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1.
To investigate the fracture behavior of wood, the porosity and heterogeneities of its microstructure should be taken into
account. Considering these features of wood microstructure in a continuum-based model is still a difficult problem and the
lattice model might be an alternative. In the developed mixed lattice-continuum model, the probable crack propagation volume
was modeled by defining a three-dimensional lattice of different beam elements and the other regions were considered as continuum
medium. Different beam elements of lattice represented the earlywood fibers, latewood fibers, ray cells and bonding medium
between the fibers. The proposed model was used to investigate the mechanism of mode I fracture in a small notched wood specimen
in RL orientation. The resulting pre-peak and softening curve and also the crack opening trajectory in both cross-section
and longitudinal-section in model were in good agreement with the experimental observations. This model shows the importance
of considering the three-dimensional and distributed propagation of microcracks and main cracks in fracture stability. It
was also shown that in mode I fracture, RL orientation, the main crack propagates in the earlywood ring. 相似文献
2.
In the process of wood drying inevitable stresses are induced. This often leads to checking and undesired deformations that
may greatly affect the quality of the dried product. The purpose of this study was to propose a new rheological model representation
capable to predict the evolution of stresses and deformations in wood cantilever as applied to wood drying. The rheological
model considers wood shrinkage, instantaneous stress–strain relationships, time induced creep, and mechano-sorptive creep.
The constitutive law is based on an elasto–viscoplastic model that takes into account the moisture content gradient in wood,
the effect of external load, and a threshold viscoplastic (permanent) strain which is dependent on stress level and time.
The model was implemented into a numerical program that computes stresses and strains of wood cantilever under constant load
for various moisture content conditions. The results indicate that linear and nonlinear creep behavior of wood cantilever
under various load levels can be simulated using only one Kelvin element model in combination with a threshold-type viscoplastic
element. The proposed rheological model was first developed for the identification of model parameters from cantilever creep
tests, but it can be easily used to simulate drying stresses of a piece of wood subjected to no external load. It can therefore
predict the stress reversal phenomenon, residual stresses and maximum stress through thickness during a typical drying process. 相似文献
3.
Biogeochemical process models are increasingly employed to simulate current and future forest dynamics, but most simulate only a single canopy type. This limitation means that mixed stands, canopy succession and understory dynamics cannot be modeled, severe handicaps in many forests. The goals of this study were to develop a version of Biome-BGC that supported multiple, interacting vegetation types, and to assess its performance and limitations by comparing modeled results to published data from a 150-year boreal black spruce (Picea mariana (Mill.) BSP) chronosequence in northern Manitoba, Canada. Model data structures and logic were modified to support an arbitrary number of interacting vegetation types; an explicit height calculation was necessary to prioritize radiation and precipitation interception. Two vegetation types, evergreen needle-leaf and deciduous broadleaf, were modeled based on site-specific meteorological and physiological data. The new version of Biome-BGC reliably simulated observed changes in leaf area, net primary production and carbon stocks, and should be useful for modeling the dynamics of mixed-species stands and ecological succession. We discuss the strengths and limitations of Biome-BGC for this application, and note areas in which further work is necessary for reliable simulation of boreal biogeochemical cycling at a landscape scale. 相似文献
4.
Thomas Perot François Goreaud Christian Ginisty Jean-François Dhôte 《Annals of Forest Science》2010,67(5):502-502
5.
Three softwood samples and one hardwood sample were tested under a tension load applied along the radial direction using small
clear specimens and the local tension strain was measured using the digital image correlation method. We successfully obtained
a stress-strain curve with a strain-softening branch by calculating the stress using the strain distributions in the vicinity
where the specimen ruptured. The continuous digital imaging of the specimen proved to be very effective for measuring the
strain in quasi-brittle materials such as wood under tension. The nonlinearity of the stress-strain curve was quantified using
two parameters representing the deviation from linear elasticity, and the formula of the stress-strain curve was deduced from
the interrelation between these parameters. This formula is expressed quite simply by using the modulus of elasticity along
the radial direction and another constant that is unique to the material.
Part of this article was presented at the 56th Annual Meeting of the Japan Wood Research Society, Akita, Japan, August 2006 相似文献
6.
Aynalem Dilla Philip J. Smethurst Karen Barry David Parsons Mekuria Denboba 《Agroforestry Systems》2018,92(6):1699-1709
A number of agroforestry models have been developed to simulate growth outcomes based on the interactions between components of agroforestry systems. A major component of this interaction is the impact of shade from trees on crop growth and yield. Capability in the agricultural production systems simulator (APSIM) model to simulate the impacts of shading on crop performance could be particularly useful, as the model is already widely used to simulate agricultural crop production. To quantify and simulate the impacts of shading on maize performance without trees, a field experiment was conducted at Melkassa Agricultural Research Centre, Ethiopia. The treatments contained three levels of shading intensity that reduced incident radiation by 0 (control), 50 and 75% using shade cloth. Data from a similar field experiment at Machakos Research Station, Kenya, with 0, 25 and 50% shading were also used for simulation. APSIM adequately simulated maize grain yield (r2 = 0.97) and total above-ground biomass (r2 = 0.95) in the control and in the 50% treatments at Melkassa, and likewise in the control (r2 = 0.99), 25% (r2 = 0.90) and 50% (r2 = 0.98) treatments at Machakos. Similarly, APSIM effectively predicted Leaf Area Index attained at the flowering (r2 = 0.90) and maturity (r2 = 0.94) stages. However, APSIM under-estimated maize biomass and yield at 75% shading. In conclusion, the model can be reliably employed to simulate maize productivity in agroforestry systems with up to 50% shading, but caution is required at higher levels of shading. 相似文献
7.
8.
A simulation model of the flight activity ofCydia funebrana was constructed on the basis of data on male captures in pheromone traps placed in a plum orchard in Central Bohemia in 1988 to 1990. The model was verified from 1991 to 1993 as a method of predicting the course of the emergence flight activity of particular generations in dependence on degree day values. The nonlinear model based on Richards' transformation was more suitable for the simulation than the linear one based on probit transformation. A technique has been proposed to use the model ofC. funebrana flight activity for optimal timing of ovicide applications based on fenoxycarb and diflubenzuron in systems of integrated pest management in plum orchards.With 3 figures and 4 tables 相似文献
9.
Majid Azizi 《林业研究》2008,19(4):323-328
The objective of this research is to design a method for an appropriate alternative to supply poplar wood as raw material for wood and paper factories in Iran.Due to the destruction of the forests and lack of proper plantation,replacement of the forest wood by the fast growing wood is vital to satisfy all requirements of these factories.A hierarchy was used to prioritize benefits,opportunities,costs and risks (BOCR) using the Analytic Hierarchy Process (AHP) ratings approach.To evaluate the "control criteria" of the system,a control hierarchy was also created and prioritized by applying the Analytic Network Process (ANP).A total of four major control criteria in the system are prioritized where each one controls a network structure evaluated by using ANP.The final synthesis results of the system showed that internal poplar tree farming supplied by the forest product factories was the best choice among three potential alternatives (factory procurement,external procurement and mix procurement). 相似文献
10.
Summary Aim of this study is to determine the basic equations concerning the second boundary value problem of the theory of elasticity in particular for construction wood. For rhombic orthotropic materials, as represented for example by straight grained wooden constructions, the equations are calculated in Cartesian co-ordinates. For cylindrical orthotropic materials, as represented for example by curved laminated beams, the equations are calculated in cylindrical co-ordinates. In both cases the equations are determined for space stress problems. Assuming the plain stress condition the basic equations of plain stress problems can then be derived from the spatial equations. 相似文献
11.
以湖南省林业科学院院内林荫道杜英修剪枝材、试验林场板栗林间伐、修剪产生的低值材为原料,利用香菇、木耳、灵芝等腐生性食药用菌菌种之不同菌型就地接种上述常规作业剩余物,测定各菌种在三种基质上的生长和产菇情况,结果表明:1用于原位接种林荫道杜英修剪枝、板栗林修剪枝及林间非目的树种泡桐的菌种可选毛木耳、香菇、野生平菇、糙皮侧耳等食用菌品种。2不同菌种不同菌型应用效果有差异。香菇宜选用子弹头菌型,其萌动时间、菌丝布满段木时间和子实体平均发生时间最短,分别为17.2天、45.6天和80.4天,第一批生物学转化率最高,为37.49%。毛木耳、野生平菇可用牙签菌型,子实体平均发生时间分别为85.8天、53.1天,第一批生物学转化率分别为39.58%、32.40%。以树皮封口的方式,牙签菌型的污染率最低。3利用食用菌原位接种板栗枝可达到板栗修枝的目的,节约人工,提高效益。 相似文献
12.
Summary Wood-cement panels have commanded renewed interest during the last decade because of their potential application in the building industry. Several methods to classify wood of various species regarding its compatibility with cement have been established in the literature based on hydration measurements in Dewar flasks. These ranking methods lack consistency in the classification of species because the hydration conditions vary among laboratories. Three techniques for evaluating wood-cement compatibility were established and compared. The best technique is based on a wood-cement compatibility factor which is the ratio of the area under the wood-cement hydration heat rate curve to that of neat cement. The area is calculated on 24-h basis starting from the initial cement set time. This area ratio method ranks species over a 100% scale and accounts well for species that are totally incompatible.This research was supported in part by the U.S. Agency for International Development under contract number 608-0160 administered by the Hassan II Institute of Agronomy and Veterinary Sciences, Morocco, and the University of Minnesota, USA 相似文献
13.
H. Yamamoto 《Wood Science and Technology》1999,33(4):311-325
Summary The anisotropic mechanism of wood swelling and shrinkage was investigated theoretically. The reinforced-matrix hypothesis
offered by Barber and Meylan (1964) was reformulated by using a multi-layered wood fiber model, and the formula describing
the dynamical behavior of swelling and shrinking wood fiber was derived. For modelling, the moisture content changes were
taken into consideration as an explicit parameter. It is expected to predict the anisotropic swelling and shrinking process
of wood dynamically and quantitatively, as well as to initiate elucidating the interaction between the moisture and cell wall
components on the basis of the present model. Some concrete calculations will be demonstrated in the following report, and
the results will be compared with the experimental ones.
Received 2 September 1997 相似文献
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16.
Manuel Raul Pelaez-Samaniego Vikram Yadama Eini Lowell Raul Espinoza-Herrera 《Wood Science and Technology》2013,47(6):1285-1319
The objective of this paper is to review the published literature on improving properties of wood composites through thermal pretreatment of wood. Thermal pretreatment has been conducted in moist environments using hot water or steam at temperatures up to 180 and 230 °C, respectively, or in dry environments using inert gases at temperatures up to 240 °C. In these conditions, hemicelluloses are removed, crystallinity index of cellulose is increased, and cellulose degree of polymerization is reduced, while lignin is not considerably affected. Thermally modified wood has been used to manufacture wood–plastic composites, particleboard, oriented strand board, binderless panels, fiberboard, waferboard, and flakeboard. Thermal pretreatment considerably reduced water absorption and thickness swelling of wood composites, which has been attributed mainly to the removal of hemicelluloses. Mechanical properties have been increased or sometimes reduced, depending on the product and the conditions of the pretreatment. Thermal pretreatment has also shown to improve the resistance of composites to decay. 相似文献
17.
Juan Shi Youqing Luo Xiaosu Yan Weiping Chen Ping Jiang 《Frontiers of Forestry in China》2009,4(1):117-122
Pine wood nematode (PWN, Bursaphelenchus xylophilus), originating from North America, causes destructive pine wilt disease. Different pine forest ecosystems have different resistances
to B. xylophilus, and after its invasion, the resilience and restoration direction of different ecosystems also varies. In this study, an
interpretative structural model was applied for analyzing the response of pine forest ecosystem to PWN disturbance. The result
showed that a five-degree multi-stage hierarchical system affected the response of the pine forest ecosystem to PWN disturbance,
in which direct affecting factors are resistance and resilience. Furthermore, the analysis to the 2nd, 3rd and 4th degree
factors showed that not only does distribution pattern of plant species and pine’s ecological features affect the resistance
of pine forests’ ecosystem, but removal of attacked trees and other measures also influence the resistance through indirectly
affecting the damage degree of Monochamus alternatus and distribution pattern of plant species. As for resilience, it is influenced directly by soil factors, hydrology, surrounding
species provenance and biological characteristics of the second and jointly dominant species, and the climate factors can
also have a direct or indirect effect on it by affecting the above factors. Among the fifth elements, the elevation, gradient
and slope direction, topographical factors, diversity of geographical location and improvement of prevention technology all
influence the response of pine forest ecosystem to PWN disturbance.
__________
Translated from Scientia Silvae Sinicae, 2007, 43(8): 85–90 [译自: 林业科学] 相似文献
18.
We investigated the relationship between the crystal lattice strain and the piezoelectric response in Japanese cypress (Chamaecyparis obtusa Endl.) wood fibers subjected to tension stress in the fiber direction. As a result, the piezoelectric voltage was very sensitive to the mechanical behavior (deformation) of the wood crystalline regions obtained from the x-ray stress measurement. Thus, by investigating the behavior of piezoelectric voltage, it was possible to simply estimate the mechanical behavior of the crystalline regions in the wood. 相似文献
19.
John F. Siau 《Wood Science and Technology》1976,10(2):149-153
The longitudinal unsteady-state flow of compressible fluids in wood is modeled on a basis of series elements of differing permeabilities and three zones acting in parallel. Two of these zones having widely differing permeabilities are capable of varying degrees of intermixing while a third low-permeability zone is isolated from the other two. A good agreement between theoretical and experimental results was achieved by the appropriate selection of the parameters.This research was supported by the National Science Foundation, Grant No. ENG 69-0056-A03. 相似文献
20.
To elucidate the origin of the shrinking anisotropy of wood during the drying process, as well as to begin to gain an understanding
of the interaction between the moisture and the cell wall components, the shrinking process of a single wood fiber regarding
water desorption was simulated by using an analytical model which was developed in the previous report (Part 1). Resulting
data were compared with the experimental ones in this paper. The following conclusions were obtained: (1) The matrix substance,
as a skeleton in the secondary wall, tends to shrink isotropically. However, the cellulose microfibrils, as a rigid framework
of the cell wall, almost did not shrink at all due to the water desorption. As result, wood shrinks anisotropically during
a drying process. The microfibril angle in the S2 layer is one of the most important factors related to the degree of shrinking
anisotropy of the wood while drying. (2) According to the simulation, the expansive strain caused in the matrix skeleton by
the water sorption increases by 15% (= 150,000 micro-strains) from the oven-dried condition to the green condition. Based
on this value, the moisture content at the fiber saturation point is calculated to be about 35%, which is close to the experimentally
obtained one. These results give quantitative evidences that the hygroexpansion of the wood cell wall is controlled by the
mechanism of the reinforced matrix hypothesis.
Received: 28 July 1998 相似文献