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International audience; Living wood in the tree performs a muscular action by generating forces at the sapwood periphery and residual strains in dead sapwood fibres. Dissymmetric force generation around the tree trunk is the motor system allowing movement, posture control and tree reshaping after accidents. Rather young trees are able to restore the verticality of their trunk after accidental rotation of the soil-root system due to wind or landslide, leading to typically curved stems shape. The very high dissymmetry of forces for the motor action is associated with the occurrence of reaction wood on one side of the inclined stem during many successive years. A method to reconstitute this biomechanical history from observations after tree felling, on either green or dry wood, is discussed. A selection of 17 trees from 15 different species (13 different families), tropical and temperate, hardwoods and softwoods, were selected and peripheral residual strains were measured in situ before felling, on 8 positions for each stem. Matched wooden rods were sawn and measured for their mechanical and physical properties in the green and dry states, allowing the estimation of tree growth stress, i.e., the force created by the living wood. It was possible to build easy-to-use conversion coefficients between the growth stress indicator (GSI), measured in situ by the single hole method, and growth strain and growth stress with the knowledge of basic density and green longitudinal elastic modulus. Maturation strain, specific modulus (as a proxy of micro-fibril angle) and longitudinal shrinkage are properties independent from basic density, whose variation among species was very large. For the whole range of compression wood, normal wood and tension wood, strong relationships between these 3 properties were observed, but together no single model, based on cell-wall microfibril angle only, could be defined. Growth forces are the product of 4 parameters: ring width, basic density, basic specific modulus and maturation strain, all of them being the result of wood formation. Thanks to the wide range of wood types and species, simple and highly significant formulas were obtained for the relationship between basic and dry density, green and dry longitudinal modulus of elasticity, basic and dry specific modulus. To estimate ring width in the green state from values in dry state, radial shrinkage needs to be measured afterwards on dry specimens. Maturation strains is less accurately linked to late measurements on dry wood, but longitudinal shrinkage offers a rather good solution for an estimation provided that the wood type (softwood, hardwood with-G layer, hardwood without G-Layer) is known.

International audience; The anisotropy of vibrational properties influences the acoustic behaviour of wooden pieces and their dependence on grain angle (GA). As most pieces of wood include some GA, either for technological reasons or due to grain deviations inside trunks, predicting its repercussions would be useful. This paper aims at evaluating the variability in the anisotropy of wood vibrational properties and analysing resulting trends as a function of orientation. GA dependence is described by a model based on transformation formulas applied to complex compliances, and literature data on anisotropic vibrational properties are reviewed. Ranges of variability, as well as representative sets of viscoelastic anisotropic parameters, are defined for mean hardwoods and softwoods and for contrasted wood types. GA-dependence calculations are in close agreement with published experimental results and allow comparing the sensitivity of different woods to GA. Calculated trends in damping coefficient (tanδ) and in specific modulus of elasticity (E′/ρ) allow reconstructing the general tanδ-E′/ρ statistical relationships previously reported. Trends for woods with different mechanical parameters merge into a single curve if anisotropic ratios (both elastic and of damping) are correlated between them, and with axial properties, as is indicated by the collected data. On the other hand, varying damping coefficient independently results in parallel curves, which coincide with observations on chemically modified woods, either "artificially", or by natural extractives.

International audience; Tension wood of Laetia procera (Poepp.) Eichl. (Flacourtiaceae), a neo-tropical forest species, shows a peculiar secondary wall structure, with an alternance of thick and thin layers, while opposite wood of this species has a typical secondary wall structure (S1 + S2 + S3). Samples for the study of microstructural properties were collected upon the estimation of growth stresses in the living tree, in order to analyze the correlation of the former with the latter. Investigation using optical microscopy, scanning electron microscopy and UV microspectrophotometry allowed the description of the anatomy, ultra-structure and chemistry of this peculiar polylaminate secondary wall. In the thick layers, cellulose microfibril angle is very low (i.e., microfibril orientation is close to fibre axis) and cellulose microfibrils are well organized and parallel to each other. In the thin layers, microfibrils (only observable in the inner layer) are less organized and are oriented with a large angle relative to the axis of the cell. Thick layers are lightly lignified although thin layers show a higher content of lignin, close to that of opposite wood secondary wall. The more the wood was under tensile stress, the less the secondary wall was lignified, and lower the syringyl on guaiacyl lignin units' ratio was. The innermost layer of the secondary wall looks like a typical S3 layer with large microfibril angle and lignin occurrence. The interest of this kind of structure for the understanding of stress generation is discussed.

Shrinkage of green cane (Arundo donax L.) was measured during air-drying at room temperature. The cane began to shrink at 150% moisture content due to a remarkable collapse of parenchyma cells. The collapse recovered after boiling in water, but more serious collapse (recollapse) was induced by the following drying. On the other hand, the collapse recovered almost completely after steaming with saturated water vapor at 92°-96°C without recollapse. By comparing the thickness of cane specimens before and after steaming, the degree of cell collapse remaining in dry cane was evaluated. When the green cane was frozen prior to drying, the degree of collapse was reduced whereas the drying rate remained unchanged. The effect of prefreezing was interpreted as the generation of air bubbles in the cell lumen which hinder the effective loading of liquid tension on the cell wall. Even when the cane was carefully dried using a conventional method used by reed manufacturers, the degree of collapse was very large and it increased with elevating internode position.

This study aims at better understanding the respective influences of specific gravity (γ), microfibril angle (MFA), and cell wall matrix polymers on viscoelastic vibrational properties of wood in the axial direction. The wide variations of properties between normal wood (NW) and compression wood (CW) are in focus. Three young bent trees (Picea abies, Pinus sylvestris and Pinus pinaster), which recovered verticality, were sampled. Several observed differences between NW and CW were highly significant in terms of anatomical, physical (γ, shrinkage, CIELab colorimetry), mechanical (compressive strength), and vibrational properties. The specific dynamic modulus of elasticity (E′/γ) decreases with increasing MFA, and Young’s modulus (E′) can be satisfactorily explained by γ and MFA. Apparently, the type of the cell wall polymer matrix is not influential in this regard. The damping coefficient (tanδ) does not depend solely on the MFA of NW and CW. The tanδ – E′/γ relationship evidences that, at equivalent E′/γ, the tanδ of CW is approximately 34% lower than that of NW. This observation is ascribed to the more condensed nature of CW lignins, and this is discussed in the context of previous findings in other hygrothermal and time/frequency domains. It is proposed that the lignin structure and the amount and type of extractives, which are both different in various species, are partly responsible for taxonomy-related damping characteristics.

International audience; A feasibility study of glulam was carried out in French Guiana using local wood species. The aim was to determine gluing parameters affording satisfactory behaviour to manufactured glulam in a tropical climate. Three abundant wood species, with special properties, were selected for the study and Resorcinol-Phenol-Formaldehyde resin was used for bonding. Three industrial parameters were considered: adhesive spread rate, closed assembly time and gluing pressure. Delamination and shearing tests were carried outin accordance with European Standards. The tests revealed the influence of wood properties and manufacturing parameters on joint resistance. In fact, the results showed that specific gravity and the shrinkage coefficientgreatly influenced the gluing step. Indeed, wood with a medium specific gravity needed more adhesive and more pressure than wood with a high specific gravity. In addition, planning and lamella thicknesswere found to affect glue joint resistance.

Context : Increased knowledge on diversity in wood properties would have implications both for fundamental research and for promoting a diversification of uses as material. Aims : The objective is to contribute to overcoming the critical lack of data on the diversity of wood dynamic mechanical/viscoelastic vibrational properties by testing lesser known species and categorising sources of variability. Methods : Air-dry axial specific dynamic modulus of elasticity (E′/γ) and damping coefficient (tanδ) were measured on a wide sampling (1,792 specimens) of 98 wood types from 79 species. An experimental device and protocol was designed for conducting systematic (i.e. rapid and reproducible) characterisations. Results : Diversity at the specimens' level corroborates the "standard” relationship between tanδ and E′/γ, which is discussed in terms of orientation of wood elements and of chemical composition. Diversity at the species level is expressed on the basis of results for normal heartwood, with specific gravity (γ) ranging from 0.2 to 1.3. Axial E′/γ ranges from 9 to 32GPa and tanδ from 4 × 10−3 to 19 × 10−3. Properties distribution follows a continuum, but with group characteristics. The lowest values of tanδ are only found in certain tropical hardwoods. Results can also suggest alternative species for musical instruments making. Conclusion : The variations in specific gravity, in stiffness or in "viscosity” appear to be predominantly linked to different levels of diversity: between species or between wood types (reaction wood or taxonomy-related differences in heartwood extractives)

The radial trends of vibrational properties, represented by the specifi c dynamic modulus (E /?) and damping coeffi cient (tan ?), were investigated for three tropical rainforest hardwood species (Simarouba amara, Carapa procera, and Symphonia globulifera) using free-free fl exural vibration tests. The microfi bril angle (MFA) was estimated using X-ray diffraction. Consistent patterns of radial variations were observed for all studied properties. E /? was found to decrease from pith to bark, which was strongly related to the increasing pith-bark trend of MFA. The variation of tan ? along the radius could be partly explained by MFA and partly by the gradient of extractives due to heartwood formation. The coupling effect of MFA and extractives could be separated through analysis of the log(tan ?) versus log(E /?) diagram. For the species studied, the extractive content putatively associated with heartwood formation generally tends to decrease the wood damping coeffi cient. However, this weakening effect of extractives was not observed for the inner part of the heartwood, suggesting that the mechanical action of extractives was reduced during their chemical ageing.

International audience; The viscoelastic properties of the cell wall of the tropic hardwood Carapa procera are investigated by means of nanoindentation tests. Three types of nanoindentation tests are undertaken: creep, continuous stiffness measurement (CSM) and nanoscale dynamic mechanical analysis (Nano-DMA), corresponding to the increased loading rate and so the response of wood cell wall to the loading in a relatively large time scale. It is found that the creep rate is dependent on the applied stress and the relation can be described by the rule of power law. Regarding the dynamic properties (i.e., storage modulus and damping coefficient) in the frequency range of 10-240 Hz, it is shown that the storage modulus increases monotonically, while the damping coefficient decreases. By using the traditional dynamic mechanical thermal analysis as a reference method, the phase transition behavior of wood cell wall can be successfully characterized by the Nano-DMA in a large frequency scale. A dependence of the storage modulus and damping coefficient on the penetration depth is quantified by the CSM tests.

Grain deviations and high extractives content are common features of many tropical woods. This study aimed at clarifying their respective impact on vibrational properties, referring to African Padauk (Pterocarpus soyauxii Taub.), a species selected for its interlocked grain, high extractives content and uses in xylophones. Specimens were cut parallel to the trunk axis (L), and local variations in grain angle (GA), microfibril angle (MFA), specific Young’s modulus (E′ L /ρ, where ρ stands for the density) and damping coefficient (tanδL) were measured. GA dependence was analysed by a mechanical model which allowed to identify the specific Young’s modulus (E′3/ρ) and shear modulus (G′/ρ) along the grain (3) as well as their corresponding damping coefficients (tanδ3, tanδG). This analysis was done for native and then for extracted wood. Interlocked grain resulted in 0–25° GA and in variations of a factor 2 in E′L/ρ and tanδL. Along the grain, Padauk wood was characterized, when compared to typical hardwoods, by a somewhat lower E′3/ρ and elastic anisotropy (E′/G′), due to a wide microfibril angle plus a small weight effect of extracts, and a very low tanδ3 and moderate damping anisotropy (tanδG/tanδ3). Extraction affected mechanical parameters in the order: tanδ3 ≈ tanδG > G′/ρ > > E′3/ρ. That is, extractives’ effects were nearly isotropic on damping but clearly anisotropic on storage moduli.

COST Action E35 : Fracture mechanics and micromechanics of wood and wood composites with regard to wood machining; 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.

Few studies have been conducted about relation between cellulose para-meters and biomechanical properties of wood in tropical angiosperms species. For this purpose, on 13 trees from 3 species of French Guyana tropical rainforest in a clear active process of restoring verticality, i) growth strains were measured in situ in order to determine the occurrence of tension wood within samples and ii) cellulose structural parameters were estimated on all the samples using X-ray diffraction method. Crystallite size was estimated from the full-width at half-maximum of the Miller index (002) arc diffraction and angle T was measured following Cave’s method. Relationships between these parameters and growth stresses were good and the variations between normal and tension wood were significant, i.e. a lower angle T and a larger crystallite size in tension wood. In order to have a good estimation of the microfibril angle in the main layer of the secondary wall for each species, an experimental calibration was done between angle T and microfibril angle observed with scanning electron microscopy.

Accepted as original article in Journal of Wood Science (6th may 2004); The detachment of gelatinous layer (G-layer), often observed on microtome cross sections, leads some authors believe that G-layer cannot act as the driving force of longitudinal shrinkage in tension wood. The aim of this study was to observe the detachment of G-layer along fibres. Green wood block was cut transversely into two samples. One sample was kept in water and the other oven-dried. One face being common to both samples, detachment of G-layer has been studied on the same fibres. Observations have been performed after blocking deformation by embedding. It reveals that the detachment of G-layer is a cutting effect produced during the first making of the transverse face of the wood block to be embedded. After 100 µm far from this primary surface of the sample no detachment can be observed. Drying shrinkage does not affect or little this detachment. The result seems to explain well why the detachment of G-layer occurs during sectioning using a conventional sliding microtomy. These observations prove the adhesion of G-layer in massive wood and confirm the active role of G-layer in tension wood properties.

Longitudinal four-point creep bending tests were performed on small clear-wood spruce specimens having various microfibrillar angles. Cell-wall compliance was deduced from macroscopic data by accounting for porosity. Time-dependent compliance was converted into complex compliance and rigidity using the value and the slope of the compliance versus logarithm of time. Complex rigidity plots of all specimens, for the time range 103–106 s, could be superimposed by a horizontal shift depending on the microfibrillar angle. The shape of complex trajectories allowed a decomposition of the cell-wall relaxation modulus as the sum of an elastic contribution function of the microfibrillar angle and a time-dependent term unrelated to it, and suggested a discussion on the contribution of the various cell-wall layers to the observed relaxation process. To cite this article: J. Gril et al., C. R. Biologies 327 (2004).

The influence of temperature on transverse mechanical properties of 10 tropical species in green condition was studied in radial compression (0 to 99 °C), transverse shear with longitudinal-radial shearing plane and rupture of the longitudinal-tangential plane (20 to 80 °C). Basic density ranged from 0.21 to 0.91 g cm -3. Load-displacement curves were characterised by initial rigidity, yield stress, yield strain and strain energy at 20% strain level. The relation between each criterion and basic density was expressed by a power law. The dependency on temperature evidenced a sharp glassy transition, except for the fracture energy only slightly influenced by temperature. An empirical model allowed evaluating a transition temperature between 51 and 69 °C, depending on the species and the criterion, which was attributed to lignin. Detailed analysis of the apparent modulus in radial compression suggested that complex relaxation phenomena occur around 10 °C and that the rubbery state is not fully reached at 80 °C., L'influence de la température sur les propriétés mécaniques transverses du bois vert de 10 essences tropicales a été étudiée. Trois types d'essais ont été réalisés : compression radiale (entre 0 et 99 °C), cisaillement transverse suivant le plan longitudinal-radial et rupture dans le plan longitudinal-tangentiel (entre 20 et 80 °C). L'infradensité des essences est comprise entre 0,21 et 0,91 g cm -3. Les courbes force-déplacement ont été caractérisées par la rigidité initiale, la contrainte de flambement, la déformation de flambement et l'énergie de déformation pour 20 % de déformation. La relation entre chaque critère et l'infradensité est exprimée par une loi puissance. La dépendance des critères avec la température met en évidence une transition vitreuse très prononcée, excepté pour l'énergie de rupture peu influencée par la température. Un modèle empirique permet d'évaluer une température de transition entre 51 et 69 °C, selon les essences et les critères. Ce phénomène est expliqué par la transition vitreuse des lignines. Une analyse détaillée du module radial apparent en compression suggère qu'un phénomène de relaxation complexe a lieu autour de 10 °C et que l'état caoutchoutique n'est pas complètement atteint à 80 °C.

Atomic Force Microscopy in force modulation mode was used to study the elastic properties of the different fibre wall layers of the tension wood of holm oak and normal wood of boco. The method is based on the measurement of the resonance frequency of the microscope lever in contact with the sampie. This frequency is related to the reduced Young modulus E* = E/(I-v2) of the material, supposed to be isotropie. 'Elastic' images of the cell are obtained simultaneously with the topographie images, which allows the observation of the mechanical properties of the cells at a nanometric scale. Layers G, S1, S2 and ML can clearly be distinguished. By comparison with known materials an estimation of the absolute modulus is given in the range 5-20 GPa, but should be considered with caution, because the inherent anisotropy of the materials has not been taken into account.

Erratum to: Wood Sci Technol (2010) 44:355–367DOI:10.1007/s00226-010-0348-0Due to a processing error the caption of Fig. 1 and the third sentence of theconclusion were incorrect. The description of a system of axis [1, 2, 3] was wronglyreplaced by (Akitsu et al. 1993; Aramaki et al. 2007; Bodig and Jayne 1982). Thecorrected caption and the sentence are given below.Fig. 1 Schematic view of the sampling procedure and of the systems of axis relatedto the trunk and specimens [R, T, L] and to the grain direction [1, 2, 3].Conclusion.(…) Mechanical parameters in the fibre-related system of axis [1, 2, 3]were identified for untreated and extracted Padauk wood. (…)

Chapitre 6; International audience; Reaction wood produces very peculiar maturation stresses at the tree periphery i.e. compressive stress or very high tensile stress, for compression and tension wood respectively, as compared to moderately high tensile stress for normal wood. This means that both its mechanical state and its mechanical and physical properties differ from normal wood. Compression wood shows big differences from normal wood in conifers, for all physical and mechanical properties: higher density and axial crushing strength (MOR) but lower modulus of elasticity (MOE), far higher axial (longitudinal) shrinkage but lower radial and tangential shrinkage, sometimes even lower than the axial shrinkage. For tension wood things are less simple and can vary a lot from hardwood species to species. Globally there are no systematic differences in density and transverse shrinkage, MOE tends to be a little higher while MOR is slightly lower. However, axial shrinkage is much higher for tension wood with a gelatinous layer (G layer) than normal wood due to the specific gel like organization of cellulose microfibrils in the G layer. For tension wood without a G layer(which is rather frequent) axial shrinkage in the first drying operation is around two times higher than in normal wood mainly because of a kind of hygro-thermal release of locked in maturation stress. Overall the very high tensile stress and stored elastic energy in tension wood lead to problemsin wood processing (end splitting and board warping), which is far less the case for compression wood. But due to the large difference in properties relative to normal wood, compression wood occurrence is always a big problem for the in service behaviour of timber, which is seldom the case for tension wood.

Accepted for publication in IAWA Journal (3rd June 2004); The gelatinous layer (G-layer) of tension wood fibres in hardwood contributes to the mechanical function of the living tree and has significant consequences on properties of solid wood. Its size, shape and structure observed by optical or electron microscopy exhibits characteristic anatomical features. However, we found that sectioning of non-embedded wood samples results in an uncontrolled swelling of the G-layer. In order to assess this artefact, the shape and thickness of the G-layer was monitored by serial sections from an embedded wood sample, from its trimmed transverse face to that located several hundreds of micrometers deep. The results revealed that the initial cutting before embedding produced a border effect responsible for the swollen aspect, which is similar to sections from non-embedded material. After a conventional embedding technique was applied, a section of at least 30 micrometers deep in the trimming surface is required to observe an un-swollen G-layer.

Macroscopic longitudinal shrinkage in beech or poplar tension wood is higher than in normal wood. This shrinkage is the result of cell walls layers mechanical interactions. In order to complete the basic data with a view to modelling the cell wall, we are interested in shrinkage differences between cell wall layers and especially of G-layer in poplar and beech. Wood samples in green condition are cut with a razor blade, and then dried before observation. SEM observation shows longitudinal shrinkage much more important in gelatinous layer than in other layers. AFM topographic images of same cells, both in water and in air-dry conditions, confirm this result. Measurements on thin sections allow quantitative results around 4.7 % longitudinal shrinkage for G-layer.