Chemical emissions from adhesive-bonded wood products at elevated temperatures

Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was employed to examine the chemical emissions from the glued wood samples at elevated temperatures (150, 200, 250, 300, 350, 400, 450, and 500°C). Two wood species, Douglas-fir (Pseudotsuga menziesii) and southern pine (Pinus palustris), one structural adhesive, phenol–resorcinol–formaldehyde (PRF), PRF-bonded Douglas-fir, and PRF-bonded southern pine were evaluated. The volatile compounds with a mass range of 35–300 m/z emitted from the pyrolysis samples were separated by gas chromatography (GC) and identified by mass spectrometry (MS). The results indicated that compared to neat wood and resin film samples, several additional pyrolysis products were observed for the PRF-bonded wood samples including (1) acetaldehyde and butanedial for PRF-bonded Douglas-fir and (2) acetaldehyde, furfural, 2-furanmethanol, butanedial, 2,3-butanedione, cyclopropyl carbinol, 1,2-benzenediol, and 1-(acetyloxy)-2-propanone for PRF-bonded southern pine. These additional compounds were possibly associated with the interaction between wood and PRF resin. The results also indicated that bonded wood products would be less thermally stable than those of neat wood and PRF resin samples.     

Shear strain distribution in PRF and PUR bonded 3–ply wood sheets by means of electronic laser speckle interferometry

The distribution of shear strain across a 3–ply wood sheet bonded with PRF and PUR, respectively, was measured by means of electronic laser speckle interferometry. Owing to the higher shear modulus of PRF compared to PUR, shear strains were more evenly distributed in PRF bonded specimens than in PUR bonded specimens. However, considering the different shear moduli of earlywood and latewood bands in the core layer of the 3–ply sheet, it is concluded that shear stress is distributed more evenly along PUR bond lines compared to PRF bond lines. Since it is less prone to stress concentrations, PUR should provide more durable adhesive connections than stiff adhesives such as PRF.     

Influence of Ammoniacal Copper Quaternary treatments on mechanical properties of blue-stained Lodgepole Pine wood

Three concentrations （2.8%, 2.0%, 1.2%） of Ammoniacal Copper Quaternary （ACQ） was selected to treat Lodgepole pine wood for evaluating ACQ treatment on mechanical properties of blue-stained wood. The bending modules of elasticity （MOE）, modules of rupture （MOR）, toughness and shearing strength parallel to grain on tangential surface, are tested according to the criteria GB1927-1943-91. Non-treated sample were also tested according to the same procedure. The results showed that the three groups specimen impregnated by different concentrations of ACQ solution met the AWPA standard 2003 of America （UC4A 6.4g/cm^3）. There were significant difference of toughness between treated wood and non-treated wood （p=0.01）, but there were no statistically significant differences among three concentrations in terms of toughness, and toughness of treated wood was approximately 20% lower than non-treated. MOR, MOE as well as sheafing strength parallel to grain were found to be not significantly different between treated wood and non-treated one, and there were no statistically significant difference among three concentrations of ACQ too. Toughness, MOR, MOE and sheafing strength parallel to grain increased with decrease of concentration of ACQ, but they were hardly affected by ACQ preservatives.     

Shear resistance and failure modes of nailed joints loaded perpendicular to the grain

Shear tests were conducted on nailed joints in wood that were loaded perpendicular to the grain; these joints had 21 specifications depending on different combinations of wood species, nail dimensions, number of nails, and edge distances of the main members, and their effects on the shear resistance of the nailed joints were also investigated. The nailed joints with CN75 nails had higher initial stiffness than the joints with CN50 nails, provided the initial stiffness of nailed joints connected with 3 or 5 nails was not always a simple product of the number of nails and the initial stiffness of nailed joints connected with a nail, and instead depended on the combination of wood species of the main member and nail dimensions. When the edge distance decreased, the maximum load and energy capacity decreased, thereby affecting the energy capacity. The maximum load of the nailed joints with CN75 nails may be smaller than those with CN50 nails depending on the combination of wood species and nail dimensions. When the edge distance of the nailed joints was less than 26 mm, the energy capacity of the nailed joints with CN75 nails was less than or similar to those with CN50 nails.     

Finger-jointing green softwood: Evaluation of the interaction between polyurethane adhesive and wood

Existing European standards for finger-jointing of load-bearing lumber require the wood to be dried before gluing. This article presents a study on the properties of green-glued finger joints, wet wood being bonded prior to drying. Issues to consider, in comparison to finger-jointing of dry wood, are mechanical performance of the joint, absorption of the polymer by the wood in its natural/wet state, and the chemical reactions of the adhesive on contact with water. Finger-jointed samples were tested in bending, and the glue joints analysed by optical microscopy, scanning electron microscopy and microdensitometry. A patented one-component polyurethane adhesive developed for gluing-green wood which has a moisture content usually higher than 70% was used in the study. The resulting green-glued joints showed improved strength properties in comparison to dry-jointed joints. The results confirm that green-glued joints provide a wide, continuous wood/adhesive interface from one substrate to the other. The adhesive penetrates several cells deep and the density of the wood adjacent to the joint surfaces is increased. The results also indicate that the patented adhesive forms covalent bonds to the wood substrate.     

Finger-jointing green softwood: Evaluation of the interaction between polyurethane adhesive and wood

Abstract

Existing European standards for finger-jointing of load-bearing lumber require the wood to be dried before gluing. This article presents a study on the properties of green-glued finger joints, wet wood being bonded prior to drying. Issues to consider, in comparison to finger-jointing of dry wood, are mechanical performance of the joint, absorption of the polymer by the wood in its natural/wet state, and the chemical reactions of the adhesive on contact with water. Finger-jointed samples were tested in bending, and the glue joints analysed by optical microscopy, scanning electron microscopy and microdensitometry. A patented one-component polyurethane adhesive developed for gluing-green wood which has a moisture content usually higher than 70% was used in the study. The resulting green-glued joints showed improved strength properties in comparison to dry-jointed joints. The results confirm that green-glued joints provide a wide, continuous wood/adhesive interface from one substrate to the other. The adhesive penetrates several cells deep and the density of the wood adjacent to the joint surfaces is increased. The results also indicate that the patented adhesive forms covalent bonds to the wood substrate.     

Tensile strength of softwood butt end joints. Part 1: Effect of grain angle on adhesive bond strength

To study the effect of grain angle on the adhesive bond strength in wood, three-part Norway spruce wood specimens were bonded and tested in tension. The two axially orientated outer parts of the specimens were joined with the middle part by means of three adhesives typically used for load-bearing constructions, i.e. one-component polyurethane (PUR), melamine-urea-formaldehyde (MUF) and phenol-resorcinol-formaldehyde (PRF). The grain angle of the middle part was varied from 0° (end grain to end grain) to 90° (flat grain to end grain) in incremental steps of 10°. In general, PRF- and MUF-bonded samples exhibited highest tensile strength at end grain to end grain orientation of the three parts, while specimens bonded with PUR showed only 25% of the strength measured for PRF and MUF, respectively. At high grain angles (90°) all specimens showed similar strength values in the range of 30% of maximum strength of MUF- and PRF-bonded specimens. To explain the changing strength levels at different grain angle a composite failure criterion was applied.     

Numerical analysis on tensile performance of bolted glulam joints with initial local cracks

Under varying climate conditions, cracks are commonly observed in bolted joints, owing to the shrinkage of wood and confinement from slotted-in steel plates and bolts. A three-dimensional finite element model was developed to investigate the mechanical behavior of bolted glulam joints with initial cracks. Wood foundation was prescribed in the model to simulate the local crushing behavior of wood surrounding the bolts. The behavior of wood in compression and the foundation were defined as transversely isotropic plastic in the software package ANSYS. Cohesive zone model was applied in the numerical analysis to consider the propagation of initial cracks and brittle failure of wood in the bolted joints under tension load. The numerical model was validated by the experiments conducted on full-scale specimens and it is indicated that the numerical model has good ability in predicting the failure modes and capacity of tension joints with local cracks. To further investigate the influence of crack number, length and locations, a parametric study was conducted with the verified model. Moreover, to study the effects of cracks on the behavior of bolted joints with different failure modes, another bolted joint including bolts with different strength grades and diameters was designed and analyzed in the parametric study, which was expected to have bolt yielding failure mode. It was found that the initial cracks can decrease the capacity and initial stiffness of tension joints by up to 16.5 and 34.8%, respectively.     

间伐小径木指接技术研究

间伐材指接胶合加工，变短材为长材，弯材为直材，为合理，有效利用间伐材开辟了新路。对间伐材木材含水率，指接端压的大小和时间，指接后养护时间，涂胶量，指榫规格等对指接材抗弯强度的影响进行了研究，并测定了苯酚，间苯二酚，甲醛树脂胶胶合的指接材快速老化性能。     

Helical thickenings and helical cavities in normal and compression woods of Taxus baccata

Summary The longitudinal tracheids in compression wood of Taxus baccata contain helical thickenings but no helical cavities. The thickenings are as frequent and well developed and have the same ropelike appearance as in normal wood of this species. They are an integral part of the S₃ in normal and of the S₂ in compression wood and have the same orientation as the innermost microfibrils in these layers. Except for the absence of cavities and presence of thickenings, compression wood tracheids of Taxus baccata possess all the anatomical features typical of such cells, including a rounded outline, intercellular spaces, a thick S₁ layer, a highly lignified S₂ (L) layer, and no S₃ layer. Pronounced compression wood of Pseudotsuga menziesii contains helical cavities but no helical thickenings. Thickenings and cavities seem to be mutually exclusive in Pseudotsuga and Taxus.This investigation was carried out under the McIntire-Stennis Program, Cooperative State Research Service. I am indebted to Mr. A. Rezanowich of the Pulp and Paper Research Institute of Canada for kindly providing the scanning electron micrographs.     

Tensile strength of softwood butt end joints. Part 1: Effect of grain angle on adhesive bond strength

Abstract

To study the effect of grain angle on the adhesive bond strength in wood, three-part Norway spruce wood specimens were bonded and tested in tension. The two axially orientated outer parts of the specimens were joined with the middle part by means of three adhesives typically used for load-bearing constructions, i.e. one-component polyurethane (PUR), melamine–urea–formaldehyde (MUF) and phenol–resorcinol–formaldehyde (PRF). The grain angle of the middle part was varied from 0° (end grain to end grain) to 90° (flat grain to end grain) in incremental steps of 10°. In general, PRF- and MUF-bonded samples exhibited highest tensile strength at end grain to end grain orientation of the three parts, while specimens bonded with PUR showed only 25% of the strength measured for PRF and MUF, respectively. At high grain angles (90°) all specimens showed similar strength values in the range of 30% of maximum strength of MUF- and PRF-bonded specimens. To explain the changing strength levels at different grain angle a composite failure criterion was applied.     

Effect of grain angle on shear strength of glued end grain to flat grain joints of defect-free softwood timber

To evaluate the effect of grain orientation on the adhesive bond strength, three-layered Norway spruce wood specimens were tested in shear. The two axial-oriented outer layers were jointed with the middle layer using three typical glues for load bearing constructions, i.e. one-component polyurethane (PUR), melamine–urea–formaldehyde (MUF) and phenol–resorcinol–formaldehyde (PRF). The grain orientation of the middle layers was varied from 0° (parallel to grain of the surface layer) to 90° (perpendicular to grain of the surface layer) in incremental steps of 10°. Samples with middle layers oriented parallel to the outer layers showed shear values in the range of solid spruce wood. Decreased shear strength values were expected for increased grain angles of the middle layer. However, no explicit tendency was observed for the shear strength in dependence of the grain direction. In general, MUF-bonded samples showed slightly higher shear strength values compared with PUR- and PRF-bonded specimens.     

微米木纤维模压制品螺纹牙受力分布曲线的数学描述

分析了微米木纤维模压制品螺纹连接的受力,提出了其力学分析的基本假设,建立了螺纹牙的力学模型,推导了螺纹牙受力分布函数,对螺纹牙受力分布曲线进行了数学描述,提出了微米木纤维模压制品螺纹牙受力分布的百分比。应用本文提出的理论,可以根据微米木纤维模压制品的刚度和螺纹的几何参数定量计算螺纹牙受力分布,进而计算其螺纹连接的强度,为微米木纤维模压制品螺纹连接的强度计算提供了重要的理论依据。     

Methods for determination of duration of surface moisture and presence of water in gaps in wood joints

If water stays on wood surfaces or is trapped in gaps, the wood is supplied with water for a long period of time and high local moisture contents are reached. This can lead to decay by rot fungi, and it is therefore important to avoid such water traps in order to limit the decay rate. This paper presents two methods: one for determination of duration of surface moisture on wood surfaces and one for determination of duration of water trapped in gaps in wood joints. Both these methods are based on electrical conductance measurements that are commonly used to measure moisture content of wood. Both sensor types consist of insulating tubing, electrically conductive adhesive and copper wire. The sensors monitor the presence of moisture on the actual material surface, and because of their small size, they do not influence the amount and duration of moisture that stays on the surface or in the gap.     

Technological heterogeneity of adhesive bonds in wood joints

 Gluing of wood is among the most effective methods for the permanent joining of furniture elements or building woodwork manufactured from wood. Technological errors occurring during the preparation process of the glue material may lead to variations in the strength of adhesive/wood joints. The purpose of the described research project was to investigate the effect of the heterogeneity of the glue bond on the distribution of tangential stresses in furniture joints, especially the effect of gas cavities, faulty glue bonds and glue outflows on the distribution of tangential stresses in adhesive bonded overlap, cross and angle wood joints. Using developed numerical models, it was shown that shear stresses in bonds of cross and angle joints reach their maximum values in corners of joints. The torsion center of cross joints is situated in the geometrical center of the bond, while in angle joints – it is found half-way through the length of one of the perpendicular edges of the joint. It was also proven that gas cavities present in the glue bond contribute to increased stresses in the neighbourhood of the source of heterogeneity. This phenomenon initiates a process of de-cohesion and, hence, reduces the overall strength of the joint. Faulty gluing, similar to gas cavities, constitutes a potential source of stress-breaking processes and reduces the strength of joints. On the other hand, glue outflows present in wood bonds increase their strength by expanding the initiation threshold of fractures even in situations where technological heterogeneity of the glue bond occurs. In furniture constructions as well as in large-size building woodwork constructions or, wherever grace and elegance of the finished product is of lesser importance, glue outflows can be treated as a positive and desirable phenomenon. Received 13 March 1999     

Fracture of multiply-bolted joints under lateral force perpendicular to wood grain

The crack initiation and propagation of multiplybolted joints subjected to lateral forces perpendicular to the grain were analyzed. Two types of bolted joint were subjected to lateral loads perpendicular to the grain. One had joints of two bolts aligned with the wood grain (type H), and the other had joints of two or three bolts aligned perpendicular to the grain (type V). The crack initiation and propagation were analyzed by means of the average stress method (ASM) and linear elastic fracture mechanics (LEFM), respectively. The maximum loads calculated by LEFM agreed comparatively well with the experimental results, and it was proved that the LEFM was an appropriate tool to analyze the fracture of multiply-bolted joints subjected to a force perpendicular to the grain. It was also found that the multiply-bolted joints failed with the fracture of the wood before the joints yielded, and that it caused a considerable decrease of the maximum loads. The reduction of strength should be considered in the design of multiply-bolted joints subjected to lateral forces perpendicular to the grain.Part of this work was presented at the annual meeting of the Architectural Institute of Japan, Hikone, September 1996     

Effect of decay on shear performance of dowel-type timber joints

Monotonic and reversed cyclic loading tests were conducted on dowel-type timber joints with varying degrees of wood decay due to Fomitopsis palustris (Berk. et Curt.), a brown rot fungus, and the effect of decay on various shear performances of dowel-type joints was investigated. For joints affected by the brown rot fungus, the initial stiffness, yield load, and maximum load of dowel-type joints were significantly decreased, even with a small mass loss of wood. The reductions in shear performance were the largest for initial stiffness, followed by yield load and maximum load, in that order. For a 1% reduction of the yield load, initial stiffness and maximum load showed reductions of 1.15% and 0.77%, respectively. When dowel-type joints that had been exposed to the brown rot fungus were subjected to reversed cyclic loading, the gap between the dowel and the lead hole of the wood was increased and equivalent viscous damping was decreased. These results indicate that decay around the dowel lead hole especially affects the load-displacement behavior at small displacement level, and dowel-type joints under cyclic loading have very low resistance to forces acting on the wooden structure. Part of this report was presented at the 5th Symposium on Timber Bridges of the Japan Society of Civil Engineers, Tokyo, July 2006; the 56th Annual Meeting of the Japan Wood Research Society, Akita, August 2006; and the Annual Meeting of the Architectural Institute of Japan, Fukuoka, August 2007     

人工林杉木和杨树木材物理力学性质的株内变异研究

按照中国国家标准研究杉木和I-214杨树木材的抗弯弹性模量、抗弯强度、顺纹抗压强度和密度,同时按照日本国家标准研究2个树种的顺纹抗剪强度.结果表明:杉木的抗弯强度、顺纹抗压强度和密度由胸高直径处向上呈波浪形增加,抗弯弹性模量则稳定降低,但不同高度间杉木的物理力学性质没有显著差异;近树皮处木材的物理力学性质高于近髓心处木材,并有极显著差异.对于I-214杨树,只有抗弯弹性模量从髓心到树皮逐渐增加,其他的物理力学性质,最小值在从髓心到树皮的过渡区,最大值在近树皮处,从髓心到树皮,杨树的物理力学性质有极显著的差异.杉木和杨树的径面顺纹抗剪强度从髓心到树皮有极显著差异,并且近树皮的高于近髓心的木材,而弦面顺纹抗剪强度从髓心到树皮没有显著差异.木材密度与力学性质有很好的线性相关关系,木材密度是一个很好的力学强度的预测手段.     

红锥木材性质与UF胶固化时间关系的研究

对红锥木材性质与UF胶固化时间的测定和分析结果表明：红锥木材呈弱酸性，心边材不同部位、树干不同高度木粉的pH值和缓冲容量无显著差异；UF胶的固化时间与pH值在0．01水平上相关极显著，与酸缓冲容量在0．05水平上相关显著，与碱缓冲容量相关不显著。研究结果为红锥木材的培育和合理利用提供了科学依据。     

End grain water absorption and redistribution in slow-grown and fast-grown Norway spruce (Picea abies (L.) Karst.) heartwood and sapwood

Abstract

Wood is susceptible to decay by rot fungi if it is exposed to high-moisture contents during long periods of time and it is therefore important to limit the duration of such periods. Critical points in outdoor wood structures are, for example, end grain surfaces in joints where water can get trapped after a rain. It is therefore of interest to study both absorption and redistribution of moisture in wood. This paper presents moisture content profiles during end grain water absorption and redistribution in Norway spruce (Picea abies (L.) Karst.) measured by computed tomography with the specimens in individual climate boxes. Heartwood and sapwood of two provenances (slow-grown and fast-grown wood) were included. No major differences were seen between the water uptake of the slow-grown and the fast-grown wood since the densities were similar despite of the large difference in growth ring width. However, for the sapwood specimens, the moisture content was higher further into the specimens than for the heartwood specimens in agreement with previous studies. For the slow-grown wood, the redistribution was also generally more rapid for the sapwood specimens than for the heartwood specimens.