Volume 63, Issue 2

No abstract available

The mechanical performance of wood and wood products is highly dependent on the structural arrangement and properties of the polymers within the fibre cell wall. To improve utilisation and manufacture of wood materials, there is an increasing need for a more detailed knowledge regarding structure/property relations at the micro- or nanostructural level. In this article, recent developments regarding our understanding of the wood cell wall structure and its mechanical performance are summarised. The new results are interpreted in relation to property performances of wood fibres and wood tissues. Suggestions are made for future requirements for research in this field.

Wood exhibits a hierarchical architecture. Its macroscopic properties are determined by microstructural features at different scales of observation. Recent developments of experimental micro-characterisation techniques have delivered further insight into the appearance and the behaviour of wood at smaller length scales. The improved knowledge and the availability of increasingly powerful micromechanical modelling techniques and homogenisation methods have stimulated rather comprehensive research on multiscale modelling of wood. Linking microstructural properties to macroscopic characteristics is expected to improve the knowledge of the mechanical behaviour of wood and to serve as the basis for the development of innovative wood-based products and for biomimetic material design. Moreover, understanding fundamental aspects of wood machining requires multiscale approaches which can take into account the heterogeneity, anisotropy and hierarchies of wood and wood composites. In this review, recent developments in the field of hierarchical modelling of the hygroelastic behaviour of wood are discussed, and a short outline of the theoretical background is given. Much focus is placed on composite micromechanical models for the wood cell wall and on multiscale models for wood resting upon hierarchical finite element models and on the application of continuum micromechanics, respectively. These models generally lead to the specification of equivalent homogeneous continua with effective material properties. Finally, current deficiencies and limitations of hierarchical models are sketched and possible future research directions are specified.

Fracturing of wood and its composites is a process influenced by many parameters, on the one hand coming from the structure and properties of wood itself, and on the other from influences from outside, such as loading mode, velocity of deformation, moisture, temperature, etc. Both types of parameters may be investigated experimentally at different levels of magnification, which allows a better understanding of the mechanisms of fracturing. Fracture mechanical methods serve to quantify the fracture process of wood and wood composites with different deformation and fracturing features. Since wood machining is mainly dominated by the fracture properties of wood, knowledge of the different relevant mechanisms is essential. Parameters that influence the fracture process, such as wood density, orientation, loading mode, strain rate and moisture are discussed in the light of results obtained during recent years. Based on this, refined modelling of the different processes becomes possible.

Predictive simulation of crack growth and fracture of wood and wood composites has always been difficult, and it has been limited by the availability of appropriate fracture models. As summarized herein, progress has been made on several fronts. First, a variety of fictitious crack model refinements have been made, along with the corresponding effects on bulk load-deformation response and R -curve behavior. Second, progress has been made on several different discrete element approaches that can explicitly represent material heterogeneity and variability. While progress has been substantial, a universal fracture law for wood remains elusive.

The data available in the literature concerning wood cutting forces permits to build models or to simulate the main wood machining processes (milling, sawing, peeling, etc.). This approach contributes to a better understanding of formation of wood surfaces and chips and the data may be helpful to optimise cutting geometry, reduce tool wear, improve tool material, and to size tool-machines. The models may also be useful for industrial application in two ways: (1) providing data to optimise the settings for a given operation (batch approach), and (2) building predictive models that could be the basis of an online control system for the machining processes (interactive approach). A prerequisite for this is that numerous machining tests on different wood materials are performed based on experiences with different kind of tools and experimental devices. With a focus on potential industrial applications, the emphasis of this review was on the wood peeling process, which is a very demanding special case of wood cutting. Although not so many industrial machines are equipped with expensive force sensors, there is a lot of high quality information available about cutting forces which may be useful to improve the scientific or technological knowledge in wood machining. Alternative parameters, such as vibration or sound measurements, appear to be promising substitutes in the praxis, particularly to feed online control systems of any wood cutting process.

Historical studies for predicting cutting forces in wood processing are based on the Piispanen/Ernst-Merchant theory employed in metal cutting where the offcut/chip is formed in shear. This analysis has been recently improved to include significant work of surface separation and formation (i.e., the fracture toughness of the workpiece, as well as the shear yield stress and friction). The new theory is applied here to wood cutting experiments. It is well known that chip formation and surface damage depend on grain orientation and chip thickness, but experiments reveal that chip formation alters with cutting speed as well. During the COST E35 action a series of experiments and special devices to orthogonally cut wood at high and low speed have been developed. In this paper, an overview of the cutting devices and the main results are given.

Recent progress in research on the cutting processes of wood is presented, particularly in regard to new methods of accomplishing an increase in volume recovery by sawmills. The suitability of modern fracture mechanics was demonstrated for the estimation of the cutting power for saws in which kerfs are different from saws used in cutting tests, and whereby the data of sawn material – such as the specific work of surface formation (toughness) and the shear yield stress – were determined. Cutting tests were carried out on the modern frame sawing machine (sash gang saw) for narrow-kerf sawing with the hybrid dynamically balanced driving system and elliptical teeth trajectory movement.

Wood, like all polymeric materials, shows viscoelastic behaviour. The time dependent behaviour of wood depends on material anisotropy, temperature, moisture and stresses. To predict the behaviour of wood, numerous mathematical models have been developed largely relying on experimental results. In this paper, time dependent viscoelastic behaviour of wood is reviewed under constant and cyclic climatic conditions, separately. More emphasis is given on results obtained in recent years on the behaviour of thin wood tissues, single fibres, thermo-viscoelasticity of wood, influence of hemicelluloses and the modelling of the effect of transient moisture at the molecular level on the mechanical response.

Wood surfaces might be considered as geometrical structures combined with position dependent chemical properties. It is possible to access surface properties only from a methodical point of view, e.g., by measuring roughness, colour, surface energy and other features. This type of characterisation remains incomplete because it does not account for the complex interactions of the properties, which are caused by the machining process. A better approach might be to describe the surface as a system of time dependent properties including the history of machining operations performed for a specific purpose. This work presents the advancements in the understanding of wood surfaces achieved in the course of the COST Action E35. The interaction between raw material, machining, and techniques of measurement will be discussed in terms of surface characteristics including the tactile properties of wood surfaces.

An analytical model has been applied to calculate the acquired strain energy density in order to achieve a certain damage state in a softwood fibre by uniaxial tension or shear load. The energy density was found to be dependent on the microfibril angle in the middle secondary wall, the loading case, the thicknesses of the fibre cell wall layers, and conditions, such as moisture content and temperature. At conditions, prevailing at the entrance of the gap between the plates in a refiner and at relative high damage states, more energy is needed to create cracks at higher microfibril angles. The energy density was lower for earlywood compared to latewood fibres. For low microfibril angles, the energy density was lower for loading in shear compared to tension for both earlywood and latewood fibres. Material parameters, such as initial damage state and specific fracture energy, were determined by fitting of input parameters to experimental data.

The asymmetric four-point bending test for obtaining shear properties of wood was examined. Rectangular bars with various notches on top and bottom surfaces of western hemlock ( Tsuga heterophylla Sarg.) were prepared as a specimen. The shear modulus, proportional limit stress, and shear strength were measured. The influence of notch configuration on the shear properties was examined independently by finite element calculations. The influence of notch configuration was not significant in experiments, while it was found relevant in finite element analyses. Although the pure shear stress condition could not be realized after the initiation of failure, the measurement of shear properties of solid wood simply by the asymmetric four-point bending test was feasible.

In this work, the identification of the shear properties of maritime pine ( Pinus pinaster Ait.) wood was investigated by applying the Arcan test method. For this purpose, an ad hoc Arcan fixture was designed for which clear wood samples could be tested along all the material planes of symmetry – LR , LT and RT planes. For the accurate evaluation of the shear moduli, correction factors taking into account the non-uniformity of the shear stress and the shear strain distributions at the gauge section were determined with finite element analyses. Experimentally, two 0/90 rosettes were glued on both frontal and back surfaces of the specimens in order to take into account any through-to-thickness shear strain variation. It was found that the Arcan test is suitable for the identification of the shear moduli of wood in all its natural planes. The failure of the Arcan specimens systematically occurred under a concentrated stress state nearby the V-notches. Nevertheless, it was shown that an accurate estimation of the shear strengths of wood can be achieved by the Arcan test.

This study examined the effects of inactive ions – ions not reacting with wood – during kraft cooking, which thus far have received modest attention. Six different sodium salts were added to kraft cooking liquors at two different levels of alkalinities. Delignification as well as the formation and degradation of hexenuronic acid (HexA) were observed of Scots pine sapwood meal. The delignification rate was greatly affected by the presence of additional anions. Chloride ions had the greatest retarding effect, while the addition of polyacrylate ions had almost no effect. When carbonate, sulphate and lactate ions were added to the liquors, the delignification rates were in-between the series with chloride and polyacrylates ions. We suggest that the anions affect the solubility of lignin fragments in analogy to the Hofmeister effect observed in various macromolecular systems in the presence of dissolved salts. When the reactions involving HexA were examined, the opposite results were obtained. In that case, the highest reaction rates were in the presence of chloride ions, and the lowest rates were obtained when no extra ions were added, and the second lowest rates were obtained in the presence of polyacrylate ions. As for delignification, the cooking series containing carbonate, sulphate and lactate ions had a reaction rate in-between the series with the highest and lowest rates. The differences obtained with various inorganic ions can be qualitatively explained by the Donnan equilibrium theory and by the variation in activity coefficients of hydroxide ions. Findings imply that knowledge of kraft cooking is far from complete. The effects of inactive ions on reaction kinetics should also receive more attention.

Lignin α-carbonyl groups play an important role in light-induced yellowing as initial absorber of UV light. The model compounds vanillin, verataldehyde, and acetovanillone were studied and the reaction rate coefficient k was calculated for the addition of bisulphite to carbonyls. UV-Vis spectra of the model compounds in bisulphite solution were measured between 200 and 400 nm and at a single wavelength of 305 nm. The reaction was fastest with vanillin, but only a minor part of acetovanillone reacted, which can be explained with different reactivity of aldehydes and ketones. Calculated reaction rate coefficients were 26×10 -3  s -1 and 18×10 -3  s -1 for vanillin and verataldehyde, respectively. Sheets from chemi-thermomechanical pulp (CTMP) were treated with bisulphite solutions (10 mM, 20 mM and 0.1 M; for 1 and 5 min) in order to study the potential of this treatment in retarding yellowing of CTMP. Initial brightness was improved with the treatments, but as the brightness loss was even more drastic, stabilization was not achieved. FT-IR spectra of bisulphite treated samples revealed that only part of the carbonyl groups reacted. The unsuccessful light stabilization of the pulp originated probably from an inadequate carbonyl addition, a problem which could not be resolved.

The performance of a fiber-reinforced polymer composite depends not only on the properties of the fiber and matrix polymer but also on the interfacial properties. Nanoindentation and numerical simulation in our previous study revealed that the transition zone of the interfacial region between the regenerated cellulose fiber (lyocell fiber) and polypropylene (PP) was less than 1 μm. Interfacial zone was modified with maleic anhydride-grafted PP (MA-g-PP) and γ-amino propyl trimethoxy silane (γ-APS). In the present study, the interfacial zone is investigated by means of contrast imaging techniques based on phase and thermal conductivity in the context of atomic force microscopy. According to the obtained images, the widths of the interfacial zone modified with MA-g-PP were approximately 113–128 nm and modification with MA-g-PP and γ-APS led to an interfacial zone of 109–173 nm.

The pyrolytic conversion of pine wood at mild temperatures between 200°C and 300°C was investigated by transmission electron microscopy (TEM). Based on TEM imaging and image analysis, a novel method was developed for determining the local orientation of the cellulose microfibrils in the secondary wall S2 which gives a measure for the progression of pyrolytic conversion of the cell wall. Elemental composition of pyrolysed specimens was determined up to 600°C. TEM imaging together with the evaluation of the elemental composition shows that first the polyoses are degraded, while the cellulose microfibril orientation is still visible up to 225°C. The cellulose microfibrils could not be observed at temperatures higher than 250°C, while lignin containing compound middle lamella (CML) was still visible. After a gradual decrease of the CML up to 275°C, the cell wall became entirely isotropic beginning at 300°C. Based on the presented results, we propose an early degradation of the supramolecular structure of the cell wall.

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