共查询到20条相似文献，搜索用时 46 毫秒
1.
介绍了抛物面天线整体旋压成型的工艺过程，针对旋压过程铝板拼接焊缝开裂的原因进行了初步分析。并对氩弧焊接工艺控制过程的经验进行了总结。 相似文献
2.
Mojgan Vaziri Sven Berg Dick Sandberg Iman Tavakoli Gheinani 《Wood material science & engineering》2014,9(2):102-109
Finite element numerical analyses were performed to determine three-dimensional heat flux generated by friction to wood pieces during linear friction welding. The objective was to develop a computational model to explain the thermal behaviour of welded wood material rather than experimental methods, which are usually expensive and time consuming. This model serves as a prediction tool for welding parameters, leading to optimal thermo-mechanical performance of welded joints. The energy produced by the friction welding of small wood specimens of Scots pine (Pinus sylvestris L.) was determined by thermocouples and used as input data in the model. The model is based on anisotropic elasticity and the thermal properties were modelled as isotropic. This numerical simulation gave information on the distribution of the temperature in the welding interface during the entire welding process. A good agreement between the simulation and experimental results showed the appropriateness of the model for planning welded wood manufacture and prediction of thermal behaviour of wood during other mechanically induced vibration processes. The specimens presented in this model required a heat flux of 11 kW/m2 to achieve a satisfactory welding joint. 相似文献
3.
This study examined the temperature distribution during rotation welding process using birch (Betula spp.) wood dowel and Chinese larch (Larix gmelinii) substrates. Wood dowels were divided into two categories including an untreated group and a group pretreated with cupric chloride. The mechanics test results indicated that the pullout resistance of the pretreated group with welded time 3 s showed the best performance. As a fitting analyses result, both the untreated group and pretreated group showed a significant nonlinear relationship among temperature, welded depth and welded time. In the untreated group case, a linear regression relationship was found between the highest temperature of the welding interface and the depth. However, two-stage fitting was used to fit the regression for the pretreated group. Compared with the untreated group, thermogravimetric (TG) analysis of the pretreated group welding interface presented two pyrolytic peaks, and it illustrated that the pretreatment promoted the depolymerization and pyrolysis of wood constituents. 相似文献
4.
Till Vallée Thomas Tannert Christelle Ganne-Chedville 《Wood Science and Technology》2012,46(1-3):333-347
Linear vibration welding of timber structural elements provides new opportunities to potentially achieve structural joints. This paper investigates to which extent welded joints can be considered for load-bearing structural joints. On the basis of a series of experimental and numerical investigations on a series of welded single-lap joints, failure modes were identified, and the associated failure criterion was quantified. A probabilistic method subsequently allowed accurately predicting the capacity of the tested wood welded joints exclusively based on objective input data, including an estimate of the scattering due to the material’s inherent variability. 相似文献
5.
B. Belleville T. Stevanovic A. Cloutier A. Pizzi A. Salenikovich P. Blanchet 《Wood Science and Technology》2013,47(5):1005-1018
This study examines the suitability of wood welding technology for producing composite panels for furniture applications with two Canadian hardwood species, sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis). For each species, twelve 30 × 225 × 300 mm3 panels were manufactured using a panelling machine specifically designed for rotational wood-dowel welding with optimized parameters obtained from a previous study. Six edge-glued panels of the same size were manufactured from each species using a non-structural polyvinyl acetate adhesive and tested for comparative purposes. The experimental programme included three-point bending at 255-mm span and visual inspection of the panels to assess performance at standard moisture conditions and after an ageing cycle with variable relative humidity. Average breaking load of 1.79 and 1.70 kN was obtained at standard moisture conditions for welded panels of yellow birch and sugar maple, respectively. Fractures consistently occurred in the dowel’s cross section, whereas no slippage was observed along the welded interface. Delamination between wood slats occurred after the ageing cycle, but did not affect the bending properties. Results confirm the suitability of wood-dowel welding for producing furniture panelling with Canadian hardwood species. Further research is needed to design panels with a more efficient position and use of welded dowels and with panel product properties that are comparable or superior to those of glued counterparts. 相似文献
6.
C. Ganne-Chedeville J.M. Leban M. Properzi F. Pichelin A. Pizzi 《Wood Science and Technology》2006,40(1):72-76
Nondestructive evaluation of wood joints welded by linear vibration welding was performed with an infrared (IR) thermography
technique, which allowed the maximal and average peak temperature profile/distribution to be measured. The density profile/distribution
at the joint interface was measured by X-ray microdensitometry. The results show that the width of the welded zone also varies
as a function of the maximum temperature reached during welding and that the maximum temperature reached at the ends of the
specimens is lower than that obtained in the central part of the specimens.
This paper is dedicated to Prof. Dr. Dr. h.c. mult. Walter Liese on the occasion of his 80th birthday. 相似文献
7.
End-grain-to-end-grain welding gave butt joints of good strength with three high density Australian eucalyptus woods, namely
Sidney blue gum (Eucalyptus saligna), Spotted gum (Eucalyptus maculata, Corymbia maculata spp.) and Black butt (Eucalyptus
pilularis). The absence or limited tendency to defibration in end-grain-to-end-grain welding for wood densities as high as these, around
800–900 kg/m3, indicates that end-grain-to-end-grain welding is possible and yields good joint strengths, but with different appearance
of the welded interphase. At anatomical level, three features were noticeable: (1) the paintbrush-like appearance of the cell
tips bent so by the frictional movement during welding. This leaves this cell tips free to intertwine with the same one from
the surface of the opposite wood piece. (2) The absence of any great amount of molten material between the straight shafts
of the parallel bundles of cells, showing that the greater part of the intercellular material has molten and has flowed away
from the cells towards the interphase. (3) The amount of molten intercellular material found on the upper surface of the cell
tips, giving the joint the appearance of an anatomical fingerjoint. 相似文献
8.
W. B. Banks 《Wood Science and Technology》1973,7(4):271-284
Summary Water in contact with wood surfaces is able to penetrate into the cellular structure by three routes: 1. As liquid water into cell lumena, by capillarity. — 2. As water vapour, by diffusion into cell lumena. — 3. As bound water, by diffusion within the cell wall.Transport from cell lumena into adjacent cell walls occurs rapidly by diffusion. In Scots pine (Pinus sylvestris) sapwood, it is shown that transport over short distances occurs much more rapidly by capillarity than by either of the diffusion processes. Treatment of wood with resin/wax water repellent formulations greatly reduces the rate of water flow due to capillarity and hence significantly cuts down the rate of dimensional change of specimens exposed to wet conditions.Swelling rates due to vapour phase and bound water diffusion were measured experimentally, and these data were used to predict the water sorption rates for specimens treated with a theoretically perfect water repellent, viz. that which excludes all liquid water.It was found experimentally that specimens immersed in water, after treatment with resin/wax water repellents, swelled more rapidly than predicted by the above procedure. This more rapid swelling is probably due to a certain amount of liquid water flow made possible by displacement of the hydrophobic film from cell wall surfaces (preferential wetting). It is suggested that the use of hydrophobic agents bonded chemically to the cell wall may be necessary to attain optimum water repellent effectiveness. 相似文献
9.
Qing-ming wu Hong-fei ZOU Jian-zhang MA 《林业研究》2014,25(4):947-952
White-naped crane （Grus vipio） is a globally threatened spe- cies. It is very important to analyze its nest site selection in circum- stances where there are multiple disturbances, and also helpful to accu- mulate valuable information about this threatened species and supply scientific suggestions for conservation and management. We studied nest site selection and the effects of environmental variables on nesting habits of white-naped crane at Zhalong National Nature Reserve, Qiqihar City, Heilongiiang, China, during March-May of 2002-2008. White-naped crane responded and adapted to changes in the quality of the spatial environments of landscape and microhabitat under multiple environ- mental disturbances. Nest site selection included two scales and two choices, namely the choice of nest site habitat type within the macro-habitat scale and nest site micro-habitat selection within the mi- cro-habitat scale. Nest sites were recorded only in reed marshes. The choice of nest site micro-habitat included three basic elements and six factors, namely incubation element （nest parameters factor, incubation temperature factor and incubation humidity factor）, safety element （pro- tection factor and concealment factor）, and food element （water factor）. Water, remnant reed clusters, and fire were major resource management challenges during the breeding period for the white-naped crane in this Reserve. 相似文献
10.
The rejuvenation ecology of three main tree species in anthropogenic pine (Pinus sylvestris L.) forests is explored in our study. We focus on the scale of micro-plots, which provide the safe sites for tree rejuvenation. We thrive on the multi-factorial relationship of tree establishment and driving ecological factors using a large dataset from pine stands in NE Germany and applying multivariate analyses. The success of the establishment of the investigated focal tree species Fagus sylvatica L., Quercus petraea Liebl. and Pinus sylvestris L. is, on general, mostly affected by three factors, i.e. water balance of the upper soil layers, browsing pressure, and diaspore sources. Our investigations on the micro-plot scale revealed species-specific differences. For beech saplings <50 cm growth height, primarily the availability of water, indicated by available water capacity (AWC), thickness, quality, and structure of the organic layer, silt and humus content in the topsoil, and the lack of a dense competitive herb layer, were identified as most important factors. On the contrary, oak seems hardly be restricted by hydrologic and/or trophic deficits in the topsoil or humus layer. In conclusion and comparison to Fagus sylvatica L., we assume for Quercus petraea Liebl. advantages in natural regeneration processes under sub-continental climate conditions and thus under the scenarios of climate change. Pinus sylvestris L. regeneration in our investigation area occurs only in a narrow niche. We conclude with regard to future forest development and the objective of stand conversion with low management intensity that oak should be favoured within natural stand regeneration. 相似文献
11.
Summary Sorption isotherms of ammonia were measured on cellulosic materials, such as beech and birch wood, as well as on cotton cellulose, the object being to obtain information on the nature of interaction between the wood and the ammonia by means of application of various sorption theories. As a result several analogies between the sorption of ammonia vapour and water vapour could be observed.—The isotherms displayed the typical S-shape and developed a hysteresis along the adsorption and desorption lines. The sorbate films reached a thickness of 4–6 molecular layers. The lowering of enthalpy of the ammonia vapour appeared to be the propelling force of the sorption process, as in the case with the uptake of water vapour.—In contrast to water isotherms, however, ammonia isotherms cannot be reproduced with the same specimen. Each sorption cycle brought a loss of substance and a reduction of the fibre saturation capacities of the adsorbent. In ammonia vapour the fibre saturation points were found at sorbate concentrations twice as high as in water vapour. The hysteresis between the adsorption and desorption processes also appeared with ammonia vapour; however, the continuous changes of the adsorbent, caused by chemical interaction with ammonia, produced heavy displacements. The specific surface area of the samples in ammonia was approximately twice the size of that in water, but the extensions varied greatly during the sorption cycles.These investigations have been supported by the Zentenarfonds of the Swiss Federal Institute of Technology. 相似文献
12.
钢结构梁柱节点破坏原因的探讨 总被引：2，自引：2，他引：0
从焊接质量、塑性铰范围、梁翼缘应力分布3个方面阐述钢结构梁柱节点脆性破坏的形态特征，并分析了钢结构梁柱节点发生脆性破坏的原因，且从节点连接强度和采用3种新型节点两个方面简要阐述了保证节点强度的措施，为钢结构梁柱设计与研究提供参考。 相似文献
13.
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15.
Xudong Zhu Songlin Yi Ying Gao Jirong Zhang Chun Ni Xiangya Luo 《Journal of Wood Science》2017,63(5):445-454
This study examined the influence of welded depth and CuCl2 pretreated dowels on wood dowel welding. In untreated group without pretreatment (group A), test results indicated that welded depth 40 mm exhibited higher pullout resistance than the other welded depths. In the same welded depth of 30 mm, specimens with dowels immersed in CuCl2 solution for 30 min (group B) exhibited the highest pullout resistance than the other specimens. According to the failure behavior, the pullout resistance of group B was considered to be the maximum theory pullout resistance in the welded depth of 30 mm. Weibull distribution could be applied reasonably to analyze pullout resistance of different welded depth. The linear simulation and Eckelman formula could not fit the relation of pullout resistance and welded depth. While the nonlinear simulation of sine function could fit the relation accurately. Based on the Weibull distribution, 95% reliability pullout resistance was calculated. The nonlinear simulation of sine function also existed between 95% reliability pullout resistance and welded depth. The temperature difference of group A-30 and group B was tested to study the reason of different pullout resistance. Both of the two groups, the temperature of point 1 was the highest, and the point 3 was the lowest. The pullout resistance was affected significantly by the temperature of point 2 and 3. For point 1, 260 °C was an excessive temperature, while 224.3 °C was the better choice for welding in this study. 相似文献
16.
The aim of this study was to evaluate the chemical composition and the dynamic water vapour sorption properties of Eucalyptus pellita wood thermally modified in vacuum. For this purpose, wood samples were thermally modified in a vacuum oven at 160–240 °C for 4 h. Chemical composition were investigated by wet chemical analysis, elemental analysis, as well as Fourier transform infrared (FTIR) analysis, and dynamic water vapour sorption properties were evaluated by dynamic vapour sorption apparatus. The results showed that holocellulose and alpha-cellulose contents decreased and lignin and extractives contents relatively increased during the heat process. Elemental analysis showed a reduction in hydrogen content and an increase in carbon content. FTIR analysis indicated that the degradation of hemicellulose and condensation reactions of lignin occurred. In addition, the thermo-vacuum resulted in a reduction in the equilibrium moisture content of wood during the adsorption or desorption process. And the sorption hysteresis had a decreasing trend with increasing treatment temperature. The development of the hygroscopicity was related to the increase in the relative content of lignin, the degradation of the carbonyl groups in xylan and the loss of carbonyl group linked to the aromatic skeleton in lignin after heat treatment. 相似文献
17.
《Forest Ecology and Management》1997,95(1):53-61
Water use by eucalypts has received a lot of attention in tropical countries during the past decade because of the large-scale introduction of these trees for afforestation. Eucalyptus grandis, widely used as a plantation item in tropical southern India, is the subject of a detailed ecophysiological study in this paper. A 4-year-old coppiced plantation was used for measurements. Microclimate data collected above the canopy were used along with stomatal conductance measurements to estimate the transpirational water loss by the Penman-Monteith equation assuming a two-layer canopy model. Leaf photosynthesis was measured diurnally and seasonally to understand the limitations in photosynthesis in the field. Results show that the water loss from the plantation ranges between 2.5 and 6.5 mm day−1 depending on the season. When suitably extrapolated, this amounts to 1181 mm annually in the study location, where annual rainfall averages 1302 mm. The stomatal conductance measurements showed that the increase in atmospheric vapour pressure deficit induced stomatal closure. This was probably regulated by the leaf water potentials also. Based on the above results it is concluded that E. grandis need not be a high water consumer because of its good stomatal control of transpirational water loss, especially during the dry season when the atmospheric vapour pressure deficit is high. The photosynthesis measurements led to a conclusion that the dry period experienced in the study location does not seriously affect the photosynthetic rate of the trees on a leaf unit area basis. 相似文献
18.
Preparation of active carbons from a commercial holm-oak charcoal: study of micro- and meso-porosity 总被引：1，自引：0，他引：1
We studied the influence of the degree of gasification and the choice of activating agent (carbon dioxide, water vapour, or both carbon dioxide and water vapour acting successively) on the activation of samples of a commercial holm-oak wood (Quercus rotundifolia) charcoal. To this end, we prepared the active carbon samples using the activating agents at 800, 850, 900, and 950°C for the time required to gasify 20, 40, or 60% of the mass of the charcoal at the moment when the set gasification temperature had been reached. The active carbons were characterised by physical gas adsorption and densimetry. Those prepared with carbon dioxide or water vapour alone had textural characteristics that were better than those of the precursor charcoal. The micropore volume was greater in the samples activated with carbon dioxide than with water vapour. The activation with both carbon dioxide and water vapour successively led to a major increase in porosity, taking into account that these samples presented a 40% burn-off percentage which endowed them with good textural characteristics. In general, as the burn-off percentage increased, so did the micropore and mesopore volumes.To sum up, holm-oak wood is a good raw material, not only to get barbecue coal, which has been used as a precursor to obtain activated coal, but it also allows the activated coal to develop its microporosity and mesoporosity in a good way, which is suitable for new applications as it is absorbent in liquid phase, gas absorbent, is a constituent part of combustible batteries, etc.The main interest of this research is the preparation of activated coal and the determination of the size pore distribution obtained, given its great influence in the quality of the activated coal obtained starting from holm-oak wood, what gives a great economic and industrial value in the Southwest of Spain for this raw material. 相似文献
19.
《Scandinavian Journal of Forest Research》2012,27(1-4):316-322
Previous investigations have shown different growth and root/shoot ratio increases of beech seedlings (Fagus sylvatica) with increased light intensity. In the present investigation both light intensity and soil water content were regulated on four levels in a factorial experiment. At the highest level of soil water content the seedling growth increased linearly with light intensity while the root/shoot ratio decreased with light intensity. At low levels of soil water content both the growth and root/shoot ratio slightly increased with increased light. The soil water content was also affected by the light intensity indicating that the soil water content may have caused unobserved effects in responding to increased light in previous investigations. It is concluded that both light intensity and soil water content should be quantified or manipulated even if only the effect of one of the two growth factors are investigated. 相似文献
20.
The physiological responses to water deficits of Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) were studied under Mediterranean mountain climate. Minimum leaf water potentials were ?3.2 MPa for oak and ?2.1 MPa for pine, with higher predawn values for pubescent oak. Relative sap flow declined in both species when vapour pressure deficit (D) went above ca. 1.2 kPa, but stomatal control was stronger for pine during the 2003 summer drought. P. sylvestris plant hydraulic conductance on a half-total leaf area basis (k L,s?1) was 1.2–2.6 times higher than the values shown by Q. pubescens, and it showed a considerably steeper decrease during summer. Leaf-level gas exchange was positively related to k L,s?1 in both species. Scots pine was more vulnerable to xylem embolism and closed stomata to prevent substantial conductivity losses. The results of this study confirm that pubescent oak is more resistant to extreme drought events. 相似文献