Mechanical behaviour of saturated wood under compression

Summary The mechanical behaviour of three species of hardwoods soaked in different swelling liquids, compressed at high rates of strain, was investigated using a split Hopkinson pressure bar system. Variations in elastic moduli, proportional limit and maximum stress with respect to the treatments were studied. It was found that the saturated specimens could be as stiff as the dry ones. This result was explained by the behaviour of the liquid present in the large cavities of the wood, i.e. the lumens of the cells, which must be different from that observed at low rates of strain. At large rates of strain, this liquid cannot flow out of the pores and must behave like a solid; therefore the structure of the material is reinforced and, as a consequence, the softening effect of the soaking agent can be masked.     

Deformation of wet wood under combined shear and compression

During refining of mechanical pulp, a process occurring at high speed at temperatures of 140–160°C, the flexibility and bonding ability of wood fibres are increased. To understand the mechanical behaviour of the fibres in this operation, deformations at low speed of wet wood specimens at 50°C were studied under two different combinations of shear and compression loadings. The results were compared with the behaviour of wet wood in pure compression. Some features of the deformation that occurred in earlywood were analysed using an image analysis procedure. During pure compression the cell walls bend independently of the shape of the fibre cells and their cell wall thickness. Under combined shear and compression, however, mainly the corners of the fibre cells get deformed. In a second deformation performed in compression, the fibre cells follow the same deformation pattern as given by the first deformation type whether in compression or in combined shear and compression. The interpretation was that permanent defects in the cells themselves were introduced already in the first load cycle of the wood samples. The load combination with lower shear gave the same permanent strain as the case of pure compression but using less energy.     

Effect of temperature and compression on the mechanical behavior of steam-treated wood

The mechanical behavior of steamed spruce wood changes dramatically with compression along the grain, the change being much more moderate perpendicular to the grain. The stiffness decrement due to increased temperature is greatest in the tangential material direction. The stiffness decrement due to compression is greatest along the grain. Compression to 80% compressive strain at 131°C inverts the order of the material directions regarding stiffness, the stiffness being the least along the grain. Plastic strain due to compression is greater at higher temperatures. The compression-induced decrement of stiffness along the grain is greater at higher temperatures, but the off-axis decrement of stiffness is less at higher temperatures.     

Corrected heat capacity of wood for sap flow calculations

Values for the temperature-dependent heat capacity of wood were applied to correct the coefficient used to calculate sap velocity from heat pulse velocity, leading to values at 20 degrees C about 5% smaller than values calculated with the widely employed algorithm of Edwards and Warwick. Errors are larger at lower temperatures (7% at 10 degrees C) and so are more problematic at night and during cold periods.     

Mechanical behaviour of saturated wood under compression Part 2: Behaviour of wood at low rates of strain some effects of compression on wood structure

Summary The mechanical behaviour of three species of hardwoods, soaked in different liquids, had been investigated at high rates of strain using a Hopkinson pressure bar system. In order to determine the influence of the rate of strain, samples from the same species were subjected to compression-tests at low rates of strain. It was noticed that, at low rates of strain, the saturated samples were always less stiff than the dry ones, which is in agreement with the literature, but differs from the behaviour at high rates of strain. This difference is attributed to the behaviour of the liquid present in the large cavities of the material, which must depend on the rate of strain. It was also noticed that the samples could support higher stresses at high rates of strain. Although permanent sets were measured after the tests, the samples were not always visibly damaged, but some typical failures were detected by means of microscopy. The damaged zones presented similar aspects, whatever the rate of strain.     

Dynamic viscoelastic behavior of wood under drying conditions

In this study heartwood from a Chinese fir [Cunninghamia lanceolata (Lamb.) Hook] plantation was treated using a high-temperature drying (HTD) method at 115°C, a low-temperature drying (LTD) method at 65°C, and freeze vacuum drying (FVD), respectively. The dynamic viscoelastic properties of dried wood specimens were investigated. The measurements were carried out at a temperature range of −120 to 250°C at four different frequencies (1, 2, 5, and 10 Hz) using dynamic mechanical analysis (DMA). We have drawn the following conclusions: 1) the storage modulus E′ and loss modulus E″ are the highest for HTD wood and the lowest for FVD wood; 2) three relaxation processes were detected in HTD and LTD wood, attributed to the micro-Brownian motion of cell wall polymers in the non-crystalline region, the oscillations of the torso of cell wall polymers, and the motions of the methyl groups of cell wall polymers in the non-crystalline region in a decreasing order of temperatures at which they occurred; and 3) in FVD wood, four relaxation processes were observed. A newly added relaxation is attributed to the micro-Brownian motions of lignin molecules. This study suggests that both the HTD and the LTD methods restrict the micro-Brownian motion of lignin molecules somewhat by the cross-linking of chains due to their heating history. __________ Translated from Journal of Beijing Forestry University, 2008, 30(3): 96–100 [译自: 北京林业大学学报]     

Bending creep behavior of wood under cyclic moisture changes

This study examined the bending creep behavior in the longitudinal direction of six species under cyclic moisture content (MC) changes. For each species, tests were made at 20°C with five cyclic relative humidity changes between 65% and 95%, beginning from moisture adsorption. A load corresponding to 25% of short-term breaking load of the species was applied to the radial section of each specimen with four-point bending. The effect of MC change on instantaneous compliance was also investigated under the same condition. The quantitative relation between mechano-sorptive (MS) compliance and MC change was examined, and the material parameter KM for the relation in specific sorption was determined. Results indicated that the total compliance in the six species with different behavior increases with sorption time. As an integral part of total compliance, instantaneous compliance changes linearly with MC and influences to a greater or lesser extent the total compliance behavior. In general, with increasing MC change, the MS compliance linearly increases during the first adsorption and all desorption and decreases slightly during subsequent adsorption. The material parameterK _M varies markedly not only with species but also with specific sorption. The first adsorption causes the largest deformation, followed by desorption.This paper was presented at the 48th annual meeting of the Japan Wood Research Society in Shizuoka, Japan. April 1998     

Effect of adding steam-exploded wood flour to thermoplastic polymer/wood composite

The effect of steam-exploded wood flour (SE) added to wood flour/plastic composite was examined using SE from beech, Japanese cedar, and red meranti and three kinds of thermoplastic polymer: polymethylmethacrylate, polyvinyl chloride, and polystyrene. Addition of SE increased the fracture strength and water resistance of the composite board to an extent dependent on the polymer species and the composition of wood/SE/polymer. However, water resistance decreased with the increasing proportion of SE when SE meranti was added. Effects of the wood species of SE on the properties of resulting board were small. An increased moisture content of wood flour or SE (or both) increased the variation of board performance.     

Strain-softening behavior of wood under tension perpendicular to the grain

Three softwood samples and one hardwood sample were tested under a tension load applied along the radial direction using small clear specimens and the local tension strain was measured using the digital image correlation method. We successfully obtained a stress-strain curve with a strain-softening branch by calculating the stress using the strain distributions in the vicinity where the specimen ruptured. The continuous digital imaging of the specimen proved to be very effective for measuring the strain in quasi-brittle materials such as wood under tension. The nonlinearity of the stress-strain curve was quantified using two parameters representing the deviation from linear elasticity, and the formula of the stress-strain curve was deduced from the interrelation between these parameters. This formula is expressed quite simply by using the modulus of elasticity along the radial direction and another constant that is unique to the material. Part of this article was presented at the 56th Annual Meeting of the Japan Wood Research Society, Akita, Japan, August 2006     

Effect of wood species on thermal flow behavior and physical properties of thermoplastic moldings

The thermoplastic flow behavior of cedar flour steamed at different temperatures in the range of 160–220°C was measured using a rheometer and compared with that of beech flour. The temperature at which the cedar flour starts to flow was approximately 70°C higher than that of beech flour, and the cedar flour exhibited low flowability. Furthermore, thermoplastic moldings were prepared from cedar and beech flours, and their physical properties were examined. Similar to the case of beech, a resin-like molding with a density of approximately 1.45?g/cm³ was obtained from the cedar flour steamed at 180°C or higher, and it was revealed by SEM observation that in these moldings wood flour particles adhere to each other. The specific bending strength was maximum for the moldings obtained from 180°C-steamed flour for both types of wood.     

Bending creep behavior of hot-pressed wood under cyclic moisture change conditions

This study examined the bending creep behavior of hot-pressed wood during cyclic moisture changes. Sugi (Cryptomerica japonica D. Don) specimens were pressed in the radial direction under six combinations of nominal compressive strain (33% and 50%) and press temperatures (140°C, 170°C, 200°C). Creep tests were conducted at 20°C with three cyclic relative humidity changes between 65% and 95% under 25% of short-breaking stress. The effect of moisture content (MC) change on elastic compliance and mechanosorptive (MS) compliance was investigated. The relation between MS compliance and thickness swelling was studied. The results indicated that total compliance increased over the history of cyclic moisture changes; and its behavior was closely related to the changes in MC and thickness swelling. The total compliance increased during adsorption and decreased during desorption. Elastic compliance increased linearly with MC and was dependent on press temperature and compression. With increasing MC change, MS compliance increased during adsorption and decreased during desorption. The first adsorption led to greater MS compliance than did the subsequent adsorption with the same amount of MC change. In general, the elastic parameterK _E and the MS parameterK _Mincreased with compression and decreased as the press temperature increased. The MS parameterK _M was apparently greater than the elastic parameterK _E. The MS parameterK _M increased with swelling coefficient K_SW of the hot-pressed specimen during adsorption and decreased with an increasing shrinkage coefficientK _SH during desorption.     

Binding effect of cellulose nanofibers in wood flour board

Wood-based materials are extensively used for residual construction worldwide, especially in Japan. Most wood-based materials are fabricated using adhesives, some of which are not environmentally friendly. As an alternative to chemical adhesives, we explored this issue using nanofiber technology, especially the use of cellulose nanofibers (CNF), as reinforcement in wood flour (WF) board to replace chemical adhesives. We found that CNF could be easily made by pulverization in a ball mill. The physical and mechanical properties of WF board were improved by the three-dimensional binding effects of the CNF.     

Changes of crystallinity in wood cellulose by heat treatment under dried and moist conditions

The different effects of heat treatment on wood, especially on the cellulose crystallites of wood under ovendried and highly moist conditions, were investigated by X-ray diffractometer. Heat was found to increase significantly the crystallinity of wood cellulose; moreover, almost twice as much crystallization was observed after heat treatment of spruce and buna under a highly moist condition than under the oven-dried condition. In pure cellulose almost the same crystallization was observed under both the conditions, whereas more crystallization occurred in wood cellulose than in pure cellulose. Absolute crystallization was observed for the wood species and pure cellulose under both conditions, considering the thermal decomposition of the amorphous region in addition to crystallization. Our results suggested that other components accomparying wood cellulose were involved in the increase of crystallinity by heat treatment, and that wood cellulose contained more quasicrystalline regions than pure cellulose. Moreover, calculated apparent activation energies revealed that crystallization and decrystallization in pure and wood cellulose under heat treatment of highly moist condition were some-what easier than those under the oven-dried condition. The behavior of the piezoelectric modulusd ₂₅ almost paralleled that of crystallinity.     

Dynamic-mechanical analysis and SEM morphology of wood flour/polypropylene composites

Introduction It is well known that over the past few decades, polymers have replaced many conventional materials, such as metal and wood in many applications. This is due to the advantages of polymers over conventional materials (Maurizio et al. 1998; Adr…     

Ultrastructure of compression wood in Ginkgo biloba

Summary Compression wood in the ancient Ginkgo biloba differs from that in most of the younger gymnosperms in the more angular outline of its tracheids, their thinner walls, and their lack of helical cavities. Both normal and compression woods of Ginkgo contain two types of tracheids, one wide, with a thin wall, and another, narrow, with a thicker wall. In all other respects the compression wood tracheids in Ginkgo are ultrastructurally similar to those in other gymnosperms. Helical cavities probably developed relatively late in the evolution of compression wood, since they are missing not only in Ginkgo but also in the Taxales and the Araucariaceae. The occurrence of compression wood in Ginkgo biloba indicates that this tissue probably has existed since the Devonean period. Very likely, the arborescent habit of the gymnosperms has always been dependent on their ability to form compression wood.This investigation was carried out under the McIntire-Stennis Program, Cooperative State Research Service. I am indebted to Mr. A. C. Day of this College and to Mr. A. Rezanowich of the Pulp and Paper Research Institute of Canada for kindly providing the scanning electron micrographs.     

Significance of laricinan in compression wood tracheids

Summary After a -D-1,3-linked glucan had been isolated from compression wood, identified, and named laricinan, other researchers suggested that it occupied the helical cavities in the S₂ layer of those tracheids. They postulated that the glucan was responsible for the capacity of compression wood to generate the large forces associated with reorientation of displaced stems and branches, and also caused its severe shrinkage with drying.Analyses herein indicate that it is improbable that such a glucan could be the primary factor responsible for those characteristics of compression wood. An alternative significance is proposed, namely that its presence strengthens the already well-supported deduction that the helical cavities have a schizogenous origin.