Éditeur(s) : HAL CCSD Elsevier
Résumé : To contribute to the understanding of how opening-mode fractures (joints) form and open or close at depth in layered rocks, we present a 2D numerical study aiming to determine whether tensile stress can develop in pre-fractured elastic multilayers submitted to biaxial compressive strain conditions. First, we investigate the role of the elastic and geometrical properties of the layers on the development of tensile stress in models with five bonded layers and containing one open fracture in the central layer. Our results indicate that, in absence of elastic contrast (in Young's modulus) between the layers, no tensile stress develops in the models. However, when the fractured layer is stiffer than the two adjacent layers directly above and below, a lobe of horizontal tensile stress develops centered on the pre-existing fracture. The creation of this tensile stress is contingent upon the partial closing of the fracture. The levels of tensile stress and the thickness of the lobe of tensile stress increase logarithmically with an increase in the elastic contrast and are systematically larger for a larger Soft/Stiff ratio (ratio of the total thickness of the soft layers with the total thickness of the stiff layers). Second, we investigate the role of fracture interaction in the development of tensile stress in models containing a pair of open fractures. We observe that the levels of tensile stress in the region between the fractures are systematically higher than those observed in identical models containing a single fracture. This increase in tensile stress is very large for small elastic contrasts between the layers but diminishes when the elastic contrast increases. Furthermore, the spacing between the pre-existing fractures plays an important role in the stress distribution in the region between them. When the fracture spacing is equal to or lower than 1.15 times the height of the fractured layer for the experimental conditions chosen, the lobes of tensile stress centered on the fractures coalesce. This results in the formation of vast areas of tensile stress in models under remote compressive loading conditions. Such tensile areas are likely to allow the initiation and propagation of subsequent opening-mode fractures. The results obtained provide new insights into the formation of joints in layered rocks in compressive environments, with important consequences on fluid flow.
ISSN: 0040-1951

hal-00407576
https://hal.archives-ouvertes.fr/hal-00407576
DOI : 10.1016/j.tecto.2006.12.003

Éditeur(s) : HAL CCSD
Résumé :  
Wood samples were taken from the upper and lower sides of 21 naturally tilted trees from 18 families of angiosperms in the tropical rain forest in French Guyana. The measurement of growth stresses ensured that the two samples were taken from wood tissues in a different mechanical state: highly tensile stressed wood on the upper side, called tension wood, and lower tensile stressed wood on the lower side, called opposite wood. Eight species had tension wood fibres with a distinct gelatinous layer (G-layer). The distribution of gelatinous fibres varied from species to species. One of the species, Casearia javitensis (Flacourtiaceae), showed a peculiar multilayered secondary wall in its reaction wood. Comparison between the stress level and the occurrence of G-layer indicates that the G-layer is not a key factor in the production of high tensile stressed wood.
IAWA Journal

hal-01032051
https://hal.archives-ouvertes.fr/hal-01032051

Éditeur(s) : HAL CCSD Springer Verlag (Germany)
Résumé : International audience
*Context: Tree orientation is controlled by asymmetric mechanical stresses set during wood maturation. The magnitude of maturation stress differs between longitudinal and tangential directions, and between normal and tension woods. * Aims: We aimed at evaluating patterns of maturation stress on eucalypt plantation trees and their relation with growth, with a focus on tangential stress evaluation. * Methods: Released maturation strains along longitudinal and tangential directions were measured around the circumference of 29 Eucalyptus nitens trees, including both straight and leaning trees. * Results: Most trees produced asymmetric patterns of longitudinal maturation strain but more than half of the maturation strain variability occurred between trees. Many trees produced high longitudinal tensile stress all around their circumference. High longitudinal tensile stress was not systematically associated with the presence of gelatinous layer. The average magnitude of released longitudinal maturation strain was found negatively correlated to the growth rate. A methodology is proposed to ensure reliable evaluation of released maturation strain in both longitudinal and tangential directions. Tangential strain evaluated with this method was lower than previously reported. * Conclusion: The stress was always tensile along longitudinal direction and compressive along tangential direction, and their respective magnitude was positively correlated. This correlation does not result from a Poisson effect but may be related to the mechanism of maturation stress generation.
Annals of Forest Science

hal-00913611
https://hal.archives-ouvertes.fr/hal-00913611
https://hal.archives-ouvertes.fr/hal-00913611/document
DOI : 10.1007/s13595-013-0318-4

Éditeur(s) : HAL CCSD
Résumé : International audience
Trees can control their shape and resist gravity thanks to their ability to produce wood under tensile stress. This stress is known to be produced during the maturation of wood fibres but the mechanism of its generation remains unclear. This study focuses on the formation of the secondary wall in tension wood produced in artificially tilted poplar saplings. Thickness of secondary wall layer (SL) and gelatinous layer (GL) were measured from cambium to mature wood in several trees sampled at different times after tilting. Measurements on wood fibres produced before tilting show the progressive increase of secondary wall thickness during the growing season. After the tilting date, SL thickness decreased markedly from normal wood to tension wood while the total thickness increased compared to normal wood, with the development of a thick GL. However, even after GL formation, SL thickness continues to increase during the growing season. GL thickening was observed to be faster than SL thickening. The development of the unlignified GL is proposed to be a low cost, efficient strategy for a fast generation of tensile stress in broadleaved trees.
IAWA journal / International Association of Wood Anatomists.

hal-01229277
https://hal.archives-ouvertes.fr/hal-01229277
https://hal.archives-ouvertes.fr/hal-01229277/document
https://hal.archives-ouvertes.fr/hal-01229277/file/AbediniClair-Iawa-TensionCellWallThickening_preprint_HAL.pdf
DOI : 10.1163/22941932-00000084

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In petroleum industry, the difference between pore pressure (Pp) and minimum horizontal stress Sh (termed the seal or retention capacity) is of major consideration because it is often assumed to represent how close a system is to hydraulic failure and thus the maximum hydrocarbon column height that can be maintained. While Sh and Pp are often considered to be independent parameters, several studies in the last decade have demonstrated that Sh and Pp are in fact coupled. However, the nature of this coupling relationship remains poorly understood. In this paper, we explore the influences of the spatial pore pressure distribution on Sh/Pp coupling and then on failure pressure predictions and trap integrity evaluation. With analytical models, we predict the fluid pressure sustainable within a reservoir before failure of its overpressured shale cover. We verify our analytical predictions with experiments involving analogue materials and fluids. We show that hydraulic fracturing and seal breach occur for fluid pressure greater than it would be expected from conventional retention capacity. This can be explained by the impact of the fluid overpressure field in the overburden and the pressure diffusion around the reservoir on the principal stresses. We calculate that supralithostatic pressure could locally be reached in overpressured covers. We also define the retention capacity of a cover (RC) surrounding a fluid source or reservoir as the difference between the failure pressure and the fluid overpressure prevailing in shale at the same depth. In response to a localized fluid pressure rise, we show that the retention capacity does not only depend on the pore fluid overpressure of the overburden but also on the tensile strength of the cover, its Poisson's ratio, and the depth and width of the fluid source.
ISSN: 0264-8172

hal-00590497
https://hal.archives-ouvertes.fr/hal-00590497
DOI : 10.1016/j.marpetgeo.2010.08.007

Éditeur(s) : HAL CCSD
Résumé : Wood samples were taken from the upper and lower sides of 21 naturally tilted trees from 18 families of angiosperms in the tropical rainforest in French Guyana. The measurement of growth stresses ensured that the two samples were taken from wood tissues in a different mechanical state: highly tensile stressed wood on the upper side, called tension wood and lower tensile stressed wood on the lower side, called opposite wood. Eight species had tension wood fibres with a distinct gelatinous layer (G-layer). The distribution of gelatinous fibres varied from species to species. One of the species, Casearia javitensis (Flacourtiaceae), showed a peculiar multilayered secondary wall in its reaction wood. Comparison between the stress level and the occurrence of the G-layer indicates that the G-layer is not a key factor in the production of high tensile stressed wood.
IAWA Journal

hal-00112579
https://hal.archives-ouvertes.fr/hal-00112579
https://hal.archives-ouvertes.fr/hal-00112579/document
https://hal.archives-ouvertes.fr/hal-00112579/file/IAWA2006_ClairRuelleBeauchenePrevostFournier_ccsd.pdf
IRD : PAR00001136

Éditeur(s) : HAL CCSD Springer
Résumé : International audience
We present here a synthesis of the evolution of rifted continental margin systems in the Gulf of Aden. These margins are volcanic to the west of the Gulf of Aden, where they are influenced by the Afar hotspot, and non-volcanic east of longitude 46° E. The combined use of magnetics, gravity, seismic reflection, field observations (tectonic, stratigraphic and sedimentological) and oil well data allowed us to obtain better constraints on the timing of continental rifting and seafloor spreading. From the Permo-Triassic to the Oligocene, the Arabian-African plate was subject to distributed extension, probably due, at least from the Cretaceous, to tensile stresses related to the subduction of the Tethysian slab in the north. In Late Eocene-Early Oligocene, 34-33 Ma ago, rifting started to localise along the future area of continental breakup. Initially guided by the inherited basins, continental rifting then occurred synchronously over the entire gulf before becoming localised on the northern and southern borders of the inherited grabens, in the direction of the Afar hotspot. In the areas with non-volcanic margins (in the east), the faults marking the end of rifting trend parallel to the inherited grabens. Only the transfer faults cross-cut the inherited grabens, and some of these faults later developed into transform faults. The most important of these transform faults follow a Precambrian trend. Volcanic margins were formed in the west of the Gulf, up to the Guban graben in the southeast and as far as the southern boundary of the Bahlaf graben in the northeast. Seaward dipping reflectors can be observed on many oil industry seismic profiles. The influence of the hotspot during rifting was concentrated on the western part of the gulf. Therefore, it seems that the western domain was uplifted and eroded at the onset of rifting, while the eastern domain was characterised by more continuous sedimentation. The phase of distributed deformation was followed by a phase of strain localisation during the final rifting stage, just before formation of the Ocean-Continent Transition (OCT), in the most distal graben (DIM graben). About 20 Ma ago, at the time of the continental break-up, the emplacement of the OCT started in the east with exhumation of the subcontinental mantle. Farther west, the system was heated up by the strong influence of the Afar hotspot, which led to breakup with much less extension. In the Gulf of Aden (s.str), up to the Shukra El Sheik fracture zone, oceanic spreading started 17.6 Ma ago. West of this fracture zone, oceanic accretion started 10 Ma ago, and 2 Ma ago in the Gulf of Tadjoura. Post-rift deformation of the eastern margins of the Gulf of Aden can be seen in the distal and proximal domains. Indeed, the substantial post-rift uplift of these margins could be associated with either the continental break-up, or activity of the Afar hotspot and related volcanic/magmatic activity. Uplift of the northern proximal margin was still active (e.g. stepped beach rocks exposed at 60 m of 2 Ma; 30 m of 35,200 years; 10 and 2 m) and active volcanoes can be inferred at depths of between 70 and 200 km beneath the margin (at 5-10 km distance from the coast). On the distal margin, heat flow measurements show a high value that is associated with post-rift volcanic activity and the development of a volcano (with flows and sills) shortly after the formation of the OCT. The Afar hotspot is therefore important for several reasons. It allows the localisation of deformation along the Red Sea/Aden system and the rapid opening of the Gulf after the continental break-up; its influence also seems to persist during the post-rift period.
ISSN: 1866-7411

hal-01053089
https://hal.archives-ouvertes.fr/hal-01053089
DOI : 10.1007/s12517-011-0475-4

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Fractures or discontinuities perpendicular to the least stress σ3 were generated in a synthetic rock analogue (granular, frictional, cohesive, and dilatant) material in axisymmetric extension tests. These fractures are of two types defined by the mean stress σ. When σ is very small, the fractures form through the mode I cracking at tensile σ3 equal to the material tensile strength. The fracture walls have smooth surfaces. At higher σ, these surfaces become rougher, with the topography features forming faint or delicate plumose patterns. The amplitude of the plumose topography increases with σ. The axial stress σ3 at fracturing reduces in magnitude and changes in sign with the σ increase. Thus σ3 orthogonal discontinuities can form at compressive σ3. SEM observations show that these discontinuities are deformation localization bands where the material is characterized by the heterogeneous decohesion and volume and porosity increase due to dilatancy. The band thickness is several grain sizes. At formation, the bands are not opened, so they are not mode I fractures. They become fractures with plumose fractography after the separation of the sample parts along the band. The formation mechanism of these discontinuities or fractures is not completely clear, but it is suggested that it represents a running constitutive instability in the form of dilatancy banding (with further σ increase the bands become inclined to σ1, i.e., shear). The similarity between the experimentally generated plumose surface fractures and natural joints is discussed, and it is suggested that they can be formed as propagating constitutive instabilities.
ISSN: 0148-0227

hal-00585846
https://hal.archives-ouvertes.fr/hal-00585846
DOI : 10.1029/2010JB008104

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Both oedometric and plane-strain tests were performed with parallelepipedic samples made of synthetic granular, cohesive, frictional and dilatant rock analogue material GRAM2. For the first time parallel sets of fractures that have all the characteristics of natural joints were reproduced in the laboratory. The fractures are regularly spaced, normal to σ3, and have plumose morphology very similar to that of natural joints. These fractures can form at tensile stress σ3 much smaller in magnitude than the tensile strength of material and even at slightly compressive σ3. When mean stress σ exceeds a certain value, the fractures become oblique to σ1 (the obliquity increases with σ), forming networks of conjugate shear bands/fractures. These results of plane-strain experiments are in good agreement with those of better controlled conventional axisymmetric tests on a similar material in Chemenda et al. (2011b) and are closer to real geological situations. Both types of experiments are complementary. Their results lead to the conclusion that at least certain categories of natural fractures (including joints, and conjugate shear fractures/bands) were initiated as deformation localization bands. The band orientation is defined by the constitutive properties/parameters (notably the dilatancy factor) that are sensitive to σ.
ISSN: 0040-1951

hal-00795555
https://hal.archives-ouvertes.fr/hal-00795555
DOI : 10.1016/j.tecto.2011.11.021

Éditeur(s) : HAL CCSD
Résumé : International audience
A key element of the biomechanical design of trees is their ability to generate largemechanical stresses in wood at the stem periphery. This function is necessary for the tree tocontrol the orientation of its axes, and therefore to grow in height, maintain its branches at anoptimal angle or achieve adaptive reorientations (Fournier et al., 2013). This “maturationstress” appears in wood fibres during their cellular maturation when their secondary cell wallis thickening. In hardwoods, the stress asymmetry is generated by the formation of specificfibres with a very high tensile growth stress on the upper side of the inclined axis. Theresulting tension wood has microstructural features highly different from the normal wood.Tension wood in almost all temperate species has a specific gelatinous cell wall layer, calledG-layer, that is not lignified and highly mesoporous (Chang et al., 2015). It has been shownrecently, at the macroscopic scale, that cellulose tension appears during maturation and issynchronous with the development of this specific layer (Clair et al., 2011). Tropical species,like simarouba, are able to generate lignified tension wood fibres. The mechanisms of stressgeneration in tension wood fibres are not yet well known. The aim of our study is tounderstand these mechanisms and to know the spatial and temporal kinetics of the differentcell wall layers stiffening during maturation, in parallel with tensile stress generation.Contact-Resonance Atomic Force Microscopy, CR-AFM, was used here to address thisquestion on embedded samples (Arnould and Arinero, 2015) using dual resonance frequencytracking (DRFT, Rodriguez et al., 2007). Change in the contact modulus between, and within,each layer of the cell wall has been measured on several radial lines of developing fibres atdifferent steps. In order to understand the obtained results, and to estimate the sensitivity ofthe AFM indentation-like technique to the different cell wall components stiffness, an elasticanisotropic indentation model was used (Vlassak et al., 2003; Jäger et al., 2011). Finally,these mechanical measurements have been compared to topochemical data obtained on thesame fibres.
8th Plant Biomechanics International Conference
Nagoya, Japan

hal-01436113
https://hal.archives-ouvertes.fr/hal-01436113

Éditeur(s) : HAL CCSD Springer-Verlag Berlin Heidelberg
Résumé : 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.
The biology of reaction wood

ISBN : 978-3-642-10813-6
hal-01135273
https://hal.archives-ouvertes.fr/hal-01135273

Éditeur(s) : HAL CCSD
Résumé : International audience
The anatomy of tension wood and opposite wood was compared in 21 tropical rain forest trees from 21 species belonging to 18 families from French Guyana. Wood specimens were taken from the upper and lower sides of naturally tilted trees. Measurement of the growth stress level ensured that the two samples were taken from wood tissues in a different mechanical state: highly tensile-stressed wood on the upper side, called tension wood and normally tensile-stressed wood on the lower side, called opposite wood. Quantitative parameters relating to fibres and vessels were measured on transverse sections of both tension and opposite wood to check if certain criteria can easily discriminate the two kinds of wood. We observed a decrease in the frequency of vessels in the tension wood in all the trees studied. Other criteria concerning shape and surface area of the vessels, fibre diameter or cell wall thickness did not reveal any general trend. At the ultrastructural level, we observed that the microfibril angle in the tension wood sample was lower than in opposite wood in all the trees except one (Licania membranacea).
IAWA Journal

hal-00194915
https://hal.archives-ouvertes.fr/hal-00194915
https://hal.archives-ouvertes.fr/hal-00194915/document
https://hal.archives-ouvertes.fr/hal-00194915/file/IAWA2006_RuelleClairBeauchenePrevostFournier_ccsd.pdf
IRD : PAR00001321

Éditeur(s) : HAL CCSD American Physical Society
Résumé : International audience
This is the first of two papers investigating the mechanical response of cemented granular materials by means of contact dynamics simulations. In this paper, a two-dimensional polydisperse sample with high-void ratio is constructed and then sheared in a simple shear numerical device at different confinement levels. We study the macroscopic response of the material in terms of mean and deviatoric stresses and strains. We show that the introduction of a local force scale, i.e., the tensile strength of the cemented bonds, causes the material to behave in a rigid-plastic fashion, so that a yield surface can be easily determined. This yield surface has a concave-down shape in the mean: deviatoric stress plane and it approaches a straight line, i.e., a Coulomb strength envelope, in the limit of a very dense granular material. Beyond yielding, the cemented structure gradually degrades until the material eventually behaves as a cohesionless granular material. Strain localization is also investigated, showing that the strains concentrate in a shear band whose thickness increases with the confining stress. The void ratio inside the shear band at the steady state is shown to be a material property that depends only on contact parameters.
ISSN: 1539-3755

hal-00523453
https://hal.archives-ouvertes.fr/hal-00523453
DOI : 10.1103/PhysRevE.82.011303

Éditeur(s) : HAL CCSD Elsevier Masson
Résumé : Extension fractures of two types defined by the mean stress σ were generated in a synthetic rock analogue material. When σ is very small, the fractures are mode I cracks with smooth surfaces. At higher σ, these surfaces have plumose topography, with the amplitude increasing with σ. Both SEM observations and mechanical measurements show that fractures/discontinuities in the latter case are initiated as dilatancy localization bands. They form under tensile or slightly compressive normal stress and can be seen as running constitutive instabilities. The similarity between the plumose fractography of the experimental fractures and natural joints suggests similarity in the formation mechanism.
ISSN: 1631-0721

hal-00585886
https://hal.archives-ouvertes.fr/hal-00585886
DOI : 10.1016/j.crme.2011.01.002

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
The mechanism for tree orientation in angiosperms is based on the production of high tensile stress on the upper side of the inclined axis. In many species, the stress level is strongly related to the presence of a peculiar layer, called G-layer, in the fibre wall. The structure of G-layer has been recently described as a hydrogel thanks to N2 adsorption-desorption isotherms of supercritically dried samples showing a high mesoporosity (pores size from 2 to 50 nm). This led us to revisit the concept of G-layer that was until now only described from anatomical observation. Adsorption isotherms of both normal wood and tension wood have been measured on six tropical species. Measurements show that mesoporosity is high in tension wood with typical thick G-layer while it is much less with thinner G-layer, sometimes no more than normal wood. The mesoporosity of tension wood species without G-layer is as low as in normal wood. Not depending on the amount of pores, the pore sizes distribution are always centred around 6-12 nm. These results suggest that, among species producing fibres with G-layer, large structural differences of G-layer exist between species
ISSN: 0022-0957

hal-00387861
https://hal.archives-ouvertes.fr/hal-00387861
DOI : 10.1093/jxp/erp133

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Large sand intrusions often exhibit conical morphologies analogous to magmatic intrusions such as saucer-shaped or cup-shaped sills. Whereas some physical processes may be similar, we show with scaled experiments that the formation of conical sand intrusions may be favoured by the pore-pressure gradients prevailing in the host rock before sand injection. Our experiments involve injecting air into a permeable and cohesive analogue material to produce hydraulic fractures while controlling the pore pressure field. We control the state of overpressure in the overburden by applying homogeneous basal pore pressure, and then adding a second local pore pressure field by injecting air via a central injector to initiate hydraulic fractures near the injection point. In experiments involving small vertical effective stresses (small overburden, or high pore fluid overpressure), the fracturing pressure (lambda(fract)) is supralithostatic and two dipping fractures are initiated at the injection point forming a conical structure. From theoretical considerations, we predict that high values of lambda(fract), are due to strong cohesion or high pore fluid overpressure distributed in the overburden. Such conditions are favoured by the pore pressure/stress coupling induced by both pore pressure fields. The dips of cones can be accounted for elastic-stress rotation occurring around the source. Contrary to magmatic chamber models, the aqueous fluid overpressure developed in a parent sandbody (and prevailing before the formation of injectites) may diffuse into the surrounding overburden, thus favouring stress rotation and the formation of inclined sheets far from the parent source. For experiments involving higher vertical effective stresses (thick overburden or low pore fluid overpressure), the fracturing pressure is lower than the lithostatic stress, and a single fracture is opened in mode I which then grows vertically. At a critical depth, the fracture separates into two dilatant branches forming a flat cone. We make use of a P.I.V. (Particle Imaging Velocimetiy) technique to analyse plastic deformation, showing that these inclined fractures are opened in mixed modes. Close to the surface, they change into steep shear bands where fluids can infiltrate. The final morphology of the fracture network is very similar to the common tripartite architecture of various injection complexes, indicating that different mechanisms may be involved in the formation of dykes. Feeder dykes under the sill zones may open as tensile fractures, while overlying dykes may be guided by the deformation induced by the growth of sills. These deformation conditions may also favour the formation of fluid escape structures and pockmarks. (C) 2011 Elsevier B.V. All rights reserved.
ISSN: 0012-821X

hal-00745640
https://hal.archives-ouvertes.fr/hal-00745640
DOI : 10.1016/j.epsl.2011.10.029

Éditeur(s) : HAL CCSD Wiley
Résumé : 13 pages, 4 figures, to be published on J. Am. Cer. Soc
International audience
Since crack propagation in oxide materials at low crack velocities is partly determined by chemical corrosion, proper knowledge of the crack tip chemistry is crucial for understanding fracture in these materials. Such knowledge can be obtained only from in situ studies because the processes that occur in the highly confined environment of the crack tip are very different from those that take place at free surfaces, or that can be traced post mortem. We report the occurrence of hydrous liquid condensate between the two fracture surfaces in the vicinity of the tip of tensile cracks in silica. Observations are performed in real-time by means of atomic force microscopy (AFM) at continuously controlled crack velocities in the regime of stress corrosion. Condensate formation and changes in extent and shape are demonstrated for a wide range of macroscopic humidity at different crack speeds. Its liquid character is confirmed by the study of AFM phase-contrast data. It is believed that this evidence of a nanoscale liquid hydrous phase at the crack tip will enable novel insights in the chemistry of failure of oxide materials.
ISSN: 0002-7820

hal-00424984
https://hal.archives-ouvertes.fr/hal-00424984
ARXIV : cond-mat/0509209
DOI : 10.1111/j.1551-2916.2005.00765.x

Éditeur(s) : HAL CCSD
Résumé :  
The anatomy of tension wood and opposite wood was compared in 21 tropical rain forest trees from 21 species belonging to 18 families from French Guyana. Wood secimens were taken from the upper and lower sides of naturally titled trees. Measurement of the growth stress level ensured that the two samples were taken from wood tissues in a different mechanical state: highly tensile-stressed wood on the upper side, called tension wood and normally tensile-stressed wood on the lower side, called opposite wood. Quantitative parameters relating to fibres and vessels were measured on transverse sections of both tension and opposite wood to check if certain criteria can easily discriminate the two kinds of wood. We observed a decrease in the frequency of vessels in the tension wood in all the trees studied. Other criteria concerning shape and surface area of the vessels, fibre diameter or cell wall thickness did not reveal any general trend.At the ultrastructural level, we observed that the microfibril angle in the tension wood sample was lower than in opposite wood in all the trees except one (Licania membranacea).
IAWA Journal

hal-01032149
https://hal.archives-ouvertes.fr/hal-01032149

Éditeur(s) : HAL CCSD Oxford University Press (OUP): Policy B - Oxford Open Option A
Résumé :  
The mechanism for tree orientation in angiosperms is based on the production of high tensile stress on the upper side of the inclined axis. In many species, the stress level is strongly related to the presence of a peculiar layer, called the G-layer, in the fibre cell wall. The structure of the G-layer has recently been described as a hydrogel thanks to N2 adsorption-desorption isotherms of supercritically dried samples showing a high mesoporosity (pores size from 2-50 nm). This led us to revisit the concept of the G-layer that had been, until now, only described from anatomical observation. Adsorption isotherms of both normal wood and tension wood have been measured on six tropical species. Measurements show that mesoporosity is high in tension wood with a typical thick G-layer while it is much less with a thinner G-layer, sometimes no more than normal wood. The mesoporosity of tension wood species without a G-layer is as low as in normal wood. Not depending on the amount of pores, the pore size distribution is always centred around 6-12 nm. These results suggest that, among species producing fibres with a G-layer, large structural differences of the G-layer exist between species.
Journal of Experimental Botany

hal-01032161
https://hal.archives-ouvertes.fr/hal-01032161
DOI : 10.1093/jxb/erp133

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : 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.
ISSN: 0931-1890

hal-00194923
https://hal.archives-ouvertes.fr/hal-00194923
https://hal.archives-ouvertes.fr/hal-00194923/document
https://hal.archives-ouvertes.fr/hal-00194923/file/Trees_RuelleYoshidaClairThibaut_submitted2006.pdf
DOI : 10.1007/s00468-007-0128-0