greater for large specimens prepared
by conventional planing compared to fixed-knife pressure-bar planing. Small specimens showed similar magnitudes of this phenomenon
with both planing methods. No differences between planing methods were found for the radial compliance coefficient measured
on either specimen size. Therefore, the second-order effects of moisture sorption appeared to be a bulk phenomenon and not
restricted to the superficial layers of wood.
Received 9 December 1997 相似文献
3.
The equilibrium moisture content (EMC) of six wood species under desorption conditions of 20°C and 100% 0% relative humidity (RH), and the rate of adsorption at various depths of three wood species blocks under 98% RH at 22.5°C were studied. There were no significant differences among the EMC values for these six wood species over the RH range 40% 0%, but there were highly significant differences over the RH range 100% 50% at constant 20°C. The amount of moisture absorbed in the wood decreased curvilinearly with the increase of depth in the specimens as sorption time increased, and their relation could be represented by a semilogarithmic equation. Time-dependent adsorption behavior at various depths of the wood specimens could be represented by an exponential equation as a function of the product of the difference between moisture contents at equilibrium and initial conditions and the term (1 – e–t/). The value of of various wood species was found to increase linearly with the increased depth of the specimen and showed the following trend: hard maple (Acer sp.) > China fir (Cunninghamia lanceolata) > Japanese cedar (Cryptomeria japonica D. Don).Part of this report was presented at the 47th annual meeting of the Japan Wood Research Society, Kochi, April 1997. 相似文献
4.
Variation of stress wave velocity with MC and temperature 总被引:5,自引:0,他引:5
The effect of moisture content (MC) and temperature on the stress wave velocity and signal frequency spectrum through sapwood
has been investigated. It was discovered that in 2.5 m long green boards only low frequencies were present in the transmitted
signal, while for boards less than 500 mm long, the much higher resonance frequency of the transducer was dominant. For green
boards between 0.5 and 2.5 m both low and high frequency components were present. The frequency spectrum was monitored for
a 540 mm long board over a range of moisture contents and temperatures. When the MC was below 30% the transmitted signal waveform
consisted almost entirely of the transducer resonance frequency, while at higher moisture contents, low frequency components
predominated. The frequency spectrum of the transmitted signal was little affected by temperature, but it was affected by
the type of transducers used. The effect of temperature and moisture content on stress wave velocity was studied and is displayed
in the form of a three dimensional graph.
Received 3 May 1999 相似文献
5.
《Wood material science & engineering》2013,8(3):105-111
Abstract Compression creep experiments of hybrid poplar (Populus deltoides×Populus trichocarpa) were performed in a pressurized vessel equipped with a heated hydraulic press. The viscoelastic response at various stress levels (2–7 MPa), a temperature of 170°C and transient steam conditions was studied. Moisture content and oven-dry density of compressed specimens were determined. While some recovery of compression strain occurred, compression resulted in permanent deformation and increased wood density. The influence of stress level on the amount of set recovery of compressive deformation was evaluated after 24 h water soaking. Applied stress level had a significant effect on the compression deformation. The initial strain, as well as creep strain, varied depending on the applied stress level. The highest oven-dry density was obtained at a stress level of 6.9 MPa. Lower stress levels resulted in lower moisture content after the compression process, while the equilibrium moisture content of compressed specimens was not significantly affected by stress level. Set recovery increased from 20% to 65% with increased stress level from 1.7 MPa to 4.1 MPa, then decreased to 53% for specimens compressed at 6.9 MPa. Moisture content after the compression process significantly affected the set recovery. 相似文献
6.
Jeroen H. van Houts Siqun Wang Huiping Shi George W. Kabalka 《Wood Science and Technology》2006,40(6):437-443
The previous paper in this series demonstrates the use of a nuclear magnetic resonance (NMR) micro-imaging system to observe the movement of liquid water through small specimens (11 × 16 mm2 cross-section) of oriented strandboard (OSB) and solid wood. In the present paper, a NMR body scanner is utilized to obtain images of moisture penetration into ASTM D 1037–99 standard sized (152 × 152 mm2) thickness swelling specimens at different intervals during water soaking. As with the micro-imaging study, silicone was applied to the surfaces and edges of some of the specimens in order to observe the directional characteristics of moisture movement through OSB. Specimens with commercially applied edge sealant were also included in the study in order to observe the effectiveness of the sealant of reducing moisture penetration and thickness swelling. The presence of voids and areas of low density were shown to contribute significantly to moisture penetration into the specimens. 相似文献
7.
Experimental determination of the convective heat and mass transfer coefficients for wood drying 总被引:1,自引:1,他引:0
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
8.
Relaxation mechanism of residual stress inside logs by heat treatment: choosing the heating time and temperature 总被引:1,自引:0,他引:1
Some methods to reduce residual stress inside logs have been reported, although the conditions for stress relaxation are
not yet clarified. Our study using precise experiments revealed that residual stress relaxation occurs only when both heat
and moisture exist inside the logs. We then determined the heating time and temperature required to relax the residual stress
inside the logs. Short air-drying treatments did not relax residual stress even though free water in the logs was greatly
reduced. The residual stress of the 33-h 80°C-heated bolts was relaxed, whereas that of the 48-h 70°C-heated bolts was not.
As for the influence of treatment time, bolts heated at 100°C were relaxed after 18 h of treatment. The 13-h heated bolts
did not show any relaxation. Therefore, residual stress relaxation occurred rapidly owing to the thermomechanical change of
the individual wood components comprising the cell wall. The moisture content inside all the bolts was much higher than the
fiber saturation point. This is because relaxation occurs only when the heating temperature is maintained above 80°C for a
particular duration of treatment.
Received: December 12, 2001 / Accepted: February 18, 2002
Present address: Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Sciences and Technology, Independent
Administrative Institution, Nagoya 463-8560, Japan Tel. +81-52-736-7320; Fax +81-52-736-7419 e-mail: m.nogi@aist.go.jp
Part of this report was presented at the 50th Annual Meeting of the Japan Wood Research Society, Kyoto, April 2000
Correspondence to:M. Nogi 相似文献
9.
Roger E. Hernández 《Wood Science and Technology》2007,41(3):249-265
Wood samples of nine tropical hardwoods from Peru and sugar maple wood from Quebec were selected to perform moisture sorption
tests associated with parallel-to-grain and tangential compression tests using a multiple step procedure at 25°C. Cold-water
and hot-water extractives, sequential cyclohexane (CYC), acetone (ACE) and methanol (MET) extracts, ash content (ASH), wood
density and interlocked grain (IG) were evaluated on matched samples too. Wood density corrected for the accessory substances
was by far the major factor positively affecting the compressive properties of tropical hardwoods. The total amount of accessory
substances is required in order to establish better relationships between physico–mechanical properties and density of tropical
hardwoods. For a given wood density, the ultimate stress in parallel-to-grain compression was higher in tropical hardwoods
than in temperate hardwoods. However, the compliance coefficients for both types of woods were quite similar. Sequential extraction
with organic solvents was the most suitable method for evaluating the effect of extractives on compressive properties of tropical
hardwoods. The CYC and ACE fractions did not contribute to variation in these mechanical properties. The substances dissolved
in MET affected positively the compliance coefficient s
11 in parallel-to-grain compression and negatively the compliance coefficient s
33 in tangential compression. The IG decreased the compliance coefficient s
11 but also decreased the ultimate stress in parallel-to-grain compression. Finally, variations in compressive properties that
were due to changes in equilibrium moisture content (EMC) were clearly influenced by wood density; denser woods were more
sensitive to changes in EMC than lighter woods. 相似文献
10.
Static bending tests and compressive test parallel to the grain of sugi (Japanese cedar, Cryptomeria japonica) green round timber were conducted to confirm whether its strength would satisfy the referenced strength determined by the Construction Ministry. The strength of green round timber and air-dried round timber were compared for bending and compression parallel to the grain. The strength change ratio in response to a 1 % change in the moisture content of round timber was compared with that of small clear specimens and timber. The results revealed that a 5 % parametric tolerance limit of bending and compressive strength parallel to the grain satisfied the referenced strength, even when using green round timber. The average strength of air-dried round timber was higher than that of green round timber, in both bending and compression parallel to the grain, with significant differences indicated at a 5 % significance level. The relation between the cross-section area that includes round timber, timber and the small clear specimens, and the strength change ratio in response to a 1 % change in moisture content change was fitted to a logarithm curve. Thus, the size of the specimen was considered to affect the strength change ratio. 相似文献
11.
Junko Nagakura Akio Akama Takeo Mizoguchi Hiroaki Okabe Hidetoshi Shigenaga Takashi Yamanaka 《Journal of Forest Research》2006,11(5):299-304
To investigate the potential effects of nitrogen (N) deposition on Japanese forests, a chronic N-addition experiment that
included three treatments (HNO3, NH4NO3, and control) was carried out in a 20-year-old Japanese cedar (Cryptomeria japonica D. Don) stand in eastern Japan over 7 years. The amount of N applied was 168 kg N ha−1 year−1 on the HNO3 plots and 336 kg N ha−1 year−1 on the NH4NO3 plots. Tree growth, current needle N concentration, and soil solution chemistry were measured. Nitrogen application decreased
the pH and increased NO3
−, Ca2+, Mg2+, and Al concentrations in the soil solution. The needle N concentration increased in both of the N plots during the first
3 years. Nevertheless, the annual increments in height and in the diameter at breast height of the Japanese cedars were not
affected by N application, and no visible signs of stress were detected in the crowns. Our results suggest that young Japanese
cedar trees are not deleteriously affected by an excess N load. 相似文献
12.
Test samples of Japanese larch (Larix leptolepis) heartwood and Japanese beech (Fagus crenata) sapwood were heated for 22 h at constant temperatures (50°–180°C) under three water content conditions. Raman spectra of
the samples were recorded before and after the heat treatments, and spectral changes in the range from 1000 cm−1 to 1800 cm−1 were evaluated using the difference spectrum method. For both wood species, the Raman band intensity at 1655–1660 cm−1 due mainly to the C=C and C=O groups in lignin clearly decreased with increasing heat-treatment temperature (HTT). The spectral
change was thought to reflect the progress of condensation reactions of lignin molecules during the heat treatment. Moreover,
the decrease in band intensity was considerably facilitated by the presence of water in the cell wall, suggesting that the
condensation is closely related to the softening of lignin. From the spectral changes in the wavenumber region of 1200–1500 cm−1, it was considered that wood constituents are partially decomposed at the higher HTT.
Part of this article was presented at the 53rd Annual Meeting of the Japan Wood Research Society, Fukuoka, March 2003 相似文献
13.
Fikret Demirci Murat Muştu M. Bora Kaydan Selma Ülgentürk 《Journal of pest science》2011,84(3):337-342
Citrus mealybug Planococcus citri (Risso) (Hemiptera: Pseudococcidae) is one of the main pests on citrus trees. Biological control of the pest is based on
the release of hymenopterous parasitoids and coccinellid predators at present. The effectiveness of entomopathogen fungus
Isaria farinosa (Holmsk.) Fries ([Sordariomycetes: Hypocreales] (Syn: Paecilomyces farinosus), as an alternative biological control agent on citrus mealybug, was investigated using four different inoculum densities
and different relative humidities (RH). The entomopathogen caused 89.39% mortality in ovisacs, 84.07% mortality in second
larval stage, 84.53% mortality in adult females, and 78.71% mortality in first larval stage at 95% RH and at 1 × 108 conidia ml−1 inoculum concentration. Percent mortalities were between 50 and 60 in ovisac, first and second larval stage at 95% RH and
at 1 × 107 conidia ml−1. Percent mortality was decreased parallel to the decrease in humidity level and inoculum densities; however, the fungus caused
significant infection in 70% and 80% RH. These results demonstrated that the effectiveness of the entomopathogen is promising
for biocontrol of citrus mealybug. 相似文献
14.
A study was conducted to determine the fitting soil moisture for the normal growth of two-year-old W. sinensis (Sims) Sweets by using gas exchange technique. Remarkable threshold values of net photosynthetic rate (Pn), transpiration rate (Tr) and water use efficiency (WUE) were observed in the W. sinensis leaves treated by various soil moisture and photosynthetic available radiation (PAR). The fitting soil moisture for maintaining a high level of Pn and WUE was in range of 15.3%-26.5% of volumetric water content (VWC), of which the optimal VWC was 23.3%. Under the condition of fitting soil moisture, the light saturation point of leaves occurred at above 800μmol.m^2.s^-1, whereas under the condition of water deficiency (VWC, 11.9% and 8.2%) or oversaturation (VWC, 26.5%), the light saturation point was below 400μmol.m^-1.s^-1. Moreover, the light response curves suggested that a special point of PAR occurred with the increase in PAR. This special point was considered as the turning point that indicated the functional transition from stomatal limitation to non-stomatal limitation. The turning point was about 600, 1000, 1000 and 400 μmol.m^-2.s^-1, respectively, at VWC of 28.4%, 15.3%, 11.9% and 8.2%. In conclusion, W. sinensis had higher adaptive ability to water stress by regulating itself physiological function. 相似文献
15.
Raman spectra of Japanese cedar (Cryptomeria japonica D. Don) treated by vacuum impregnation with aqueous boric acid solutions (8.1 × 10−2 to 7.29 × 10−1 mol dm−3) were recorded using a near-infrared laser as an excitation source. Raman spectroscopic measurements were carried out on
treated wood blocks of two sizes: 20(T) × 20(R) × 5(L) mm (A-type) and 15(T) × 15(R) × 50(L) mm (B-type). Our attention was
focused on a prominent band (ν
1) assigned to a symmetrical stretching vibration of the BO3 group because no Raman band due to boron species was observed except bands of B(OH)3. We observed a change in ν
1 band intensity with increasing boric acid concentration in the aqueous solution used to treat the A-type wood blocks and
investigated the correlation between the intensity and the peak-top wavenumber. Raman line maps in the longitudinal direction
of the treated B-type wood blocks revealed that B(OH)3 is concentrated near the cut ends. These results suggested that two groups of B(OH)3 exist in wood in terms of the chemical species in the nearest neighbor sphere.
Received: March 11, 2002 / Accepted: June 26, 2002 相似文献
16.
On the loss factor of wood during radio frequency heating 总被引:2,自引:0,他引:2
The radial direction loss factor of full-size western hemlock sapwood and heartwood, as well as western red cedar heartwood
timbers was measured using the direct calorimetric method with a laboratory-scale radio frequency/vacuum dryer at the frequency
of 13.56 MHz, moisture content range between 10 and 80%, temperature range between 25 and 55 °C, and root mean square (rms)
electrode voltages of 0.8 and 1.1 kV, respectively.
The results indicated that the moisture content, temperature, electric field strength and wood type significantly affected
the loss factor. Empirical regression equations were derived based on the experimental data that made possible the calculation
of the loss factor and power density within wood during RF heating.
Received 18 January 1997 相似文献
17.
Spatial distribution of sporocarps of Suillus pictus A.H. Smith and Thiers was studied in a plot of 6 × 12 m in size established in a stand of Pinus koraiensis Sieb. et Zucc. in Kyoto, Japan for 4 years, and the biomass of mycorrhizas was examined in the last year. S. pictus was dominant in both sporocarp and ectomycorrhizal community in the study plot. The number of S. pictus sporocarps ranged from 0.94 to 1.26 m−2 (surface area) in the study plot and did not vary very much during the study period. Sporocarps of S. pictus occurred in clumps and the distributions of clumps were generally random. As the spots of sporocarp occurrence changed gradually
from year to year, the distributions of sporocarps that occurred successively in 2-year periods overlapped, especially when
analyzed in 9-m2 unit size using the m
*
–m method. Mycorrhizas of S. pictus were distributed in more subplots than its sporocarps. Distribution of mycorrhizas and sporocarps of S. pictus generally overlapped well. The biomass of mycorrhizas and mycelia in the mycorrhizas of S. pictus was estimated at 15.5 g DW m−2 and 6.2 g DW m−2 (surface area) in this plot, respectively. The biomass of mycorrhizas and mycelia in the mycorrhizas supporting the production
of one sporocarp (average dry weight was 0.86 g) of S. pictus was evaluated as about 16.4 and 7.3 g DW, respectively, in this plot.
Received: December 20, 2001 / Accepted: August 12, 2002
Acknowledgments We thank Dr. T. Furuno, Mr. N. Kato, and Dr. I. Nakai for their help in preparing the study plot and collecting sporocarps,
and Prof. K. Yokoyama for the identification of sporocarps. Thanks are also due to Dr. E. Kuno for his suggestion about analysis.
Correspondence to:J. Kikuchi 相似文献
18.
Steam conditioning of softwood boards after kiln drying is of critical importance for relief of residual drying stresses
and to improve distribution of final moisture content. The conditioning practice in New Zealand includes two steps: immediately
after high temperature (HT) drying the load is cooled until the core wood temperature is 75 to 90°C, and then the stack is
steam conditioned for a period of 1 to 4 hours depending on the lumber thickness and moisture content after drying. In this
work, experimental and theoretical studies were performed to better understand the conditioning process and to investigate
factors which influence its effectiveness. In the experiment, 50 mm thick Pinus radiata sapwood boards were first dried at 120/70°C for 11, 12, 13, 16 and 18 hours, respectively, to varying moisture contents,
and then cooled and steam conditioned for 1 hour. To assess the effectiveness of conditioning, moisture pick-up, moisture
gradient, and transverse residual drying stress (indicated by cup and strain) were measured. It was found that drying wood
to a low moisture content (below 6%) increased the conditioning effectiveness. A separate matched stack was conditioned for
4 hours after 13 hours drying which showed better results than 1 hour conditioning. A mathematical model for wood drying was
extended to include both the cooling and conditioning phases. The model was numerically solved to examine the wood temperature
and moisture content changes during the whole process of drying, cooling and final steam conditioning. Increase in wood temperature,
moisture pickup and moisture gradient during steam conditioning were predicted and validated by the experimental data. This
information is currently being used at the New Zealand Forest Research Institute in simulation of stress development and relief
for drying of Pinus radiata lumber.
Received 6 July 1998 相似文献
19.
Robinia pseudoacacia stands act as a typical ecological protection forest in hilly semi-arid area of China. Two fields of surface runoff were
separately set up inR. pseudoacacia stand and its clearcut area in the western Liaoning Province (18°50’–122°25’ E, 40°24’–42°34’ N) for measuring the characteristics
of runoff and sediment as well as soil moisture dynamics. Contractive analysis of the two land types showed that there existed
a significant difference in volumes of runoff and sediment between the sites ofR. pseudoacacia stand and its clearcut area. The runoff volume and sediment volume in clearcut are were much bigger than those inR. pseudoacacia stand, with an increase amount of 40%–177% for runoff and 180%–400% for sediment. Hydrograph of surface runoff of typical
rainfall showed that the peak value of runoff inR. pseudoacacia stand was decreased by 1.0–2.5×10−3m3·s−1 compared with that in its clearcut area, and the occurring time of peak value of runoff inR. pseudoacacia stand was 10–20 min later than that in its clearcut area. Harmonic analysis of soil moisture dynamics indicated that the
soil moisture inR. pseudoacacia stand was 2.3% higher than that in clearcut area, and the soil moisture both inR. pseudoacia stand and its clearcut area could be divided into dry season and humid season and varied periodically with annual raifall
precipitation. It was concluded thatR. pseudoacacia stand plays a very important role in storing water, increasing soil moisture, and reducing surface runoff and soil erosion.
Foundation item: This paper was supported by Chinese “863” Plan Water-Saving Agriculture (2002AA2Z4321), the Key Knowledge Innovation Project
(SCXZY0103), and The “Tenth-five” Plan of Liaoning Province (2001212001).
Biography: GAO Peng (1967-), male, Dr. candidate, associate professor of Institute of Applied Ecology, Chinese Academy of Sciences,
Shenyang 110016, P. R. China.
Responsible editor: Song Funan 相似文献
20.
Dynamic viscoelastic properties of wood acetylated with acetic anhydride solution of glucose pentaacetate 总被引:1,自引:0,他引:1
Spruce wood specimens were acetylated with acetic anhydride (AA) solutions of glucose pentaacetate (GPA), and their viscoelastic
properties along the radial direction were compared to those of the untreated and the normally acetylated specimens at various
relative humidities and temperatures. Higher concentrations of the GPA/AA solution resulted in more swelling of wood when
GPA was introducted into the wood cell wall. At room temperature the dynamic Young's modulus (E′) of the acetylated wood was enhanced by 10% with the introduction of GPA, whereas its mechanical loss tangent (tan δ) remained almost unchanged. These changes were interpreted to be an antiplasticizing effect of the bulky GPA molecules in
the wood cell wall. On heating in the absence of moisture, the GPA-acetylated wood exhibited a marked drop in E′ and a clear tan δ peak above 150°C, whereas the E′ and tan δ of the untreated wood were relatively stable up to 200°C. The tan δ peak of the GPA-acetylated wood shifted to lower temperatures with increasing GPA content, and there was no tan δ peak due to the melting of GPA itself. Thus the marked thermal softening of the GPA-acetylated wood was attributed to the
softening of wood components plasticized with GPA.
Received: March 29, 2002 / Accepted: May 21, 2002
Correspondence to:E. Obataya 相似文献