Modelling the time-dependent behaviour of wood

International audienceThis presentation will be dedicated to the memory of David Hunt (1930-2016), who made very significant contributions to the understanding of time dependent phenomena in wood. As a hygroscopic polymer wood displays a viscoelastic behaviour strongly influenced by temperature and humidity.The time-dependency of its response to a mechanical loading results from the delayed movements of molecular components under stress, as much as from the kinetics of the sorption process or any other chemical reaction modifying the molecular structure. Mechanosorption is a typical example of such complex interaction. It refers to the phenomena observed when wood is subjected simultaneously to mechanical loading and sorption resulting from changing hygrothermal conditions. The difference between the observed response and that expected from viscoelastic data at constant moisture content (m.c.) (e.g., creep or relaxation) is defined as the mechanosorptive response – although the evaluation of that “expected” response is not straightforward*. Mechanosorption has been commonly modelled as a time-independent process, only governed by variations of the moisture content. This hypothesis is not easily verified due to the time-dependency of the sorption process, and it seems to be in contradiction with observations that it triggers viscoelastic processes. The large dominance of mechanosortive over constant-humidity creep is another belief that needs to be put in perspective: it depends on the reference being the dry or wet state [1]. Based on the work of Ranta-Maunus in the 70ties m.s. was initially modelled using viscous-like elements where time is replaced by m.c., distinguishing desorption (-), adsorption (+) and ‘adsorption above highest level reached since loading’ (++). Hunt in the 80ties introduced the m.s. ‘trajectories’ where the creep compliance is plotted against m.c., and the concept of ‘creep limit’ reached after repeated humidity cycles under load (Fig. 1a-d). He proposed that the creep limit is approached both in sorption and desorption, and explained the apparent recovery during adsorption as moisture expansion modified by strain [2]. This led to rheological models based on combinations of “m.s. dashpots” and springs, commonly used in numerical developments, with viscoelastic components modelled separately. It was also assumed by Matar [3], as a conservative hypothesis, when he derived equations of the long-term creep of softwood as a function of wood quality, in view of improved evaluation of Kdef factor in Eurocode5 (Fig. 1e)

Comparison between wood hygromechanical description and deformation modification factors of Eurocode 5

International audienceEngineers need confidence in normative standards to design timber structures. Structural design is performed to ensure safety issues and answer qualitative constructions regarding to an economic competitive objective. In order to improve and provide well matched normative rules and wood material behaviour we performed laboratory creep experiments to quantify and describe the kinetics of wood material facing hygro-mechanical history. Furthermore it allows a direct comparison with deformation modification factors of EN1995 regarding an increase of structural longevity of timber structures

La biomécanique des plantes ou " Comment les plantes tiennent debout ? "

La biomécanique des plantes est née d'une approche interdisciplinaire. Elle regroupe ainsi des biologistes, des forestiers ou des agronomes, et des mécaniciens (des matériaux et des structures et plus récemment des fluides). Elle s'intéresse à la façon dont les plantes se portent et croissent. En particulier comment les plantes terrestres croissent en hauteur " contre " la gravité et dans le vent ; mais aussi comment les plantes aquatiques et les algues se développent dans les courants

Anisotropy of wood vibrational properties: dependence on grain angle and review of literature data

International audienceThe 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

Hygro-mechanical behaviour of wooden panels from cultural heritage: effect of a coated face

International audiencePanel paintings belong to tangible heritage, and their conservation is needed to preserve artistic and historical memory. Several factors can cause damage both to the wooden support and to the paint layer. With the exception of biological decay, the principal risks originate from the microclimatic changes. As an organic material, wood responds readily to changes in ambient temperature and humidity, absorbing and desorbing moisture as environmental conditions fluctuate. The absorption and desorption of moisture results in mechanical changes such as swelling and shrinking of wood. Repeated cycles of swelling and shrinking—often occurring over centuries—will cause panels to warp, twist, or split. This type of damage to a painting's substrate can, in turn, have a dramatic effect on the paint layer, potentially resulting in paint loss. In this presentation, the dimensional changes of a raw wooden board due to hygro-thermal variations will be explained, function of its ring orientation. Then, the behaviour of a board with a coated face will be considered. Thus, a board restrained by crossbeams or reinforcements on the back will be studied. These situations will be illustrated using finite element analyses. Keywords : wood, cultural heritage, painted panels, hygrothermal variations.Les peintures sur panneaux de bois font partie du patrimoine culturel. Leur conservation doit tenir compte de la préservation de leur mémoire artistique et historique. Plusieurs facteurs peuvent endommager le support bois et la couche picturale. Mise à part la dégradation biologique, les risques principaux sont dus aux variations microclimatiques. Matériau organique, le bois réagit rapidement aux changements d'humidité et de température ambiante, par absorption ou désorption d'humidité suivant les fluctuations des conditions environnementales. L'absorption et la désorption d'humidité entraînent des changements mécaniques tels que le gonflement ou le retrait du bois. Des cycles répétés de gonflement et retrait – durant généralement plusieurs siècles – mènent au gauchissement, à la torsion ou à la fissuration des panneaux. Le support d'une peinture ainsi endommagé peut à son tour entraîner des conséquences dramatiques sur la couche picturale, menant potentiellement à des pertes de peinture. Dans cette présentation, nous expliquerons les changements dimensionnels d'une planche de bois brute soumise à des variations hygrothermiques, en fonction de l'orientation des cernes. Puis nous considérerons le cas d'une planche avec une face peinte. Pour finir, nous étudierons le cas d'une planche restreinte par des traverses ou des renforcements au dos. Ces situations seront illustrées par des analyses par éléments finis

Shrinkage of cane (Arundo donax L.) II Effect of drying condition on the intensity of cell collapse

To improve the drying method in the manufacture of woodwind reeds, green canes (Arundo donax L.) were dried under various humidity-temperature conditions and the intensity of cell collapse was evaluated from the swelling due to steaming involving the recovery of collapse. At 30 C, the intensity of collapse was increased by slower drying. It was considered that: 1) slower drying resulted in higher sample temperature in the early stage of drying to increase the collapse; 2) rapid drying stiffened the surface of sample and such "shell" prohibited the following collapse; 3) slower drying i.e. longer loading of liquid tension caused more remarkable and/or frequent viscoelastic yields of cells. Consequently the intensity of collapse increased when the cane was dried from its waxy outer surface or in the presence of node: both of them retarded the drying. On the other hand, higher drying temperature caused greater intensity of collapse in spite of faster drying. It was suggested that the thermal softening of cane cells leads to easier yield of the cell wall, at the same time the rapid drying does not allow the recovery of collapse after the disappearance of free water. These results indicated that faster drying at lower temperature is preferable for drying cane with less collapse.Comment: Will be published in J. Wood Sci. (Japanese wood research society) - follows part I: J. Wood Sci. 50 : 295-30

Studies on European beech (Fagus sylvatica L.). Part 1: Variations of wood colour parameters

Colour parameters of European beech were measured using CIELab system. 103 logs from 87 trees in 9 sites were cut into boards to study the variations of wood colour parameters. Both site and tree effect on colour were observed. Patterns of red heartwood occurrence were defined. When excepting red heartwood there was still a highly significant effect of site and tree; differences remained after veneer processing. Axial variations were small, except very near the pith or in red heartwood, suggesting possible early selection at periphery under colour criteria. Red heartwood is darker, redder and more yellow than normal peripheral wood.Comment: to be published in Annals of Forest Science reception 12.8.04; acceptation 15.2.0

Recouvrance hygrothermique du bois vert. I. Influence de la température. Cas du jujubier (Ziziphus lotus (L) Lam)

Le chauffage du bois vert met en jeu une compétition entre plusieurs mécanismes de déformation. A la déformation thermique proprement dite, réversible et caractérisée par un coefficient d'expansion négatif en dessous de la température de transition vitreuse de la lignine, se superpose lors du premier chauffage la recouvrance viscoélastique de déformations bloquées lors de l'élaboration sous contrainte du bois dans l'arbre. Il en résulte une dilatation tangentielle, associée à une contraction radiale plus faible, à l'origine des fentes à coeur induites par l'étuvage humide des grumes. La séparation des mécanismes de déformation associés à la "recouvrance hygrothermique" a été tentée pour le jujubier (Ziziphus lotus (L) Lam) à partir de mesures silmutanées, pour plusieurs modes de chargement en température, des déformations tangentielle et radiale, permettant notamment d'interpréter une décroissance transitoire de la déformation tangentielle observée lors du premier chauffage. (Résumé d'auteur

Hygro-locks modelling of the mechano-sorptive behavior based on integral formulation or internal variables

session 'Computational Material Modeling of Wood and Wood Products'Wood is a porous, hygroscopic, anisotropic and non homogeneous biopolymer. It is classified as a viscoelastic material with mechanical properties depending on temperature and moisture content. The effect of creep is an important factor for the design and the durability of timber structures. Creep evolution results from the interaction between mechanical stress and moisture content variations