共查询到20条相似文献，搜索用时 46 毫秒
1.
Hiroshi Yoshihara 《Journal of Wood Science》2001,47(1):8-12
The influence of the span/depth ratio when measuring the mode II fracture toughness of wood by endnotched flexure (ENF) tests was examined. Western hemlock (Tsuga heterophylla Sarg.) was used for the specimens. The ENF tests were conducted by varying the span/depth ratios; and the fracture toughness at the beginning of crack propagation GIIc was calculated by two equations that require the load-deflection compliance or Young's modulus. Additionally, the influence of the span/ depth ratio on the load-deflection compliance was analyzed by Timoshenko's bending theory in which additional deflection caused by the shearing force is taken into account. The following results were obtained: (1) When the span/depth ratio was small, the fracture toughness calculated with the data of load-deflection compliance was large. In contrast, the fracture toughness calculated with the equation containing Young's modulus tended to be constant. (2) In the small span/depth ratio range, the load-deflection compliance was estimated to be larger than that predicted by Timoshenko's bending theory. (3) To obtain the proper fracture toughness of wood with a single load-deflection relation, the span/depth ratio should be larger than that determined in several standards for the simple bending test method of wood, 12:16. 相似文献
2.
Thomas Goodrich Nadia Nawaz Stefanie Feih Brian Y. Lattimer Adrian P. Mouritz 《Journal of Wood Science》2010,56(6):437-443
This article presents an experimental study into thermal softening and thermal recovery of the compression strength properties
of structural balsa wood (Ochroma pyramidale). Balsa is a core material used in sandwich composite structures for applications where fire is an ever-present risk, such
as ships and buildings. This article investigates the thermal softening response of balsa with increasing temperature, and
the thermal recovery behavior when softened balsa is cooled following heating. Exposure to elevated temperatures was limited
to a short time (15 min), representative of a fire or postfire scenario. The compression strength of balsa decreased progressively
with increasing temperature from 20° to 250°C. The degradation rates in the strength properties over this temperature range
were similar in the axial and radial directions of the balsa grains. Thermogravimetric analysis revealed only small mass losses
(<2%) in this temperature range. Environmental scanning electron microscopy showed minor physical changes to the wood grain
structure from 190° to 250°C, with holes beginning to form in the cell wall at 250°C. The reduction in compression properties
is attributed mostly to thermal viscous softening of the hemicellulose and lignin in the cell walls. Post-heating tests revealed
that thermal softening up to 250°C is fully reversible when balsa is cooled to room temperature. When balsa is heated to 250°C
or higher, the post-heating strength properties are reduced significantly by decomposition processes of all wood constituents,
which irreversibly degrade the wood microstructure. This study revealed that the balsa core in sandwich composite structures
must remain below 200°–250°C when exposed to fire to avoid permanent heat damage. 相似文献
3.
Fracture toughness of wood and wood composites has traditionally been characterized by a stress intensity factor, an initiation strain energy release rate (G init) or a total energy to fracture (G f). These parameters provide incomplete fracture characterization for these materials because the toughness changes as the crack propagates. Thus, for materials such as wood, oriented strand board (OSB), plywood and laminated veneer lumber (LVL), it is essential to characterize the fracture properties during crack propagation by measuring a full crack resistant or R curve. This study used energy methods during crack propagation to measure full R curves and then compared the fracture properties of wood and various wood-based composites such as, OSB, LVL and plywood. The effect of exposure to elevated temperature on fracture properties of these materials was also studied. The steady-state energy release rate (G SS) of wood was lower than that of wood composites such as LVL, plywood and OSB. The resin in wood composites provides them with a higher fracture toughness compared to solid lumber. Depending upon the internal structure of the material, the mode of failure also varied. With exposure to elevated temperatures, G SS for all materials decreased while the failure mode remained the same. The scatter associated with conventional bond strength tests, such as internal bond and bond classification tests, renders any statistical comparison using those tests difficult. In contrast, fracture tests with R curve analysis may provide an improved tool for characterization of bond quality in wood composites. 相似文献
4.
Aurelio Guevara-Escobar Peter D. Kemp Alec D. Mackay John Hodgson 《Agroforestry Systems》2007,69(3):199-213
Traditionally, poplar (Populus spp.) have been planted to control erosion on New Zealand’s hill-slopes because of their capacity to dry out and bind together
the soil. Two systems: (1) widely spaced, planted poplar for soil conservation, and (2) non-eroded open pasture were compared
to determine the relative effect of the poplar–pasture system on the production, nutritive value and species composition of
the pasture, and on the water balance. Measurements were made at three sites with mature poplar (>29 years and 37–40 stems ha−1) and at a replicated experiment with young poplar (5 years, 50–100 stems ha−1). Soil water relations did not suggest strong competition for water between poplar and pasture. Pasture accumulation under
mature poplar was 40% less than in the open pasture, but under young poplar was similar to that in the open pasture. Chemical
composition of pasture suggested that feed quality of pasture in the open was better than under the poplar canopy, except
during spring, when most chemical components were similar. At the most, in vitro digestibility of pasture dry matter was 8.9%
lower and metabolisable energy of pasture dry matter was 1.5 MJ kg lower under the poplar canopy than in the open pasture.
Shade tolerant species were not dominant in the plant community under the poplar canopy with grasses such as browntop (Agrostis capillaris, L.) and ryegrass (Lolium perenne, L.) being a high proportion of the plant community. Differences in chemical composition were related to differences in the
botanical composition between the open pasture and the poplar understorey. It was concluded that the greatest effect of poplar
was on pasture production due to shading, and that management of this silvopastoral system needs to focus on control of the
tree canopy to lessen the decrease in pasture production. 相似文献
5.
Zephyr strand board (ZSB) and zephyr strand lumber (ZSL) were produced using zephyr made from poplar veneer to investigate the greater utilization of low-density poplar as a structural material. These materials were then compared to ordinary plywood, laminated veneer lumber (LVL) from poplar veneer, lauan plywood, and particleboard. The bending properties (moduli of rupture and elasticity) of ZSB proved superior to those of poplar plywood: and ZSL produced from poplar veneer zephyr had bending properties greater than ordinary LVL from poplar veneer. Apparently, the conversion of the poplar veneer into zephyr material had a positive effect on bending properties. Additionally, poplar ZSB had bending properties superior to those of lauan particleboard and equal to those of lauan plywood. The internal bond strength of poplar veneer ZSB was nearly two times greater than that of lauan particleboard.Parts of this report were presented at the international symposium on the utilization of fast-growing trees, Nanjing, China, October 1994. Report IV appeared inMokuzai Kogyo 49:599, 1994 相似文献
6.
The effect of tool angles on the shapes of chips generated by parallel-to-grain and end-grain milling was explored for China
fir and maple under fixed spindle and feed speeds and cutting depth. The milling path was up-milling by straight router-bits
with a diameter of 12 mm. The chip shapes could be distinguished as five types: spiral, splinter, flow, thin, and granules
or powder. The flow and thin chips were generated most often (on a weight percentage basis) for all tool angles investigated
for parallel-to-grain and end-grain milling of China fir and maple. More granule chips were produced with parallel-to-grain
milling than with end-grain milling for both woods. The measured chip thickness (t′) was thicker than the calculated thickness (t
max). Thicker and longer maple chips were produced by end-grain milling than by parallel-to-grain milling. The tool geometries
of 40°/15° (sharpness of the angle–rake angle), 50°/15°, and 60°/15° for China fir and 40°/25°, 50°/5°, and 60°/5° for maple
produced relatively more flow chips with parallel-to-grain milling. Furthermore, the tool geometries of 40°/5°, 50°/15° and
60°/25° produced more flow chips (weight percentage) by end-grain milling of China fir and maple.
Received: May 23, 2001 / Accepted: June 28, 2002
Acknowledgment This study was supported by a grant from the National Council of Science, Taiwan (NSC89-2313-B-415-011). 相似文献
7.
Christian Brischke Carola Hesse Linda Meyer-Veltrup Miha Humar 《Wood material science & engineering》2018,13(4):222-230
The overall aim of this study was to provide comprehensive durability characteristics of wood species underutilized but frequently occurring in Central and Northern Europe: Common juniper (Juniperus communis L.), Black cherry (Prunus serotina Ehrh.), English yew (Taxus baccata L.), and Rowan (Sorbus aucuparia L.). Decay resistance was tested against white and brown rot causing basidiomycetes and soft rot causing micro-fungi in terrestrial microcosms. Their wetting ability was determined in terms of capillary water uptake at the end-grain, the liquid water uptake during submersion, the water vapor uptake at high humidity, and the water release during drying. All tests were performed with unleached and leached specimens. Durability classes were assigned based on results from the different tests. Juniper and Yew were classified very durable (Durability class DC 1); Black cherry and Rowan were found to be less durable (DC 3–5). Leaching did not affect the durability classification significantly. Durability characteristics were completed with different indicators for the wetting ability of the four wood species. The combined effect of wetting ability and inherent decay resistance was considered for service life modeling based on a resistance model using dose–response relationships between material climate (dose) and fungal decay above ground (response). 相似文献
8.
《Wood material science & engineering》2013,8(2):90-95
Abstract In a previous study it was shown that the mechanical stability of an end-grain joint bonded with a one-component polyurethane adhesive (PUR) was insufficient compared with melamine–urea–formaldehyde and phenol–resorcinol–formaldehyde bonding. Based on this, the aim of this study was to improve the mechanical stability of the end-grain joint by means of a hydroxymethylated resorcinol (HMR) primer and by increasing the spreading quantity. To study the effect of HMR and the increased spreading quantity on the adhesive bond strength of end-grain to end-grain-bonded wood samples, three-part Norway spruce wood specimens were tested in tension. Before bonding, each end-grain surface was treated with an aqueous solution of HMR. The two axially orientated outer parts of the specimens were jointed with the middle part using a PUR adhesive. Compared with untreated, i.e. non-primed samples, the tensile strength of HMR-treated specimens was more than doubled. Furthermore, a positive effect of increased adhesive spread was shown for untreated PUR-bonded samples. An increase in adhesive spread by a factor of 1.6 led to an improvement in tensile strength by a factor of about 2.6. 相似文献
9.
Biomass, carbon and nitrogen dynamics of multi-species riparian buffers within an agricultural watershed in Iowa, USA 总被引：2，自引：0，他引：2
This study was conducted to determine biomass dynamics, carbon sequestration and plant nitrogen immobilization in multispecies riparian buffers, cool-season grass buffers and adjacent crop fields in central Iowa. The seven-year-old multispecies buffers were composed of poplar (Populus×euroamericana Eugenei) and switchgrass (Panicum virgatum L.). The cool-season grass buffers were dominated by non-native forage grasses (Bromus inermis Leysser., Phleum pratense L. and Poa pratensis L). Crop fields were under an annual corn-soybean rotation. Aboveground non-woody live and dead biomass were determined by direct harvests throughout two growing seasons. The dynamics of fine (0–2 mm) and small roots (2–5 mm) were assessed by sequentially collecting 35 cm deep, 5.4 cm diameter cores (125 cm deep cores in the second year) from April through November. Biomass of poplar trees was estimated using allometric equations developed by destructive sampling of trees. Poplar had the greatest aboveground live biomass, N and C pools, while switchgrass had the highest mean aboveground dead biomass, C and N pools. Over the two-year sampling period, live fine root biomass and root C and N in the riparian buffers were significantly greater than in crop fields. Growing-season mean biomass, C and N pools were greater in the multispecies buffer than in either of the crop fields or cool-season grass buffers. Rates of C accumulation in plant and litter biomass in the planted poplar and switchgrass stands averaged 2960 and 820 kg C ha–1 y–1, respectively. Nitrogen immobilization rates in the poplar stands and switchgrass sites averaged 37 and 16 kg N ha–1 y–1, respectively. Planted riparian buffers containing native perennial species therefore have the potential to sequester C from the atmosphere, and to immobilize N in biomass, therefore slowing or preventing N losses to the atmosphere and to ground and surface waters.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
10.
Hiroshi Yoshihara 《Journal of Wood Science》2003,49(3):210-215
The measurement method of mode II fracture toughness-crack propagation length relation (i.e., the resistance curve, or R-curve) was examined by end-notched flexure tests on sitka spruce (Picea sitchensis Carr.). The tests were conducted by varying the span/depth ratios under the constant loading point displacement condition.
The fracture toughness was measured from the load-crack shear displacement (CSD) and load-longitudinal strain relations. The
crack length was determined by a combination of load-CSD and load-strain compliances and Williams's end correction theory,
as well as the observation of crack propagation. When the specimen had an appropriate span/depth ratio, the fracture toughness
and crack propagation length were measured from the load-CSD compliance and combined load-CSD and load-strain compliances,
respectively, and the R-curve could be determined properly under the constant loading point displacement condition.
Received: March 15, 2002 / Accepted: July 25, 2002 相似文献
11.
Almudena Majano-Majano Mark Hughes Jose L. Fernandez-Cabo 《Wood Science and Technology》2012,46(1-3):5-21
The fracture toughness of thermally modified beech (Fagus sylvatica L) and ash (Fraxinus excelsior L) wood under Mode I loading was quantified using Compact Tension (CT) specimens, loaded under steady-state crack propagation conditions. The influence of three heat-treatment levels and three moisture contents, as well as two crack propagation systems (RL and TL) was studied. Complete load–displacement records were analysed, and the initial slope, k init, critical stress intensity factor, K Ic, and specific fracture energy, G f, evaluated. In the case of both species, thermal modification was found to be significantly affect the material behaviour; the more severe the thermal treatment, the lower the values of K Ic and G f, with less difference being observed between the most severe treatments. Moisture content was also found to influence fracture toughness, but had a much less significant effect than the heat treatment. 相似文献
12.
13.
Xiang Cun-ti Dong Ai-rong Liu Xue-feng Li Chun Yuan Shu-zhong Zhang Jing-hua 《林业研究》2001,12(3):157-164
The isolation, culture and the active determination of poplar ice nucleation active (INA) bacteria and the inoculation tests
in laboratory and field were conducted, and the varieties, distribution and number of poplar INA bacteria and its pathogenicity
and freezing injury property were determined. The study results showed that the INA bacteria widely spread on poplar in Northeast
China and caused the frozen injury for poplar under the frost condition in Spring or Autumn, which was the key factor to induce
INA bacterial canker. Through evaluation and investigation of different poplar varieties and inoculation tests, fine disease-resistant
varieties and strains of poplar suitable for Northeast China were selected. Further tests for strong seedling showed that
burying cuttings in sand and covering with plastic film could effectively avoid the frostbite, frozen and drought damage,
reduce INA bacteria infection, and promote poplar growth. INA bacterial canker was detected early by highly specialized antiserums
of INA bacteria and the agglutinated test of ring-shaped boundary surface. The inducers such as streptomycin, phenylmercuric
acetae, salicylic acid and heat-killed bacteria to immerse cuttings, have obvious induced disease-resistant effect. Before
poplar sprouted in early spring, through spraying the solution of frostbite agent, the control effect also was obvious.
Foundation item: This paper was supported by National Foundation of Ninth Five-Year Plan (No.96-005-04-01-03).
Biography: XIANG Cun-ti (1933-), male, professor in Collage of Forest Resources and Environment of Northeast Forestry University, Harbin
150040, P.R. China.
Responsible editor: Zhu Hong 相似文献
14.
We examined the applicability of end-notched flexure (ENF) tests for measuring the mode II fracture toughness of wood. Western hemlock (Tsuga heterophylla Sarg.) was used for the specimens. The fracture toughness at the beginning of crack propagationG
IIc and that during crack propagationG
IIR were calculated from the loadloading point compliance and load-crack shear displacement (CSD) relations. The obtained results were compared with each other, and the validity of measurement methods were examined. The results are summarized as follows: (1) The value ofG
IIc increased with the increase in initial crack length. When measuringG
IIc by ENF tests, we should be aware of the dependence ofG
IIc on the initial crack length. (2) The value ofG
IIR initially increased with the crack length, and it reached a constant value. (3) Measurement of the CSD is recommended when obtainingG
IIR because the crack length, which has a great influence on theG
IIR calculation, is implicitly included in the CSD. (4) We found that the crack length during its propagation should be evaluated by the final crack length. 相似文献
15.
竹片覆面胶合板的初步研究 总被引：2，自引：0，他引：2
本文研究了以竹片为外层材料、多层杨木单板为芯层材料的复合胶合板的结构和力学性能，分析了板坯结构形式、纵向纵片厚度、单板层数及板坯压缩率与产品机械强度之间的关系。初步研究结果表明，板坯结构形式对产品的静载荷抗弯曲性能影响显著；在试验范围内，纵向竹片厚度为３．５－５．０ｍｍ、板坯压缩率在２３％左右时，竹片覆盖面杨木胶合板的综合力学性能比较理想。 相似文献
16.
The relation between crack propagation based on fracture mechanics and end-check propagation during drying was evaluated in
this study. Corresponding to the direction of end-check propagation, the mode I fracture toughness of air-dried sugi specimens
in TR, TL, and intermediate systems was examined by single-edge-notched bending tests. The occurrence and propagation of end
checks on sugi (Cryptomeria japonica D. Don) blocks during drying were observed at the scale of the annual rings. It was found that the critical stress intensity
factor (K
IC) decreased as the crack propagation changed from TL to TR. The value of K
IC in the TR system was significantly lower than that in the TL system. As a measure of fracture energy, the area under the
load-crack opening displacement curve in the TR system was more than twice that in the TL and intermediate systems. These
results indicate that cracks perpendicular to the tangential direction initiate radially with ease, and then crack arrest
occurs to prevent growing. This finding provides a consistent interpretation of the end-check propagation observed during
drying as follows: tiny end checks, as an analog of TR cracks, occur easily and selectively in latewood or transition wood
and propagate toward the pith during drying. When there is no corresponding secondary check in the forward latewood, the checks
are arrested and do not propagate further. 相似文献
17.
The effect of different machining processes on surface roughness and on adhesive tensile strength of end-grain-bonded spruce
wood specimens was studied. Surfaces that had been cut with two different circular saws containing 48 and 96 teeth were compared
with those that had been further processed by smoothing with a microtome, machine planing, or sanding. Two different adhesives
and two different spreading quantities were used to join the test specimens by their end-grain surfaces. Increasing tensile
strength of the bonded specimens was observed with increased surface roughness, which was ascribed to an enlarged bonding
area in the case of circular-sawn samples with a rough surface. On the other hand, more pronounced starving of the bond line
and thus decreased bond strength was observed in the more open cells of the smoothed end-grain surfaces. A positive effect
regarding tensile strength was further observed with increased spreading quantity of the adhesives. Machining was found to
particularly affect earlywood tracheids, whereas surface roughness of latewood tracheids was comparable for the differently
treated end-grain surfaces 相似文献
18.
转基因杨树对美国白蛾幼虫中肠保护酶系统的影响(英文) 总被引：10，自引：0，他引：10
以转Bt基因欧洲黑杨(P.nigra L.)和转CpTI基因毛白杨(Populus tomentosa)叶片饲喂4-5龄美国白蛾（Hyphantria cunea Drury）幼虫，对其体内保护酶系统活性进行测定。结果表明，饲喂两种转基因杨树叶片的幼虫中肠保护酶表现出相似的变化规律，SOD、CAT和POD三种酶的活性在饲喂后数小时内逐渐增加，某一时刻达到最高值，此后突然下降。饲喂转Bt基因杨树叶片的幼虫，其中肠SOD、CAT活性峰值出现在饲喂后的24小时，POD活性峰值出现在饲喂后的12小时；饲喂转CpTI基因杨树叶片的幼虫，其中肠三种保护酶活性高峰出现时间均较前者滞后12小时。本文还比较了饲喂两种转基因叶片不同中毒程度的美国白蛾幼虫体内保护酶活性，发现不论饲喂那种转基因叶片，中毒较轻者其体内保护酶活性显著高于中毒较重者，这种差异在饲喂CpTI叶片的处理株表现尤为明显。 相似文献
19.
[目的]以无人机高清影像为数据源,结合样地实地调查数据,研究杨树冠幅提取及其与胸径和林分蓄积量的相关性,为无人机森林调查技术提供一种思路和方法。[方法]基于无人机高分影像及实地调查数据,采用面向对象法,对杨树林木冠幅进行分割与提取,通过实地测量数据建立冠幅-胸径模型,利用一元材积表计算样地蓄积量,并进行相关性分析与精度检验。[结果]影像分割效果良好,但提取得到的冠幅比实际值偏小,研究区最适宜的杨树冠幅分割尺度为10,平滑度0.1,紧致度0.5。杨树冠幅与胸径建立相关模型,其中一元线性方程拟合效果最好,相关系数为0.75。通过模型计算的样地蓄积与实测样地蓄积进行双侧T检验,结果 sig=0.0580.05,两组数据差异不显著。[结论]采用面向对象法,通过无人机高分影像能自动分割并提取了杨树林木冠幅信息,提取效果良好;利用影像提取林木平均冠幅,通过冠幅-胸径相关关系模型得到林木胸径,进而推算林分蓄积的方法可以满足森林资源调查精度要求。 相似文献
20.
Summary Fine structure of cellulose microfibrils in poplar (Populus euramericana) gelatious layer and Valonia cell walls was observed in the electron microscope by using disintegration and ultrathin section techniques with various electron stains. Staining of the gelatinous layer in poplar showed that the microfibrils had a paracrystalline region surrounding a crystalline core, but such a region was negligible in Valonia. From the facts that kinks and shortened microfibrils were observed after mechanical and hydrolysis treatments, respectively, and that the microfibrils were unaffected by negative staining, it was concluded that the microfibril core is uniformly crystalline in the longitudinal direction although there may be some crystalline dislocations.The authors are indebted to Dr. Y. Yokohama (Marine Biological Laboratory, Tokyo University of Education) for supplying the samples and to Mr. Y. Tachida (Sanyo-Kokusaku Pulp Co.) for the technical assistance of the glow discharge apparatus. 相似文献