FRP增强结构用集成材木梁胶合性能研究

FRP增强结构用集成材木梁可有效地提升梁体的抗弯强度,FRP与结构用集成材的复合首先要解决的是FRP与木材的胶合问题。本试验对比等离子体处理及羟甲基间苯二酚HMR改性处理工艺对FRP木材界面胶合性能的影响,并用胶层剥离率、剪切强度对胶合性能加以表征。试验结果表明,FRP增强结构用集成材木梁的胶合界面优化处理工艺为：FRP表面不处理,木材表面预先以2.5 m/min的送料速度进行射频功率为400 W的等离子体处理5次,然后以150 g/m2的涂胶量涂布HMR。试材的浸渍剥离率、煮沸剥离率、干剪强度、湿剪强度及木破率均满足结构用集成材国家标准GB/T 26899-2011中关于使用环境3的要求。     

不同预应力CFRP增强结构用集成材抗弯性能研究

通过对CFRP施加不同的预应力,分析在不同预应力下CFRP增强结构用集成材的抗弯性能。结果显示:随着预应力的增大,CFRP增强结构用集成材的抗弯性能得到显著提高。在50%~66%预应力施加范围内,CFRP增强结构用集成材较未增强结构用集成材,极限荷载提高了15.8%~51.8%,抗弯强度提高了38.5%~81.7%,抗弯弹性模量提高了23.9%~70.5%。对CFRP施加预应力使集成材木梁的弯曲破坏形式由脆性拉伸破坏转变为延性压缩破坏,并且使集成材木梁跨中挠度过大的现象得到改善。同时也使集成材木梁的中性轴位置向下偏移,从而使木材的抗压能力得到了充分利用。     

小径落叶松层板等级区分和集成材强度性能的研究

根据抗弯弹性模量、节径比、纹理斜度、年轮宽度和裂纹、腐朽、变色等木材缺陷,确定小径落叶松层板等级。结构集成材具有较大的规格尺寸、较高的强度和耐久性等优点,它的强度取决于层板强度和配置以及胶合性能。     

竹质工程材料的力学性能试验研究

重组竹与竹集成材是两种应用较为广泛的竹质工程材料,对其力学性能的研究有助于扩展其在工程上的应用。本研究对重组竹和竹集成材进行了抗弯和抗压力学性能试验,分别得到了它们的抗弯弹性模量、抗弯强度、抗压弹性模量和抗压强度,并分析了两种竹质工程材料的不同破坏情况。试验结果表明:重组竹的抗弯和抗压力学性能要优于竹集成材;重组竹抗弯破坏表现为脆性破坏,竹集成材则表现出一定的延性特性;竹材的力学性能与其本身材料性能和胶合面的力学性能有关。     

落叶松结构用胶合木层板指接工艺参数对力学性能的影响

以兴安落叶松刨光锯材制作胶合木指接层板,探讨指榫参数、指接压力和不同规格尺寸对锯材指接层板抗弯性能的影响。结果表明:指榫工艺参数对指接层板抗弯弹性模量影响较小,而对抗弯强度影响较大。当选用12.7、19mm和25mm指榫长度及其相应的指接工艺参数时,层板抗弯强度平均值和5%分位值均能达到国家标准GB/T 26899-2011《结构用集成材》目测分等Ⅰ_d等级和机械分等M_E14等级规定值,对于中小截面胶合木层板,尽可能考虑选用12.7~19mm范围内较小的指榫长度进行指接加工以利节材。在指接过程中应选用合适的指接压力,指接压力过大或过小都会对指接层板抗弯性能尤其强度造成不利影响,指接层板抗弯强度的尺寸效应与非指接锯材相似。     

《木结构试验方法标准》中U型挠度架的设计与应用

GB/T50329-2002《木结构试验方法标准》,对木结构梁的抗弯试验及剪切模量测试用U型挠度架装置,仅提出刚度要求,未涉及材料、构造等内容.笔者针对胶合木梁的抗弯性能测试,设计两种尺寸的U型挠度架,并将其应用于木梁抗弯性能测试.根据试验结果,阐述挠度架的材料选择、结构设计以及应用要点.     

落叶松结构集成材承载性能足尺检测与特性评价

针对我国尚无结构集成材足尺检测基础数据的现状,重点开展足尺材料测试和评价研究。研究以普遍应用的落叶松集成材为对象,按GB/T26899-2011《结构用集成材》国家标准对3种国际通用规格的结构集成材进行足尺承载特性测试。依据检测结果,按美国ASTM D2915-10《结构用木材和木质产品抽样和数据分析标准》规定的非参数法和参数法统计分析方式,确立了落叶松集成材抗弯弹性模量(MOE)和抗弯强度(MOR)的特征值数据及正态分布、对数分布和威布尔分布下的拟合优度;确定了各规格结构集成材MOE、MOR的变化趋势。针对不同组别之间MOE及MOR的尺寸效应,利用回归分析方法确立了MOE和MOR的关系;初步确定了结构用集成材的特征值和承载特性的评价方式,为国产树种结构集成材的设计和应用提供了重要的理论支撑。     

高温预处理对足尺胶合木梁力学性能的影响

【目的】研究高温预处理对足尺胶合木梁力学性能的影响,明确高温热改性和环境湿度对木材平衡含水率、木材顺纹抗剪强度和顺纹抗拉强度的影响规律,揭示高温预处理对胶合木梁抗弯性能影响的作用机制,为高温热改性技术在木结构领域中的应用提供参考。【方法】以高应力等级的兴安落叶松为研究对象,采用工业化热处理技术对落叶松木材进行高温热改性,以高温热改性后的落叶松木材为层板,制备12个足尺胶合木梁。基于EN 408标准四点弯曲方法,分析高温热改性和环境湿度对胶合木梁抗弯弹性模量、抗弯强度、破坏模式、跨中截面荷载-应变曲线和跨中截面应变分布规律等抗弯性能的影响。【结果】高温热改性会在一定程度上降低胶合木梁的抗弯强度,但可明显提高高湿度条件下胶合木梁的抗弯弹性模量,与90%环境湿度下未处理胶合木梁相比,高温热改性后,同湿度下胶合木梁的抗弯强度降低29. 79%,抗弯弹性模量提高23. 71%;高温热改性可降低胶合木梁抗弯弹性模量对环境湿度的敏感性,环境湿度从60%提高到90%,未处理胶合木梁的抗弯弹性模量降低23. 27%,经高温热改性预处理的胶合木梁抗弯弹性模量降低7. 55%; 60%和90%环境湿度下的荷载-位移曲线和跨中截面应变分布曲线表明,胶合木梁在高湿环境中具有更明显的非线性特性,高温预处理后的胶合木梁表现为线弹性。环境湿度对胶合木梁的抗弯性能具有较为明显的劣化作用,90%湿度下胶合木梁抗弯强度和抗弯弹性模量分别为43. 98 MPa和12. 191 GPa,比60%湿度下未处理胶合木梁低17. 07%和23. 27%;环境湿度对木材平衡含水率影响明显,高温热改性是降低木材平衡含水率的有效措施,环境湿度从60%提高到90%,高温热改性处理后落叶松木材平衡含水率分别从10. 74%和20. 62%降至4. 76%和11. 18%。【结论】60%和90%环境湿度条件下,未处理胶合木梁为拉伸破坏,高温热改性构件为拉剪联合破坏,高温热改性后材料的顺纹抗剪强度和顺纹抗拉强度降低是材料破坏模式改变的根本原因。高温热改性后胶合木梁在高湿环境条件下抗弯弹性模量明显改善。     

榫头局部正交层板结构对直榫节点受力性能的影响

【目的】针对直榫节点受弯时榫头横纹变形较大、刚度和承载力较弱的问题,将"正交胶合木"概念引入直榫节点中,探讨榫头局部正交层板结构对直榫节点受力性能的影响,并提出合理的直榫节点"预增强"方法和参数为传统榫卯节点在现代木结构工程中的应用提供理论与技术支持。【方法】以花旗松普通层板胶合木为材料,设计制作18个足尺直榫节点试件,其中部分试件榫头不作处理(对照组),其他试件榫头加工成正交层板结构。通过节点抗弯试验分析直榫节点的破坏模式和工作曲线,计算得到节点的抗弯刚度和极限弯矩;提出复杂应力状态下的木材本构关系模型,建立直榫节点有限元模型,对其抗弯性能进行模拟。与试验结果对比并验证模型准确性后,基于模型对局部正交层板结构直榫节点进行参数分析,探讨正交层板厚度、宽度等参数对直榫节点受力性能的影响规律。【结果】对照组直榫节点主要破坏模式为榫头端部顶面和根部底面的局部横纹压缩变形,局部正交层板结构可有效缓解榫头横纹压缩变形。与对照组相比,4组不同局部正交层板结构直榫节点的抗弯刚度平均值提升14.0%～36.9%,而极限弯矩有升有降;数值分析结果与试验结果吻合良好;随正交层板宽度增加,直榫节点的抗弯刚度先升后降,而承载力先降后升;随正交层板厚度增加,直榫节点的抗弯刚度呈上升趋势,而承载力呈下降趋势;对本研究直榫节点,建议正交层板厚度与榫头厚度比值取0.25,宽度取210 mm。【结论】局部正交层板结构可提高直榫节点的抗弯刚度,通过合理选择参数还可提高直榫节点的承载力;采用强度和韧性较好的材料,如重组竹和钢板等替代木材层板,可进一步提高直榫节点的抗弯刚度和承载力。     

GFRP增强竹木复合结构用集成材木梁胶合性能研究

通过对比等离子体处理及羟甲基间苯二酚(HMR)改性等胶合界面处理工艺对GFRP/竹、竹/木界面胶合性能的影响,并用胶层剥离率、剪切强度对界面胶合性能加以表征,以期得出胶合界面的优化改性处理工艺。试验结果表明,GFRP增强竹木复合结构用集成材木梁的胶合界面优化处理工艺为:GFRP表面不作处理;木材与竹材表面分别预先以2.5、1.25m/min的送料速度进行5次射频功率为400W的等离子体处理,随即以150g/m~2的涂布量涂布HMR。试材的浸渍剥离率、煮沸剥离率、干剪强度、湿剪强度及木破率均满足结构用集成材国家标准GB/T 2689%—2011中关于使用环境3的要求。     

Growth of Phlebiopsis gigantea in wood of seven conifer species

Heterobasidion parviporum and Heterobasidion annosum are widely distributed root‐rot fungi that infect conifers throughout Europe. Infection of conifer stumps by spores of these pathogens can be controlled by treating fresh stumps with a competing non‐pathogenic fungus, Phlebiopsis gigantea. In this study, growth of three Latvian strains of P. gigantea and the biological control agent ‘Rotstop’ strain was evaluated in stem pieces of Norway spruce, Scots pine, lodgepole pine, Douglas‐fir, Weymouth pine, Siberian larch and Sitka spruce. The growth rates of one H. parviporum and one H. annosum isolate were also measured in the same stem pieces. The growth rate of P. gigantea varied greatly in wood of different conifer species. It was higher in the three pine species, lower in Norway spruce and lowest in Sitka spruce and Siberian larch, and in Douglas‐fir, this fungus did not grow. The largest area of wood occupied by P. gigantea was in lodgepole pine. Growth of Latvian isolates of P. gigantea in the wood of Pinus and Picea species was comparable to that of the Rotstop isolate. Consequently, stump treatment with local P. gigantea isolates should be recommended. However, our results suggest that Douglas‐fir stump treatment against Heterobasidion by P. gigantea may be ineffective and other stump treatment methods should be considered.     

Growth of Mixed Birch-Coniferous Stands in Relation to Pure Coniferous Stands at Similar Sites in South-eastern Norway

Mean age, mean and top heights and yield were studied in 20 mixed stands of birch ( Betula pubescens Ehrh. and B. pendula Roth) and Picea abies (L.) Karst. and nine mixed stands of birch and Pinus sylvestris L. in south-eastern Norway. Each mixed stand and the adjacent pure coniferous stand (control) were growing under the same site conditions and had not been commercially thinned. There were no significant differences in mean age at breast height or in top heights between birch and conifers in the mixed stands, while mean height was significantly higher for birch than for spruce. A growth index was calculated based on total volume and age at breast height. For the spruce sites the growth in young mixed stands (birch < 17 m) was superior to that of pure spruce, while the difference was insignificant in older stands. The growth index correlated positively with the ratio between generatively and vegetatively regenerated birch trees, and negatively with the age of the oldest species in the mixture and with site quality. For the pine sites there was no significant difference in the growth index between mixed birch-pine and pure pine stands. A mixture effect of birch on the volume yield of spruce or pine could not be demonstrated.     

Defoliation by the Needle-shortening Pine Gall Midge,Thecodiplosis brachyntera,on Pines in Central Sweden

An outbreak of the needle-shortening pine gall midge, Thecodiplosis brachyntera , occurred on Scots pine, Pinus sylvestris (L.) and lodgepole pine, P. contorta (Dougl.) in central Sweden during 1995-1998. The larval feeding of T. brachyntera kills current-year needles and heavy infestation causes severe defoliation. A pattern of fluctuating defoliation among years and sites was found on both pine species. Defoliation was evenly distributed in the crown of P. sylvestris during years of high as well as low infestation; apical whorls had as much defoliation as basal whorls. In P. contorta , however, there was a tendency for defoliation to be concentrated on branches in the most apical whorls. Growth ring increments on P. sylvestris with high levels of defoliation (71%) did not differ from trees with moderate levels (26%) of defoliation. Shoot lengths were significantly shorter, however, on heavily infested trees the year after defoliation.     

Decomposition and nutrient loss from prunings of different contour hedgerow species in tea plantations in the sloping highlands of Sri Lanka

Contour hedgerows of multipurpose tree species in the sloping tea lands of Sri Lanka are expected to reduce soil erosion and also add significant amounts of plant nutrients to the soil via periodic prunings. The objective of this experiment was to characterize the biomass decomposition pattern and quantify the amount of nutrients added through prunings of six tree species (Calliandra calothyrsus, Senna spectabilis, Euphatorium innulifolium, Flemingia congesta, Gliricidia sepium and Tithonia diversifolia) currently being used in hedgerows associated with tea. Withered leaf and stem prunings (50 g) were enclosed in 2-mm litter bags, placed at 5-cm depth and retrieved after one, three, six, nine and 12 weeks. Loss of initial dry weight, N, P and K was measured. Single exponential decay function adequately described both dry weight and nutrient loss. Tree species differed significantly in their rate of breakdown with decomposition constants (k) varying from 0.0299 to 0.2006 week⁻¹ for leaves and from 0.0225 to 0.0633 week⁻¹ for stems. Gliricidia showed the highest k for leaves with the rest in the following descending order: Senna > Tithonia ≥ Euphatorium > Calliandra > Flemingia. A similar pattern was observed for loss of all nutrients with Calliandra and Flemingia always having lower k values than the rest. Although N immobilization was not observed, immobilization of P and K was observed during the first week of incubation in some species, particularly in stem prunings. Annual biomass of prunings differed significantly between tree species in the following descending order: Calliandra > Senna > Flemingia > Tithonia > Gliricidia > Euphatorium. Calliandra added the greatest amount of nutrients annually to the soil with Euphatorium adding the least. Calliandra prunings provided the annual total K requirement and 49% of the N requirement of mature tea. However, none of the species provided more than 5% of the P requirement. It is concluded that among the tree species tested, Calliandra and Flemingia are the most suitable for contour hedgerows in tea plantations of this agroclimatic region because of their higher soil nutrient enrichment capacity and slower decomposition rates which would minimize leaching losses. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of Seed Weight and Seed Type on Early Seedling Growth of Pinus sylvestris under Harsh and Optimal Conditions

The effects of seed weight and seed type on seedling growth of Pinus sylvestris (L.) were studied by seeding individually weighed orchard and stand seed in different mixtures under harsh (direct seeding in field) and optimal (seeding in nursery) conditions. In the nursery experiment an increase in the seed weight from 3 to 7 mg increased the seedling height by 10-27% and total weight by 27-113%, and decreased the height/diameter ratio by 5-6% after 2 yrs. With elimination of seed weight effects, orchard seedlings were 2% taller than stand seedlings in year 2. Without elimination of seed weight effects, orchard seedlings were 7-13% taller. In the field experiment an increase in the seed weight from 3 to 7 mg increased seedling height by 18-65%, stem volume by 81-274% and the number of top-buds by 23-34% in year 5. After elimination of seed weight effects, orchard seedlings were 7-13% taller than stand seedlings and without elimination of seed weight effects 20-21% taller after 5 yrs. Even after elimination of both seed weight and genetic effects orchard seedlings were 3-9% larger than stand seedlings in the field experiment. In conclusion, the influence of seed weight and seed type on growth traits and slenderness is highly significant and the influence seems to be greater in harsh conditions.     

Modelling of adequate pretwist for obtaining straight timber

Abstract

Wood in general and wooden studs in particular are often distorted owing to uneven shrinkage during the drying process in the sawmill. Twist is often the most detrimental of all types of distortion, and it is caused by spiral grain in combination with variations in moisture content. For sawmills, the objective is to produce dried, straight boards, and one method of dealing with boards with excessive spiral grain is to sort them out and then dry them in a pretwisted position to obtain straight boards after drying. A model using the finite element (FE) method for the simulation of drying twist distortions was first calibrated against laboratory experiments in which boards were dried with and without restraints and pretwists. After the calibration, the FE results were compared with industrial test results for boards that were dried without restraints or with restraints with zero pretwist, i.e. straight restraints. The FE model used an elastic–ideally plastic material model to obtain permanent deformations. The calibration was to set the yield stresses so that there was a good match between FE results and results from the laboratory experiments. The comparison between the industrial test results and the FE results showed that the FE model is capable of realistic simulations of drying boards with and without restraints and presumably also pretwists.     

Potential to harness superior nutritional qualities of exotic baobabs if local adaptation can be conferred through grafting

Baobab leaves form an important part of the local diet in Sahel countries and elsewhere in Africa. Existing leaf nutritional data and agroforestry performance information are based solely on Adansonia digitata L., the baobab of continental Africa. The introduction potential of Adansonia species from the center of diversity in Madagascar and from Australia remains untapped. To assess this potential, the mineral contents and B₁ and B₂ vitamin levels of dried baobab leaves were determined for five-year old trees of A. digitata, A. gibbosa (A. Cunn.) Guymer ex D. Baum, A. rubrostipa Jum. & H. Perrier (syn. A. fony Baill.), A. perrieri Capuron and A. za Baill. grown in an introduction trial in Mali. Nutritional data were evaluated against survival and vigor to identify promising germplasm. Leaf vitamin and crude protein contents were highest in the Madagascar species, especially A. rubrostipa (B₁ 88 mg 100 g⁻¹, B₂ 187 mg 100 g⁻¹, protein 20.7% dry weight). However, the local species far outperformed the introductions in survival, tree height, basal diameter and resistance to termites. We suggest grafting as a way of harnessing the vigor of well-adapted local baobab varieties to the superior nutritional profiles of A. rubrostipa and others. Cross-species grafting tests in Adansonia were successful, thus creating new agroforestry possibilities with different scion/rootstock combinations.     

Boron in forest trees and forest ecosystems

This review critically examines the role of boron (B) in forests in view of recent findings on B nutrition and the continuing occurrence of B deficiency. Many perceptions about the role of B in plants and its uptake and mobility have been altered since the last review on B in forest trees in 1990. Now there is evidence for a fundamental role of B in the formation of the pectic structure in primary cell walls in plants, and further roles in membrane function are being explored. In plants, channel-mediated B uptake, active B uptake and B uptake by mycorrhizas have been shown, B transporters have been identified, and B retranslocation has been shown. We explore these findings and their consequences on forest trees and on ecosystems that they dominate. Particular emphasis is placed on B retranslocation and B in mycorrhizal symbiosis, given their importance in trees.     

Survival and Growth of Picea abies Regeneration after Shelterwood Removal with Single- and Double-grip Harvester Systems

Three stands in Sweden were followed for 7 yrs to study the survival and growth of Norway spruce [Picea abies (L.) Karst.] regeneration after shelterwood removal with single- and double-grip harvester systems. The height of all seedlings within circular plots was recorded before and after final cutting. The tallest seedling and one randomly selected seedling on each circular plot were permanently marked and their annual height growth was measured each year during the period 1993-1999. No significant differences between single- and double-grip harvester systems were found with respect to their effects on regeneration density, survival or annual height growth. The annual height growth of the individual seedlings after shelterwood removal was significantly correlated with seedling height before shelterwood removal. Significant differences were found in annual height growth between seedlings with high and low vitality. Logging damage did not affect the annual height growth. Several practical conclusions were drawn. First, the choice of harvester system does not affect seedling height growth or survival after shelterwood removal. Secondly, the removal of a 200 m³ shelterwood with 200 stems ha^-1 does not seem to constitute a serious problem for seedling survival and growth. Finally, estimated vitality and seedling height both seem to be fairly good predictors of postrelease survival and growth.     

Development of Pinus sylvestris Main Stems Following Three Different Precommercial Thinning Methods in a Mixed Stand

Reducing competition by point cleaning may be an attractive alternative to conventional precommercial thinning in forestry. In this study, survival and development of main stems of Scots pine (Pinus sylvestris L.) following point cleaning were examined in a mixed stand of mainly Scots pine and birch (Betula pendula Roth and Betula pubescens Ehrh.) in northern Sweden. Treatments studied were: no precommercial thinning, conventional precommercial thinning, point cleaning of all secondary stems, and point cleaning of only the secondary stems that were higher than the main stem, within a radius of 0.8 m from the main stem, respectively. A randomised complete block design with three replicates was used. Measurements taken 13 growing seasons after these treatments showed that conventional precommercial thinning was the best treatment for retaining Scots pines as main stems. However, conventional precommercial thinning resulted in the poorest timber quality of the remaining Scots pines in terms of stem taper, thickest branch diameter and relative live crown height. In conclusion, point cleaning has to be followed by a complementary cleaning operation if the objective is to retain Scots pines as main stems in such mixed stands.