Effect of microfibril angle on the longitudinal tensile creep behavior of wood

In this report, we undertook studies of the viscoelastic properties of wood from the viewpoint of the fine structure and properties of the constituent materials in the wood cell wall. To measure the mechanical properties of the wood as the behavior of the cell wall, it is required to perform the longitudinal tensile test using a homogeneous specimen. In this study, microtomed specimens of sugi (Cryptomeria japonica D.Don) earlywood were used for the creep test, which were conducted at the fiber saturation point. The substantial creep compliance of the cell wall was simulated using a simplified viscoelastic model consisting of a Voigt element and an independent spring in series. Based on the experimental results, the values of the parameters were optimized. The results were as follows: (1) the longitudinal tensile creep deformation tends to increase with the elapsed time, similar to the bending creep behavior; (2) the magnitude of the longitudinal creep function increases with MFA; and (3) each parameter in the simplified viscoelastic model is markedly affected by the MFA. Based on these results, the mechanism of the longitudinal tensile creep deformation of wood is discussed.     

Behavior of the cellulose microfibril in shrinking woods

We measured the longitudinal and tangential shrinking processes in wood specimens from Chamaecyparis obtuse Endl. with different microfibril angles (MFAs). The shape of the shrinking curve was compared with the MFA. Only the longitudinal shrinking process of specimens with a small MFA clearly showed nonlinearity, and the degree of nonlinearity increased as the MFA decreased. In contrast, the tangential shrinking process and the longitudinal shrinking process of compression wood with a large MFA were linear. The nonlinearity is probably caused by the longitudinal shrinkage of the noncrystalline region of the cellulose microfibril (CMF) in regions of low moisture content during water desorption. When the moisture content is high, the matrix substance in the cell wall begins to dry; however, the shrinkage in the chain direction is restrained by the rigid CMF. As the wood dries further, the noncrystalline region of the CMF embedded in the matrix substance begins to shrink. Because the longitudinal mechanical behavior of wood with a small MFA is greatly affected by a rigid CMF, longitudinal shrinkage increases suddenly at about 10% moisture content; as a result, the shrinking process shows nonlinearity.     

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.     

Morphological changes in sugi (Cryptomeria japonica) wood after treatment with the ionic liquid, 1-ethyl-3-methylimidazolium chloride

We investigated morphological changes in wood tissues of sugi (Cryptomeria japonica) resulting from treatment with the ionic liquid 1-ethyl-3-methylimidazolium chloride ([C2mim][Cl]), which dissolves cellulose. Treatment with [C2mim][Cl] caused dissociation and distortion of tracheids in latewood, but not in earlywood. This difference was due to the difference in swelling behavior of the cell wall between earlywood and latewood. Many pit membranes in bordered pits were broken by treatment with [C2mim][Cl]. In addition, some chemical changes in wood components, such as cellulose and lignin, occurred before significant disruption or destruction of the cell wall. Our results show that the reaction of wood liquefaction by [C2mim][Cl] treatment is not homogeneous, both from chemical and morphological viewpoints.     

Micromechanics of wood subjected to axial tension

Summary The behaviour of a small group of wood fibers of Sitka spruce during tensile loading is investigated. The load-extension curves for both early and late wood fibers consist of three distinct segments. The first segment is almost a straight line, at some stage of loading a yield point is observed. Beyond this point the specimen becomes less stiff and undergoes a large, mainly irreversible deformation. As the load is increased further, the curve exhibits the third segment showed by a significant change in slope. These curves look different from those obtained on thick specimens. In this respect, the behaviour of a thin wood specimen subjected to cyclic type tensile loading along its longitudinal direction is also illustrated. Based on wood microstructure, a model is presented to interpret the evolution of the Young's modulus of a wood fiber during tensile loading. The model considers wood as an assembly of cylindrical fibers pasted together in a longitudinal direction. We have assumed the cell wall to comprise only an S₂ layer made of a composite material consisting of a lignin and hemicellulose matrix reinforced by helical microfibrils along the fiber. Furthermore, it is assumed that the microfibril angle a in the S₂ layer is not uniform along the fiber axis and matrix degradation occurs in the zones where the microfibril angles are bigger. The validity of this assumption is verified by using holographic interferometry to visualize the displacement field of the specimen's surface under tension.The work reported in this paper is supported by a grant from the Swiss National Science Foundation, and its support is gratefully acknowledged     

The effects of within-species and between-species variation in wood density on the photodegradation depth profiles of sugi (Cryptomeria japonica) and hinoki (Chamaecyparis obtusa)

Low density wood is more rapidly eroded than denser wood when exposed to the weather, possibly because it is more susceptible to photodegradation. Fourier transform infrared microscopy was used to examine: (1) the depth of photodegradation in earlywood and latewood of sugi (Japanese cedar) and earlywood of hinoki (Japanese cypress) exposed for up to 1500 h to artificial sunlight emitted by a xenon lamp (375 W/m² within the 300 to 700 nm spectral range); and (2) the relationship between the density of wood tissues and depth of photodegradation. The depth of photodegradation varied between species (sugi and hinoki) as well as within a growth ring (sugi earlywood and latewood), and there was an inversely proportional relationship between depth of photodegradation and wood density. These findings may explain why low density earlywood is more rapidly eroded than latewood during weathering, and more generally, why there is an inverse relationship between the density of wood species and their rate of erosion during artificial and natural weathering. Part of this work was presented at the 54th Annual Meeting of the Japan Wood Research Society, Sapporo, August 2004     

Microfibril angle and density of hinoki (Chamaecyparis obtusa) trees in 15 half-sib families in a progeny test stand in Kyushu, Japan

It was previously believed in Japan that the wood qualities of hinoki (Chamaecyparis obtusa) were superior to sugi (Cryptomeria japonica). However, few studies of wood properties such as MFA (microfibril angle of S₂ layer in secondary wall of tracheid) have been completed for hinoki. Some reports have found that hinoki plus tree families have similar mechanical properties to sugi. Here we report the characteristics of MFA and density of hinoki half-sib families in a progeny test stand. There were significant differences in MFA and density between families. The wood properties of two families, Nakatsu 3 and Kanzaki 5, are stable in radial pattern and suitable for structural use. Early selection of hinoki families by MFA and density may be difficult. Effects of MFA and density on E _d (dynamic modulus of elasticity) of logs differed between families. The effects of growth rate on MFA and density differed between families and also between juvenile and mature wood. The faster growth rate in Nakatsu 3 appeared to improve wood properties and increase E _d of logs, although in many other families, faster growth rate had negative effects on desirable wood properties for structural use.     

Microfibril angle non-uniformities within normal and compression wood tracheids

The pattern and extent of variation of microfibril angle (MFA) in normal and compression tracheids of softwood were investigated by using confocal laser scanning microscopy technique. All measurements support the idea that the orientation of microfibrils in single wood tracheids is not uniform. MFA of the radial wall of earlywood tracheids was highly non-uniform and had an approximately circular form of arrangement around the bordered pits (inside the border). Between the bordered pits the measured MFAs were less than the other parts of the tracheid. In the latewood tracheids MFA was less variable. The average orientation of simple pits in the crossfield region was consistent with the mean MFA of the tracheids; however some of the measurements showed a highly variable arrangement in the areas between the simple pits. In many cases the local measured MFAs of compression wood tracheids agreed with the orientation of natural helical cavities of compression wood. Comparing the measured results in different growth rings showed that MFAs in juvenile wood are generally larger than in perfect wood.     

Reflection and transmission of visible light by sugi wood: effects of cellular structure and densification

Transmittance and reflectance of visible light by sugi wood (Cryptomeria japonica) were investigated in the longitudinal (L) and tangential (T) directions. Transmittance was the highest in the L direction and reflectance was the highest in the T direction, suggesting that structural anisotropy influences transmittance and reflectance. Intra-ring variations observed with a microspectrometer indicated that T transmittance was higher for latewood than for earlywood, but there was no such trend in for L transmittance in which the highest levels occurred near the annual ring boundaries, on either the earlywood or latewood side, and the lowest at the transition from earlywood to latewood. Dependence of L transmittance on wavelength also showed variations according to the intra-ring position. The increasing of transmittance of earlywood at wavelengths?<?500 nm with increasing wavelength was observed, but this was not confirmed for latewood because of absorption by lignin. These observations supported a previously published finding, which was based on measurements in the radial direction, that the number of internal cell wall reflections, rather than density, determines wood lightness. Indeed, in the L direction, most of the incident light passes through lumens in earlywood and through cell walls in latewood, while it is subjected to numerous internal reflections at the interface between lumens and cell walls. This was further confirmed by the transmittance of earlywood being greatly decreased by radial compression.     

Predicting anisotropic shringkage of softwood Part 1: Theories

 The phenomenon of wood shrinkage by losing moisture can be analysed at four levels: molecular, ultrastructural, microstructural and macrostructural levels. To predict the shrinkage of wood cells, the model of Barber and Meylan is modified in the current work to reflect combined effects of shrinkage of the cell wall, changes of the lumen shape and effects of rays and bordered pits. Where a piece of wood contains a multi-layer of earlywood and latewood or multi-layer of normal and defect wood with variable properties, a model is proposed to relate the total, measurable shrinkage to the shrinkage of each layer. The model can be applied to a specimen with asymmetric properties through the thickness. In such a board, bow (or crook), cup and twist are often observed. The modified and proposed models involve several mechanical properties of the cell wall which are difficult to measure. These properties vary with wood types, such as earlywood, late wood, compression wood, or wood with spiral grain. However, an alternative method may be used to obtain these properties from experimentally measured shrinkage data, and this method will be presented in a subsequent paper. Received 25 January 1999     

Effect of annual rings on transmission of 100 GHz millimeter waves through wood

For the application of millimeter wave (MMW) technique to nondestructive evaluation of wood, the effect of annual rings on the behavior of a 100 GHz MMW transmitted through wood was examined. The complex amplitude was measured for 2 mm thick flat- (LT) and quarter-sawn (LR) specimens of several species with different annual ring structures at 11 % moisture content, of which the density distribution was measured using X-ray radiography. For the LT specimens of all species and the LR specimens with small density fluctuation or with earlywood width smaller than the wavelength of the MMW (=3 mm), the amplitude and phase of the transmitted wave were similar to those of the wave without a specimen. For the LR specimen with large density fluctuation and with earlywood width close to or larger than the wavelength, the amplitude and phase were different from those of the wave without a specimen. All the measured complex amplitudes were well expressed using a diffraction model. It was concluded that the MMW is deformed by the density distribution, and then its components with periods shorter than the wavelength by diffraction are attenuated.     

Fibre morphological effects on mechano-sorptive creep

The increased creep rate of paper under load during moisture cycling conditions as compared to that at high constant humidity is a problem in the use of packaging materials. In order to investigate the influence of morphological factors of the fibres on the occurrence and magnitude of this phenomenon, i.e. the occurrence of mechano-sorptive creep, studies on wood fibres isolated from different parts of spruce wood were performed. Thus, creep properties were studied on earlywood and latewood fibres from both juvenile wood and mature wood. In general, latewood fibres showed a higher degree of mechano-sorptive creep than earlywood fibres, and mature wood showed a higher degree of mechano-sorptive creep than juvenile fibres. The difference in mechano-sorptive creep rate between different fibres was shown to be correlated to the differences in fibril angle. The smaller the fibril angle the higher was the mechano-sorptive creep ratio. It was suggested that at fibril angles approaching 45° wood fibres do not exhibit mechano-sorptive creep.     

Morphological changes of Japanese beech treated with the ionic liquid, 1-ethyl-3-methylimidazolium chloride

The morphological changes in wood tissues of Japanese beech (Fagus crenata) upon treatment with the ionic liquid, 1-ethyl-3-methylimidazolium chloride ([C2mim][Cl]), which can dissolve cellulose, were investigated. Treatment with [C2mim][Cl] induced significant swelling of all wood tissues. However, the swelling behavior of wood fibers was different from that of vessels. Intervascular pits were occluded, and pit membranes in ray-vessel pits were broken after treatment with [C2mim][Cl]. No significant differences in swelling behavior were found between latewood and earlywood, although different morphological changes for latewood and earlywood during [C2mim][Cl] treatment were seen in our previous studies on sugi (Cryptomeria japonica). We have found that the effects of [C2mim][Cl] on Japanese beech tissues are inhomogeneous and different from those found for other wood species.     

Modeling wood fiber deformation caused by vapor expansion during steam explosion of wood

Steam explosion is a process used to enhance enzyme penetration and digestibility of wood. Wood chips are processed with high-pressure steam for a limited time, and the bonding between polysaccharides and lignin is weakened. After this processing, the pressure is rapidly reduced to induce steam explosion where the vapor inside a fiber expands and exerts pressure on the fiber walls. This pressure causes fiber deformation and breakage. In this study, fiber deformation caused by vapor expansion was simulated by single wood fibers using finite element modeling. When pressure is applied inside a fiber, it is likely to break from the corner and midway between two adjacent corners. The fiber is modeled with four layers (P, S1, S2, and S3). Although the P, S1, and S3 layers are very thin, they significantly prevent fiber deformation. The fibers with a thin wall and a low micro-fibril angle (MFA) deform more than the fibers with a thick wall and a higher MFA. It was found that the shape of the fiber plays an important role in its deformation. The areas of localized strain are the most likely places for fiber splitting. Essentially, fiber wall damage is more likely to occur in (1) thin-walled fibers, i.e., earlywood, (2) fibers with damaged P and S1 layers, (3) fibers with low MFAs, and (4) fibers with irregular cross-sections. Different chemical pretreatments, fractionation procedures, and selections of raw materials can accordingly be considered to produce easily steam-exploded materials.     

Properties of the cell wall constituents in relation to the longitudinal elasticity of wood

This study examined the origin of the moisture dependency of the longitudinal Youngs modulus of wood (E _L ) in relation to the microfibril angle (MFA) of the S2 layer of the secondary wall. Microtomed early wood specimen of sugi (Cryptomeria japonica D.Don) were used for the experiment. The following was revealed:

The effect of axial strain on crystalline cellulose in Norway spruce

The effect of strain on dry, clear Norway spruce (Picea abies [L.] Karst.) wood was studied by tensile testing along the cell axis and by in situ X-ray diffraction measurements. The mean microfibril angle (MFA) was initially 3–12 degrees and did not decrease due to strain. Based on the positions of the reflections 200 and 004 of crystalline cellulose, cellulose chains elongated and the distance between the hydrogen bonded sheets of chains decreased due to the strain. The elongation of the unit cell parallel to the cellulose chains was twice as high in juvenile wood as in mature wood. The (X-ray) Poisson ratio ν _ca for crystalline cellulose in Norway spruce was calculated from the deformation of the unit cell. The average ν _ca of earlywood was 0.28 ± 0.10 in juvenile wood and 0.38 ± 0.23 in mature wood. In latewood, the average ν _ca was 0.48 ± 0.10 in juvenile wood and 0.82 ± 0.11 in mature wood. The average ν _ca values were not directly correlated to the crystallite dimensions or to the mean MFA in juvenile and mature earlywood and latewood. The results show that the amorphous matrix has a definite effect on the deformation of cellulose crystallites.     

Transverse swelling behavior of hinoki (Chamaecyparis obtusa) revealed by the replica method

Transverse swelling and its anisotropy in hinoki (Chamaecyparis obtusa) in several kinds of organic liquids and in water were investigated by means the replica method. There was more cross-sectional swelling of cell walls and cell wall thickness in earlywood than in latewood. Marked swelling toward cell lumens was observed in wood swollen in liquids that had higher swelling potentials than water. This suggests that the swelling of cell walls in these liquids is much greater than the external swelling. Feret's diameters of the cell lumens were reduced by swelling in all the observed cases except in the tangential direction of earlywood, suggesting that cell walls swell to a much less extent in width than in thickness. Deformation of cell shapes caused by the tensile force from the latewood were observed in the earlywood and in the transitional region from earlywood to latewood. When swollen in water, transverse swelling anisotropy caused only by the swelling in cell wall thickness were calculated to be 1.2 for the whole region over an annual ring and 1.4 for the earlywood. These values could not account for the external swelling anisotropy of 2.1. Considering obvious deformations of cell shapes in the earlywood and in the transitional region, we conclude that the interaction between earlywood and latewood is one of the prime factors contributing to the transverse swelling anisotropy of coniferous wood.Part of this report was presented at the 48th Annual Meeting of the Japan Wood Research Society, Nagoya, April 1998     

Modelling moisture-related mechanical properties of wood Part II: Computation of properties of a model of wood and comparison with experimental data

Starting with simple concepts of the molecular structure and models of the stiffness and swelling behaviour of lignin, hemi-cellulose and cellulose and building up through the various levels of organisation in the wood cell wall a model has been constructed that simultaneously predicts the variation with moisture content change of both the longitudinal Young's modulus and longitudinal shrinkage of wood. The model closely predicts both longitudinal shrinkage and Young's modulus as they vary with the moisture content of the wood. The model also takes into account structural variations in the form of changes in cell wall layer thicknesses and mean cellulose microfibril orientation.     

Cantilever experimental setup for rheological parameter identification in relation to wood drying

Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed by authors for rheological parameter identification.     

Pilot Study to Examine Radial Variation in Annual Ring Width, Density and Shrinkage

The annual ring width, density and shrinkage variation from pith to bark in Chinese fir (Cunninghamia lanceolata) and Boka sugi (Cryptomeria japonicd) were studied and compared. The results show that the ring width decreased sharply from pith to bark for Chinese fir. However, the ring width variation pattern for Boka sugi followed a different way, i.e., the ring width decreased to the fifth ring, increased to the tenth ring, decreased again to the fifteenth, and then increased to the twentieth, where it became constant. The large variations of Boka sugi appeared to show the maintenance of fast growth for many years. The annual ring mean density of Chinese fir increased gradually from pith to bark. However, the density changes for Boka sugi indicated the opposite trend, i.e., the mean density decreased gradually from pith to bark. The former showed a pattern as the same as a pine and a larch, and the latter was often found in a cedar and a cypress. The longitudinal shrinkage in juvenile wood was much hig