Measuring the shear strength ratio of glued joints within the same specimen

Measuring the shear strength of both the glueline and the solid wood with the same test specimen offers the advantages that the comparison is direct and the influence of wood variability is minimized. A new specimen geometry was developed for this purpose, and the test is run with two successive loading steps. Finite element modelling showed that the stress distribution is not significantly influencing the results, whereas the compressive pre-stress on the wood adherents results in a slightly lower resistance, probably mainly caused by micro-cracks that are already present or occur during the first loading step and propagate during the second. The analysis of fracture propagation in mode I revealed that different spruce wood batches have a much different behaviour. The method will be proposed as a standard test for epoxy adhesives used in thick joints. The evaluation criterion developed might be interesting also for other types of structural adhesives.

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Messen des Scherfestigkeitsverhältnisses verklebter Brettverbindungen innerhalb der gleichen Holzart

Zusammenfassung Die Scherfestigkeit sowohl von Klebefugen als auch von Vollholz an gleichen Testproben zu messen, bietet den Vorteil, dass der Vergleich direkt und der Einfluss der Holzvariabilität minimiert ist. Eine neue Holzprobengeometrie wurde zu diesem Zweck entwickelt. Der Test erfolgt mit zwei aufeinanderfolgenden Belastungsschritten. Die FEM-Berechnung zeigte, dass die Spannungsverteilung die Ergebnisse nicht wesentlich beeinflusst, wohinegen die Druckvorspannung auf die Holzklebstoffe in einem leicht geringeren Widerstand resultiert, möglicherweise durch Mikrorisse verursacht, die schon vorhanden sind, oder beim ersten Belastungsschritt auftreten und sich durch den zweiten fortsetzen. Die Analyse der Risserweiterung in Modus I zeigt, dass unterschiedliche Fichtensortimente ein völlig unterschiedliches Verhalten aufweisen. Diese Methode wird als Standardtest für Epoxidharze für dicke Brettkverbindungen vorgeschlagen. Das entwickelte Bewertungskriterium könnte auch für andere Arten von Konstruktionsklebern interessant sein.

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Measuring the shear strength ratio of glued joints within the same specimen

Measuring the shear strength of both the glueline and the solid wood with the same test specimen offers the advantages that the comparison is direct and the influence of wood variability is minimized. A new specimen geometry was developed for this purpose, and the test is run with two successive loading steps. Finite element modelling showed that the stress distribution is not significantly influencing the results, whereas the compressive pre-stress on the wood adherents results in a slightly lower resistance, probably mainly caused by micro-cracks that are already present or occur during the first loading step and propagate during the second. The analysis of fracture propagation in mode I revealed that different spruce wood batches have a much different behaviour. The method will be proposed as a standard test for epoxy adhesives used in thick joints. The evaluation criterion developed might be interesting also for other types of structural adhesives.     

Thermal stability of antioxidants obtained from wood and industrial wastes

The thermal stability of two synthetic food antioxidants (BHA and BHT) and three biomass-derived fractions with antioxidant activity (ethyl acetate soluble-fraction from Eucalyptus globulus acid hydrolysates, ethyl acetate soluble-fraction of autohydrolysis liquors from red grape pomace after fermentation and distillation and washing water of the same feedstock) were assessed. In the case of BHA and BHT, the non-volatile fraction and the antioxidant activity were measured at 100, 150 or 200 °C in assays lasting up to 120 min. In the case of biomass-derived fractions, the percentage of recovered phenolics in solid phase was also determined. The susceptibility of synthetic antioxidants towards volatilisation was higher than those of biomass-derived fractions, which also showed a remarkable ability to retain antioxidant activity after heating.     

Nondestructive assessment of glued joints in timber applying vibration-based methods

In this study, three different shapes and glue coverage rates of joints were evaluated nondestructively using longitudinal and flexural vibration-based methods. Rectangular specimens of 20?×?40?×?360?mm3 (R?×?T?×?L) dimensions were prepared from clear eastern beech (Fagus orientalis; Lipsky) wood. Joints were located at the middle of the beams. The selected joint shapes were 45° scarf joint, 30° scarf joint and finger joint with 10?mm length and 3?mm pitch. Longitudinal vibration test proved a better assessment for gluing than flexural efficiency. Some weaker jointed timbers could demonstrate identical Young’s moduli by both longitudinal and flexural vibration-based methods. In the presence of any significant difference between Young’s moduli evaluated by two methods, it was realized that the joints were weak and required to be rejected or assigned to re-manufacturing. The R² of Timoshenko’s linear model significantly decreased when there was any un-integrity in the joints. This factor was suggested to be used as a potential subsidiary indicator. The vibration-based methods were recommended to be applied for joint assessment of old jointed timbers and for grading newly constructed jointed lumbers.     

Quality assessment of pellets and briquettes made from glued wood waste

European Journal of Wood and Wood Products - In accordance with sustainable economic and social development, Europe supports the use of energy from renewable sources to decrease the use of fossil...     

An experimental study of shape stability in glued boards

An experimental study of shape stability in wooden glued boards was performed to verify certain results of particular interest obtained earlier in numerical simulations. Possibilities for achieving products of good shape stability by gluing boards together in an optimal way are discussed. Since twist is often the most serious form of distortion defects for the user, it was the main type of board deformation considered. Three types of glued products were tested. The experiments show clearly that glued boards can be produced that remain stable in shape when exposed to extreme variations in moisture. The results agree well with the numerical simulations performed earlier. It was found that initial twist could be reduced by proper application of pressure during gluing and that through gluing the occurrence of cracking defects on visible surfaces could be substantially reduced.     

Influence of geometric and mechanical parameters on stress states caused by threaded rods glued in wood

In the construction and rehabilitation of wooden structures, timber connections are often made using glued rods. To guarantee that these connections provide adequate resistance, it is necessary to analyse the characteristics of the connections that affect their load capacity and ductility. To address this issue, the mixed strategy of developing a broad experimental program was adopted while simultaneously performing numerical analysis of models. In this paper, the finite element method is used to study the impact of several design parameters on stress transfer between materials. This approach has facilitated design strategies that improve the effectiveness of these connections.     

Thermal diffusivity of foods measured by simple equipment

This paper describes how measurements of thermal diffusivity can be made by cheap, simple equipment available in most laboratories. The food sample is filled into a cylinder and a thermogram is plotted after placing the cylinder in a thermostatically controlled water bath. The diffusivity is calculated from the graph. Diffusivities thus obtained are in good agreement with data from literature.     

Improvement of tensile shear strength and wood failure percentage of 1C PUR bonded wooden joints at wet stage by means of DMF priming

Tensile shear tests according to EN 302-1 for load-bearing timber structures were performed on European beech wood (Fagus sylvatica L.) and Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] bonded by means of a one-component polyurethane adhesive (1C PUR). Results reveal a substantial loss of tensile shear strength (TSS) and wood failure percentage (WFP) at the wet stage compared to the dry stage. As can be seen from microscopic images, this is accompanied by a loss of adhesion at the boundary layer. Therefore, the aim of this work was to find a priming fluid that improves the load transmission between adhesive and adherend at the wet stage without introducing formaldehyde into the gluing process. A substantial improvement of TSS and WFP was achieved by means of the hygroscopic organic solvent N,N-dimethylformamide (DMF). In addition, contact angle measurements were carried out, revealing that DMF heavily enhances the wettability of the joining surface. Furthermore, it was attempted to integrate the outcomes into the swelling strain model stated by Frihart in 2009. By way of comparison a hydroxymethylated resorcinol coupling agent, a mixture of diphenylmethane-4,4′-diisocyanate isomers and water were also tested as priming fluids. The data confirm that TSS and WFP of 1C PUR bonded wooden joints do not correlate, whilst WFP is mostly not normally (at wet stage often bimodally) distributed.     

Thermal degradation products of wood

Woods were degraded by analytical pyrolysis at 450°C. The degradation products were separated by gas chromatography and the polysaccharide derived degradation products were characterized by mass spectrometry using electron impact (EI) and chemical ionization (CI). The complete (EI) mass spectra and structural formula of 104 polysaccharide derived products are presented. The mass spectral recognition of anhydrosugars and the presence of structure isomers from dihydro-methyl-furanones in the product mixture is discussed.     

Thermal degradation products of wood

Thermal degradation products of woods arising at 450°C have been separated by gas chromatography on a DB-1701 capillary column. GC retention times and mass spectral data of 104 polysaccharide derived pyrolysis products are tabulated. The elemental composition of 76 identified compounds are also presented. The mass spectra were obtained by electron impact ionization (EI). However, the molecular mass of all degradation products with Mw>76 was determined by chemical ionization (CI) using iso-butane as reagent gas. The peak assignment was proved in 38 cases by mass spectrometry of authentic compounds and 32 degradation products were identified according to the literature. The spectral data are presented as a Mass Peak Index showing the intensity of the nine most abundant peaks.     

Thermal degradation products of wood

After thermal degradation of wood the lignin degradation products were separated by gas chromatography and identified by mass spectrometry. The mass spectra and chemical formulae of 82 monomeric lignin derived degradation products are presented.     

Comparative antioxidant capacities of phenolic compounds measured by various tests

The purpose of this study was to compare the antioxidant capacities of standard compounds (phenolic compounds, ascorbic acid, and glutathione) as measured by various assays. Five methods were selected so as to span a diversity of technical approaches: TEAC (radical 2,2′-azino-bis(3-ethylbenzothiazoline)-6 sulphonic acid), DPPH (radical 2,2-diphenyl-1-picryhydrazyl used to measure reducing capacity), ORAC (oxygen radical scavenging capacity), red blood cell haemolysis (protection of biological sample), and ESR (electron spin resonance for direct free radical evaluation). Most compounds showed significant differences in free radical scavenging activity according to the method used. Of the 25 tested compounds, only a few, such as myricetin and gallocatechin, gave comparable activities in the various tests. To standardise reporting on antioxidant capacity, it is proposed to use a weighted mean of the values obtained using the DPPH, ORAC, resistance to haemolysis, and ESR assays.     

Thermal degradation products of wood

The analysis of volatiles after thermal degradation of wood allows specific characterization of wood and its potential use as raw material for different purposes. Thermal degradation products from analytical pyrolysis have been separated by gas chromatography and characterized by mass spectrometry. Up to 82 different degradation products of lignins are tabulated. In following publications also degradation products derived from polysaccharides will be presented.     

Thermal characterization of pine wood by photoacoustic and photothermal techniques

Photoacoustic and photothermal techniques are used to investigate the thermophysical properties at room temperature for each of the three main planes of nine species of pine wood. The open-photoacoustic-cell approach is used to measure thermal diffusivity, α, and the photothermal technique of continuous illumination of the sample in vacuum is used to measure the product of density and specific heat capacity, ρc. Thermal conductivity and effusivity are derived from the measurements for α and ρc. We conclude that these techniques are well suited to study thermal properties of wood.     

Viscoelastic characterization of glutenins in wheat kernels measured by creep tests

The effect of high and low molecular weight glutenin subunits on viscoelastic properties of wheat kernels was investigated using creep tests with generalized Kelvin–Voigt model (r² ≈ 0.94; P < 0.0001). Glu-A1, Glu-B1 and Glu-D1 affected the creep coefficients in Glu-1 loci. Regarding LMW-GS, the locus Glu-A3 had highest influence on creep followed by Glu-B3; Glu-D3 did not showed differences. In general, the modulus of elasticity E₀ ≈ 242 MPa and viscosity was μ₀ ≈ 1.6 × 10⁷ MPa·s. Higher elastic moduli and viscosity were found in HMW-GS and LMW-GS of good quality compared to the poor quality performance genotypic groups. Samples with subunit Glu-A1 null presented lower elastic modulus and viscosity compared to Glu-A1 1 and 2^∗. Glu-B1 13 + 16 presented higher elastic modulus and viscosity, which is associated with good quality performance. The elastic moduli, especially E₀ were positively correlated with SDS-volume and rheological properties. This methodology can be applied to few kernels is easy, rapid, and nondestructive.     

木桁架齿板连接性能模拟的研究现状

从齿板连接构件材料及连接性质确定和连接性能模拟两方面介绍国内外木桁架齿板连接性能理论及模拟研究现状,并对三种常用的齿板连接性能研究的分析软件SAT、PPSAⅡ和ANSYS作了简单介绍。通过对文献资料的系统分析,认为齿板连接性能研究具有重要的科研和应用价值。