Torsional-shear stress of wood at various temperatures

Summary The maximum torsional-shear stress of 5 softwoods and 4 hardwoods were tested in the radial and tangential planes. These tests were carried out in glycerin which was preheated to different temperatures between 22°C and 150°C, inclusively. The dense hardwoods possess 2.4 times higher shear stress than softwoods at 22°C. However, at elevated temperatures, the same degree of shear stress reduction (77 %) is obtained for hardwoods and softwoods in both radial and tangential failure. Thus, the reduction in shear stress is independent of physical and structural wood variables.     

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.     

On the mechanical behaviour of water soaked wood at various states of stress

Summary The paper is concerned with an experimental investigation of the swelling strains and mechanical strains of water soaked wood under one- and two-dimensional stress states at changing moisture content. The aim of this paper is to develop an experimental basis for setting up the physical relations between strains, stresses and moisture content in wood.     

Chemical kinetics of stress relaxation of compressed wood blocks

Summary Methods of chemical kinetics are used to study stress relaxation behaviour of wood blocks made fromPinus sylvestris and compressed parallel to grain at 20°C and 65% relative humidity. The effect of stress levels on stress behaviour is investigated. It is found that both activation volume and activation energy decrease with increasing stress level. The results prove the validity of the chemical kinetics approach in describing stress relaxation of wood blocks in compression. Suggestive explanations are given for the variations in activation energy and activation volume with stress level.Part of this work was carried out at the School of Agriculture and Forestry Sciences, University of North Wales, Bangor, UK through an ODA TCT fellowship granted to one of the authors (SPSR). He would like to thank Director General, ICFRE (India) and Director, IWST, Bangalore (India) for providing him with the opportunity to work in UK. He would also like to acknowledge the help received from Mrs. Helen Simpson and Dr. Robert Brooke of School of Agriculture and Forestry Sciences, University of North Wales, Bangor, U.K     

Stress relaxation of wood at several levels of strain

Summary Stress relaxation tests were performed with six tropical American species. Stress relaxation was not found to be a linear function of strain at any level of strain. At qual low levels of strain, stress relaxation in compression was much greater than in tension.A mechanical model consisting of an isolated spring in parallel with a spring and dashpot in series was used as an aid in the derivation of equations describing stress relaxation.An attempt to apply Newtonian viscous theory to the model was unsuccessful in accounting for rate of relaxation. However, when the hyperbolic sine law of viscous flow was applied, mathematically derived curves fitted the data very well.Stress relaxation appears to be related to departure strain which may be obtained readily from static stress strain diagrams.

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Zusammenfassung Versuche über die Spannungsrelaxation wurden mit sechs tropischen Holzarten Amerikas durchgeführt. Es erwies sich, daß die Spannungsrelaxation nicht in jedem Bereich der Dehnung eine lineare Funktion dieser Dehnung ist. In vergleichbar niedrigen Dehnungsbereichen zeigte sich zum Beispiel, daß die Spannungsrelaxation bei Druck größer ist als bei Zugbeanspruchung. Mit Hilfe eines mechanischen Modells, bestehend aus einer einzelnen Feder in Parallelschaltung zu einer Feder mit Dämpfungselement wurden Gleichungen zur Beschreibung der Spannungsrelaxation abgeleitet.Der Versuch die Newtonschen Viskositätsgesetze auf dieses Modell anzuwenden, schlug aufgrund der Relaxationsgeschwíndigkeit fehl. Bei Anwendung des hyperbolischen Sinussatzes für viskoses Fließen stimmten jedoch die ermittelten Werte recht gut mit den mathematisch berechneten Kurvenwerten überein.Die Spannungsrelaxation scheint mit der sogenannten Anfangsdehnung zusammenzuhängen, wie man sie stets bei statischen Spannungs-Dehnungsschaubildern erhält.

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A condensation of a dissertation submitted to the faculty of the Yale School of Forestry as partial fulfillment of the requirements of the D. For. degree.

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This research is part of a comprehensive study being conducted at the Yale School of Forestry in cooperation with the Office of Naval Research, Department of the Navy, under Contract Nonr 609 (13), Project NR 330-001, Properties of Tropical Woods. The author acknowledges the fellowships granted by the Organization of American States, and the Instituto Nacional de la Investigación Científica de México. The author wishes to thank Professor Frederick F. Wangaard for his counsel and assistance, and Professors Robert M. Kellogg and Robert P. Vreeland for encouragement and assistance.     

Infrared spectral characteristics and surface inactivation of wood at high temperatures

Summary Spectra resulting from chemical changes in white spruce [Picea glauca (Moench) Voss] microsections heated in air or nitrogen at high temperatures (100 to 240°C) were continuously recorded on an infrared spectrophotometer.A significant change occurred in the intensity of the 1730 cm^-1 band which indicates a carbonyl absorption of carboxyl and ester groups of wood. This intensity initially decreased and the increased at a greater rate. The time periods to reach the minimum inflection point, termed the times to initiate a significant oxidative carboxylation or oxidation, showed a good curvilinear relationship with heating temperatures.Quantification of this time-temperature relationship required to reach a significant level of oxidation was achieved, using wood microsections that had extractives removed to varying degrees. It was concluded that the extractives served only as catalysts for oxidation. When drying wood at temperatures over 180°C, in addition to oxidation, pyrolytic degradation occurred.Chemical evidence was further confirmed by tests of plywood panels bonded with phenolformaldehyde glue. Three separate types of veneers were investigated—non-extracted, acetone extracted, and veneer with the surface chemically stabilized by treatment with sodium borohydride. The results suggest that the time period to reach a significant level of oxidative carboxylation is also the time period to initiate a wood surface inactivated to polymer adhesion.The author wishes to thank Miss D. Bouchard and Mr. H. N. Mukai for their assistance in the experiments. Appreciation also is due to Mr. H. MacLean for a critical review of this paper.     

Effects of periodic temperature changes on stress relaxation of chemically treated wood

In order to clarify the relationship between the microstructural changes and the rheological behaviors of four chemically treated woods (delignified wood, hemicellulose-removed wood, DMSO swollen and decrystallization treated wood), the stress relaxation of wood with three different moisture contents was determined during periodic temperature changes. The experimental results show that after wood relaxation for 4 h at 25℃, the stress decays sharply when the temperature increases and 2 h later the stress recovers again when the temperature drops back to the original point. The additional stress relaxation, produced after temperature begins to increase, is mainly caused by the thermal swelling, molecular thermal movement and the break of a part of residual hydrogen bonds. The number of hydrogen bonds and the size and amount of cavities in various treated woods greatly affect themagnitude of the additional relaxed stress and the recovery stress.     

Interaction between Epicoccum purpurascens and xylophagous basidiomycetes on wood blocks

Epicoccum purpurascens, a component of the bark micromycota, was evaluated as a potential biocontrol agent for three decay‐causing hymenomycetes frequently found in Buenos Aires city trees, Ganoderma platense, Inonotus rickii and Rigidoporus ulmarius. Treatments consisted of dual cultures of E. purpurascens and xylophagous fungi on box elder (Acer negundo) wood blocks. Two inoculation sequences were tested: (i) the biocontrol agent as the first microorganism inoculated followed by the pathogen; and (ii) the xylophagous fungus as the first microorganism inoculated followed by the biocontrol agent. After 3 months of incubation, percentage dry weight loss was calculated. When E. purpurascens was allowed to colonize wood blocks before the introduction of the xylophagous fungus, dry weight loss was significantly reduced in all instances. However, as a secondary colonizer, E. purpurascens mildly reduced the weight loss caused by G. platense, had no effect on degradation by I. rickii and increased wood weight loss by R. ulmarius.     

Potassium acetate-catalyzed acetylation of wood: reaction rates at low temperatures

Spruce wood blocks were acetylated in the presence of potassium acetate (KAc) at 20, 40, 60, 80 and 120°C. At 20°C, the weight percent gain (WPG) due to the KAc-catalyzed acetylation reached 20% in 18 days, whereas that due to pyridine-catalyzed acetylation did not exceed 8%. The hygroscopicity and dimensional stability of the KAc-acetylated wood were the same as those of conventionally acetylated wood at the same WPG, irrespective of reaction temperature. These facts suggest that the KAc enables simplified acetylation of wood at room temperature. The activation energy (E _a) of the KAc-acetylation in the lower temperature range (20–40°C, 121–131 kJ/mol) was comparable to that of the acetylation of wood meal (140–146 kJ/mol). It was speculated that diffusion became a minor factor at reduced reaction rates in the lower temperature range, thus requiring a greater E _a.     

Tensile stress relaxation of copper–ethanolamine (Cu–EA) treated wood

The tensile stress relaxation of Chinese fir wood treated with copper–ethanolamine (Cu–EA) was compared with that of untreated control to investigate the influence of Cu–EA treatment on the dimensional stability of wood in long-term application, and also for a better understanding of copper–wood–water interactions in copper containing water-borne preservative systems. The results showed that temperature and moisture conditions play important roles in the stress relaxation behavior of wood with or without Cu–EA treatment. At 25°C, Cu–EA treatment has little influence on stress relaxation; while at 35°C, Cu–EA treatment can significantly reduce the stress relaxation of wood, suggesting that Cu–EA treatment can increase dimensional stability of wood at high atmospheric temperature in long-term application. The complicated effect of copper retention on stress relaxation further confirms that copper competes for hydroxyl groups as adsorption sites with water molecules, as put forward in the previous report.     

Relationship between various extracted basic densities and wood chemical components inEucalyptus camaldulensis

Summary Relationship between various extracted basic densities and wood chemical components were investigated by their within-tree variations inEucalyptus camaldulensis for assistance in the prediction of the properties of wood or wood derived products. Within-tree variation was not observed for basic density because extraneous compounds masked it. However, extractives-free basic density, total extractives-free basic density and extraneous compounds-free basic density were high on the bark side and top parts in the trunk. These extracted basic densities were expected to have significant relationships to the fiber morphology causing the within-tree variations, and to be very important factors for wood industries. These relationships were sought by correlations between extracted basic densities and wood chemical components based on their within-tree variations. Furthermore, fair relationships between extracted basic densities and hemicellulose composition were observed and speculation made as to the relationship to the constituent ratio of cell wall layers.The authors wish to thank Conservation and Land Management (Western Australia) for the provision of samples and Ms R. Noda and Mrs. T. Furukawa for experimental assistance     

A stress transformation approach to predicting the failure mode of wood

Summary A simple model, based on the use of transformations of second-order tensors, is presented in this paper to predict the failure mode of wood members stressed in various degrees of parallel-and perpendicular-to-grain tension and parallel-to-grain shear. This type of loading is indicative of structural wood members with cross grain or grain deviations in the vicinity of knots subjected to bending or tension. The model is based on the assumptions that failure is dictated by the presence of any of the aforementioned stresses that exceed the clear wood strength in that mode and that failure does not result from stress interactions. The magnitudes of the applied stresses are normalized relative to the wood strength in that mode. The ratio of applied stress to material strength that is greatest at any particular angle of load to grain is presumed to be the failure mode at that angle. To verify model predictions, optical and microscopic analyses of surfaces of failed specimens loaded in uniaxial tension at angles between 0° and 90° to grain were compared to previously obtained, or otherwise known, surfaces of specimens tested in tension and shear. Specimens tested at various angles to grain demonstrated failed surfaces very much like those associated with specimens loaded in the modes predicted by the model.     

Overlapping of dielectric relaxation spectra in oven-dry yellow birch at temperatures from 20 to 100°C

Summary The existence of three overlapping dielectric relaxation spectra from oven dry yellow birch was found over the frequency range from 20 Hz ... 2 GHz and at the temperature range from 20 ... 100°C. Origin of the spectra is largely attributed to hindered reorientation of various dipolar groups in the material. The spectra were resolved in terms of the super-position principle for dielectric loss and permittivity, and the distribution of relaxation times. The spectral resolution was checked by the Cole-Cole plot. The values for the activation energies indicate, as expected, that the dipolar groups are imbedded by hydrogen bonding in the solid structure. The crystalline portion of native cellulose in the wood estimated from the spectra agrees with such data from other sources.     

Three mechanisms affecting the mechanical properties of spruce wood dried at high temperatures

Wood drying experiments are conducted in which the temperature and the drying rate are controlled independently. In relationship to drying processes, at least three mechanisms are believed to contribute to the properties of dried wood. However, only two of these are found to affect the properties of macroscopic specimens, the third mechanism being observable in microtomed earlywood sections, and possibly in specimens loaded in the radial direction. Degradation of structural components and irreversible hydrogen bonding (hornification) are found to contribute to both the hygroscopicity and the mechanical properties of macroscopic wood specimens. Mass loss from thermal degradation occurs predominantly in slow high-temperature drying processes. Irreversible hydrogen bonding takes place in high-temperature drying, in particular with high ultimate dryness. Regarding the effect on strength and stiffness, mass loss and hornification appear to compete. The third identified mechanism, microscopic cell wall damage caused by incompatible drying shrinkage of cell wall elements, does not seem to affect the mechanical properties of macroscopic wood specimens. Consequently, slow high-temperature drying processes do not provide much benefit regarding the mechanical behavior of dried wood. The reasons for this are discussed.     

Osmotic stress in wood — Part I: The analogy between thermal and swelling stress

Summary The analogy between thermal stress and swelling (drying) stress in wood makes it possible to give mathematical formulae for calculating the drying and swelling stresses in many situations. The models allow for viscoelastic and plastic phenomena.Valuable discussions with Dr. P. U. A. Grossman are gratefully acknowledged.     

Creep and stress relaxation behavior for natural cellulose crystal of wood cell wall under uniaxial tensile stress in the fiber direction

We investigated the temporal changes in creep and stress relaxation behavior in both microscopic crystalline cellulose and macroscopic strain of wood specimen using Japanese cypress (Chamaecyparis obtusa Endl.) to understand the viscoelastic properties of wood cell walls. Specimens 600 µm in thickness were observed by the X-ray diffraction and submitted to tensile load. The crystal lattice strain of (004) plane and macroscopic strain of specimen were continuously detected during creep and stress relaxation tests. It was found that the creep compliance based on macroscopic strain showed a gradual increase after instantaneous deformation due to loading and then the parts of creep deformation remained as permanent strain after unloading. On the other hand, crystal lattice strain showed a different behavior for macroscopic strain; it kept a constant value after instantaneous deformation due to loading and then increased gradually after a certain period of time. These differences between macroscopic and microscopic levels were never found in the stress relaxation tests in this study. Relaxation modulus at the macroscopic level only showed a decreasing trend throughout the relaxation process. However crystal lattice strain kept a constant value during the macroscopic relaxation process. In addition, the microfibril angle (MFA) of wood cell wall has a role of mechanical behavior at microscopic level; crystal lattice strains were smaller with increasing MFA at both creep and relaxation processes. Creep compliance and stress relaxation modulus at the macroscopic level decreased and increased with increasing MFA, respectively. Our results on the viscoelastic behavior at microscopic level evidenced its dependency on MFA.     

Relations between various extracted basic densities and wood chemical components in Eucalyptus globulus

Relations between various extracted basic densities and wood chemical components were investigated by their within-tree variations in Eucalyptus globulus for assistance in the prediction of the properties of wood or wood-derived products. Extraneous compounds affect the relations between various basic densities and wood chemical components such as holocellulose and the lignin syringyl/guaiacyl ratio. We also discuss the relation of various densities, the molar composition of neutral sugars constituting hemicellulose, and fiber morphology.     

Potassium acetate-catalyzed acetylation of wood at low temperatures II: vapor phase acetylation at room temperature

Ezomatsu wood blocks were impregnated with potassium acetate (KAc) and then exposed to acetic anhydride vapor at 25°C and 120°C. The KAc-impregnated wood was rapidly acetylated at 120°C, and only 6 min was needed to achieve 20% weight percent gain (WPG). The WPG increased with increasing catalyst loading (CL), but it turned to decrease above 20% CL probably because the diffusion of acetic anhydride vapor was hindered by excess KAc depositing in the cell lumina. Thus, careful control of CL is necessary in the vapor-phase acetylation. KAc was also effective in catalyzing the vapor-phase acetylation at 25°C: the KAc-impregnated wood attained 20% WPG within 7 days, whereas the WPG did not exceed 10% even after 1 month in the uncatalyzed system. Irrespective of treatment methods, the hygroscopicity of wood was reduced and its dimensional stability was improved with an increase of WPG. These results confirm that the use of KAc simplifies the acetylation process at room temperature with minimal loss of acetic anhydride.