高湿状态下单板层积材的横纹承压性能

重组加工是提高低强度木材结构性能的重要手段,以杨木重组加工的单板层积材为研究对象,从受力方式、承压方向及增强方法等方面研究了高湿状态下单板层积材的全表面、局部表面及尽端局部表面横纹承压性能。结果表明：湿环境对杨木单板层积材横纹承压性能影响较大,湿环境下其全表面横纹承压性能为干环境下的33%;杨木单板层积材横纹承压性能主要与其承压位置与承压方向有关,局部横纹承压时受到承压面周围木材纤维的支持作用显著,承压面平行于单板层时其局部承压强度最高,在干燥环境下,分别是尽端局部及全表面横纹承压强度的1.2倍和1.4倍。单板层积材结构是导致不同承压方向横纹承压性能差异的主要原因,承压面垂直于单板层时,单板层积材易过早发生分层或屈曲破坏。湿环境下单板层积材含水率较高材质较软,自攻螺钉支持作用明显,有助于改善其承压性能。杨木单板层积材横纹承压强度对环境湿度较为敏感,在工程应用时应保持环境干燥并使承压面平行于单板层。     

正交胶合木销槽承压性能研究进展北大核心

对正交胶合木(CLT)销槽承压性能研究进行了系统回顾。归纳总结了木材销槽承压性能测试标准,分析比较了不同测试方法的优缺点,梳理了销槽承压性能影响因子,并对现有CLT销槽承压性能计算公式进行整理。研究表明:CLT销槽承压性能与木材销槽承压性能有较大区别,除木材销槽承压的影响因素外,层板材料和组坯结构、销类连接件打入和加载方向、横向层厚度占比等对CLT销槽承压性能也有很大影响。建议针对主要因素进行深入研究,建立统一的CLT销槽承压性能理论预测公式,并对采用国内材种研发的CLT销槽承压性能展开系统研究。     

加拿大铁杉正交胶合木弯曲性能预测与评估

为了对正交胶合木(CLT)的弯曲性能开展有效预测与评估,验证CLT板件弯曲性能预测值的可靠性,采用自由梁横向振动法,对加拿大铁杉锯材顺纹弹性模量进行动态测试与质量分等;采用悬臂板横向振动法动态测试铁杉锯材的横纹剪切模量和弹性模量;将分等后的铁杉锯材制成足尺CLT楼板、墙板用板材,并采用四点弯曲法静态测试CLT板材的弹性模量值;然后将锯材顺纹弹性模量、横纹剪切模量和弹性模量实测值代入模型公式进行CLT板弯曲性能预测,并与CLT板材静态弹性模量实测值作比较分析。结果表明:CLT主强度方向弹性模量预测值为11 222 MPa,实测值为11 312 MPa,CLT板材的弯曲弹性模量的预测值与其实测值决定系数为0.887,具有较强的相关性;用CLT板材的预测值模型公式评估其弯曲性能指标,具有应用上的可行性和可靠性;铁杉制成的CLT板材弯曲性能达到CLT楼板、墙板等工程用的等级要求。     

ESWood复合木材料物理力学性能试验研究

为了解ESWood复合木作为结构用材的物理力学性能,对其进行木材物理力学性能试验并对各项性能进行评价。结果表明:ESWood复合木密度为0.725 g/cm~3、含水率为18.5%,顺纹销槽承压强度30.73 MPa、横纹销槽承压强度18.29 MPa,试验结果准确指数均在5%以下。顺纹抗拉强度71.61 MPa、顺纹抗压强度53.05 MPa、横纹全部抗压强度7.91 MPa、横纹部分抗压强度12.29 MPa、横纹抗拉强度1.53 MPa,试验结果准确指数均在10%以下。通过与木结构设计规范中TB13等级材料设计强度对比,可知ESWood复合木的木材性能及连接性能良好。     

正交胶合木(CLT)性能测试及其分析

简述了国内首条大幅面CLT生产线制造的铁杉正交胶合木(CLT)的工艺过程,并主要对该CLT板主强度方向的弹性模量、抗弯强度,以及胶层剪切强度、层间剪切强度和浸渍剥离等性能进行了测试与分析。表明:铁杉CLT的抗弯性能和抗剪性能测试值均能达到加拿大ANSI APA PRG320—2012标准相关等级要求;CLT垂直层滚动剪切强度是决定CLT抗弯强度和界面层剪切强度的主要因素;真空—加压浸渍处理对胶结面附近木材力学性能的影响大于对胶层的影响,其浸渍剥离率的平均值为9.75%,单组份PUR胶黏剂的胶层剪切强度和耐候性等性能符合室外环境的使用要求;铁杉CLT的抗弯性能和抗剪性能测试值达到加拿大ANSI APA PRG320—2012标准相关等级要求,满足工程要求。     

锯材/单板层积材混合正交胶合木力学性能研究

为改善正交胶合木(Cross laminated lumber,CLT)的力学性能,将加拿大云杉-松-冷杉规格材和单板层积材按照不同结构进行混合组坯,形成3种不同结构的混合结构CLT,并对CLT试件进行弯曲和剪切力学性能测试,观察试件破坏形式。试验结果表明,芯层(横向层)剪切破坏是CLT梁试件弯曲破坏的主要、关键形式,单板层积材(Laminated Veneer Lumber,LVL)显示了较低的横纹剪切性能;将LVL置于表层,能较大地改善CLT材料的力学性能,其顺纹抗弯弹性模量比普通结构CLT值提高17.6%,将LVL置于芯层(横向层),SPF作为表层形成的混合CLT材料力学性能低于普通结构CLT性能。     

自攻螺钉在竹木混合正交胶合木中抗拔性能研究

混合正交胶合木（hybrid cross-laminated timber, HCLT）可以充分发挥不同层板材料特性,改善CLT物理力学性能。研究采用重组竹和速生杉木（Cunninghamia lanceolata）,以不同组坯结构制备3类CLT：重组竹CLT、竹木混合CLT和杉木CLT,评价和比较不同直径和钉入深度自攻螺钉（self-tapping screws,STS）在三类CLT中的抗拔性能。结果表明,随着STS钉入深度或直径增加,STS在3类CLT抗拔承载力都增加,且对于重组竹CLT和竹木混合CLT,有利影响程度随着钉入直径或深度增加而增加。STS钉入深度或直径变化对其在3类CLT中抗拔强度影响规律不一致。STS在重组竹CLT和竹木混合CLT的抗拔强度明显大于速生杉木CLT,且对于竹木混合CLT,有利影响程度又随STS钉入深度增加而降低,随STS直径增加而增加。STS在竹木混合CLT中的抗拔性能,受到STS钉入深度、直径和CLT组坯结构等因素多重影响,后续应建立预测模型,量化和优化STS在竹木混合CLT中的抗拔性能。     

不同胶黏剂对南方松CLT胶合性能的影响

为探索不同胶黏剂用于正交胶合木(CLT)生产的可能性,研究采用来自不同厂家的5种胶黏剂制备南方松CLT,以浸渍剥离率、剪切强度和木破率评价南方松CLT的胶合性能。结果表明:仅A-API和B-PUR1胶黏剂制造的南方松CLT胶层耐久性达到标准要求,木破率均未达到标准要求;5种胶黏剂对南方松CLT的浸渍剥离率、剪切强度和木破率有显著影响(P<0.005);从胶层耐久性、剪切性能和生产成本考虑,宜用A-PUR和A-API制造南方松CLT。     

云南松正交胶合木胶合工艺探索

为探索云南松用于正交胶合木(CLT)制造的技术可行性,以剪切强度、木破率和剥离率研究了不同胶黏剂、施胶量和压力对云南松CLT胶合性能的影响。结果表明:云南松的物理力学性能和胶合性能达到北美ANSI/APA PRG 320-2019标准要求,云南松可用于CLT制造。不同胶黏剂、施胶量和压力对云南松CLT胶合性能影响显著(P0.05)。随着施胶量增加,云南松CLT胶层剪切性能提高;随着压力增加,云南松CLT胶层剪切性能降低。从胶合性能和生产成本综合考虑,制造云南松CLT的较优工艺为:采用国产EPI(水性高分子异氰酸酯)胶黏剂,施胶量为220 g/m2,压力为0.8 MPa。     

国产日本落叶松正交胶合木的层间剪切性能

采用国产日本落叶松制备CLT,分析层板锯材自身的弹性模量、层数、层板厚度、组坯方向对CLT板层间剪切性能的影响。结果表明:层板模量增加、层数减少、层板厚度减少,对CLT层间剪切强度影响为显著,对层间剪切弹性模量影响不显著;改变组坯纹理方向能够显著提高CLT的层间剪切性能;采用剪力类比法,能够有效预测CLT等效剪切刚度,预测值和实测值相对误差在7%以内。     

温度对人工林落叶松材物理力学性能的影响

探讨在不同的干燥温度状态下,木材抗弯强度,抗弯弹性模量、横纹抗压强度的变化规律。试验结果显示,在中、低温区域,随着温度的升高,抗弯弹性模量、横纹抗压强镁呈降低趋势;在高温区域,随着温度的升高,抗弯强度被明显削弱,而纹抗压强度则得到增强。     

Effects of semi-isostatic densification of wood on the variation in strength properties with density

The variation in strength properties with density was compared between semi-isostatically densified and non-densified wood. Strength properties were compared with published data from earlier studies using other methods for densification. Small clear specimens of eight species were analysed for compression strength in axial, radial and tangential direction, three-point bending and Brinell hardness. After densification, all tested strength properties increased with density, but especially strength perpendicular to grain became lower than expected from the density of non-densified wood. Strength of densified wood relative to what could be expected for non-densified wood of similar density was denoted as ‘strength potential index’. For axial compression strength and bending strength, strength potential index of individual wood species varied between 0.7 and 1.0, i.e. densified wood is slightly weaker than what could be expected from its density. Strength potential index was lower for properties much determined by strength perpendicular to grain. In radial direction, densified wood was rubbery with low modulus of elasticity and nearly no proportional limit or modulus of rupture. Generally, wood was apparently weakened in proportion to the degree of compression in respective direction. Strength potential index also increased with increasing original density of the species.     

竹节对竹材力学强度影响的研究

本文对毛竹和刚竹的带节材与不带节竹材的主要力学性质研究表明,带节竹材的抗弯强度、顺纹抗压、抗剪、抗拉强度和冲击韧性都有一定程度降低的趋向,但抗劈开强度和横纹抗拉强度却有明显提高。影响竹材力学强度的主要因素是维管束数量、维管束排列方向及维管束中纤维的力学强度,了解这些规律,对复合材料的结构仿生有重要参考作用。     

Determination of embedding strength of wood for dowel-type fasteners

Embedding tests parallel and perpendicular to the grain were conducted to produce a database of embedding strength of wood for the design of dowel-type joints. Dowel diameters were 8,12,16, and 20mm. Embedding strength was evaluated by the 5% off-set method and a maximum load up to 5mm displacement according to EN383. The embedding strength parallel to the grain evaluated by the former method showed values close to those obtained with the latter method, but they showed a significant difference in tests conducted perpendicular to the grain. The embedding strength parallel to the grain was 0.9 times as large as the compressive strength parallel to the grain regardless of the evaluation method. The embedding strength perpendicular to the grain evaluated by the 5% off-set method was four times as large as the compressive strength perpendicular to the grain. When the embedding strength perpendicular to the grain was evaluated by a maximum load up to 5mm displacement according to EN383, the ratio of embedding strength perpendicular to the grain to the compressive strength perpendicular to the grain decreased as the dowel diameter increased.Part of this paper was presented at the annual meeting of the Architectural Institute of Japan 1999, Hiroshima, September 1999; the 50^th annual meeting of the Japan Wood Research Society, Kyoto, April 2000; and the World Conference on Timber Engineering 2000, Whistler, Canada, July–August, 2000     

Variations in compression strength and surface roughness of heat-treated Turkish river red gum (Eucalyptus camaldulensis) wood

This article reports the effects of heat treatment on compression strength parallel to the grain, the surface roughness [average roughness (R_a)], and the air-dry den-sity of wood from the river red gum tree (Eucalyptus camaldulensis Dehn.) planted in Turkey. Eucalyptus wood was heat-treated at temperatures varying from 120° to 180°C for durations of 2–10 h. Samples cut from the heat-treated wood were tested for air-dry density, compression strength parallel to grain, and surface roughness properties. Roughness measurements by the stylus method were made in the direction perpendicular to the fiber. Based on the findings in this study, the results showed that density, compression strength, and surface roughness values decreased with increasing treatment temperature and treatment times. Eucalyptus wood could be utilized by using proper heat treatment techniques without any losses in strength values in areas where working, stability, and surface smoothness, such as in window frames, are important factors.     

Evaluating rolling shear strength properties of cross-laminated timber by short-span bending tests and modified planar shear tests

This paper presents an experimental study on rolling shear (RS) strength properties of non-edge-glued cross-laminated timber (CLT) made out of New Zealand Radiata pine (Pinus radiata) structural timber. CLT specimens with 35 and 20 mm thick laminations were studied to evaluate the influence of lamination thickness on the RS strength of CLT. Short-span three-point bending tests were used to introduce high RS stresses in cross layers of CLT specimens and facilitate the RS failure mechanism. Modified planar shear tests from the conventional two-plate planar shear tests were also used to evaluate the RS strength properties. It was found that two test methods yielded comparable RS strength properties and the lamination thickness significantly affected RS strength of the CLT specimens. The test results also indicated that the recommended characteristic RS strength values of CLT products in Europe and Canada might be over conservative. Also, it might be more efficient to specify different RS strength values for CLT with different lamination thickness given the minimum width-to-depth ratio of laminations is satisfied.     

基于换算长细比的正交胶合木稳定系数计算方法

正交胶合木(CLT)纵横向层板交叉组坯,纵向层板强度和模量较高,横向层板强度和模量较低,其截面呈强弱相间的条带状结构.当轴心受压构件失稳时,截面剪切变形对CLT稳定承载力的影响不可忽略,故不能采用一般木产品的稳定承载力计算方法.目前我国相关标准中尚无CLT轴心受压构件稳定承载力计算方法.鉴于此,从CLT产品的截面构成特...     

正交胶合木胶合性能研究现状及展望

胶合性能是胶合木制品的重要性能之一。在正交胶合木（CLT）中木材特征、胶黏剂性能以及木材与胶黏剂之间界面性能共同决定了正交胶合木的胶合性能。木材作为一种天然各向异性材料,其纹理方向不同,造成组坯方式对于胶合性能影响巨大。在胶合性能测试中,取样尺寸、取样形状对于胶合性能的测试结果都有一定的影响。胶合界面性能表征方面的研究正逐渐兴起,但是针对CLT胶合界面性能表征的相关研究报道并不多。有关CLT胶合的研究多集中在胶黏剂和胶合工艺方面,而针对不同锯材单元的特性对胶合性能尤其是耐久性影响的研究尚少有报道。文中基于现有研究对CLT胶合界面性能影响因素进行总结,梳理对比CLT胶合性能测试方法以及胶合界面性能表征技术,并对今后研究方向提出意见和建议。     

人工林落羽杉力学性能研究

对江汉平原人工林落羽杉物理力学性能进行了研究,结果表明:落羽杉的气干密度为0.413 g/cm3,气干密度等级为轻;综合强度为74 MPa,强度等级为Ⅰ级;径向横纹抗压强度略大于弦向横纹抗压强度;端面硬度最高,弦面硬度与径面硬度差别不大。落羽杉南北面近树皮处木材的密度、抗弯强度、弹性模量、顺纹抗压强度均大于近髓心处,南北方向对落羽杉的密度、顺纹抗压强度在5%水平上差异均不显著,而对其抗弯强度、弹性模量在1%水平上差异显著。     

Fracture analysis of perpendicular to grain loaded dowel-type connections using a 3D cohesive zone model

The strength and fracture behavior of dowel-type connections with stiff dowels loaded perpendicular to grain was studied by nonlinear 3D finite element (FE)-analysis. A cohesive zone model was used to model the perpendicular to grain fracture of the wood, i.e., failure by wood splitting along the grain. The influence of load eccentricity and dowel-to-loaded-edge distance was studied for a plate type of geometry loaded in tension and for a simply supported beam loaded in bending. The strength found from the FE-analysis is compared to strength from experimental tests with centric loading, showing overall good agreement. Numerical results for centric loading are further compared to strength predictions according to the linear elastic fracture mechanics (LEFM)-based design criterion present in Eurocode 5 (EN 1995-1-1:2004). The comparison showed good agreement regarding the relative influence of connection geometry, but the design criterion appears, however, to yield unconservative strength predictions. The results of the FE-analyses regarding dowel load eccentricity showed that such loading conditions may yield significantly lower strengths compared to centric loading. An approximate engineering method to account for the strength reduction due to load eccentricity is, furthermore, presented.