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Dimensional stability of Douglas fir and mixed beech-poplar plywood: Experimental measurements and simulations
Abstract
A finite element method (FEM) numerical model predicting the warping of plywood panels from a description of the intraveneer properties in response to moisture content changes is presented and compared with experimental measurements. Thirteen three-ply plywood panels of 40×40 cm2 (1.9-mm thickness per ply) were manufactured from Douglas fir or from mixed beech and poplar. Maps of the longitudinal and tangential swelling as well as the wood density were established on the veneers before the glueing process. The shape was measured on the panels conditioned first at 17% moisture content (MC), then at the fiber saturation point, in order to calculate the maximum displacements along six profiles (three longitudinal and three tangential) on each panel. These experimental data were compared with the results of simulations based on two models. First, they were compared to a "heterogeneous" model in which all measured veneer properties were used as inputs. In this case the predicted displacements showed a close correlation (R
2=0.74) to the measured ones, but were remarkably underestimated. Second, they were compared to a "homogeneous" model in which the three layers of each panel were only described by their mean density and average swelling coefficients. For this comparison the prediction of the displacements was much less accurate in the longitudinal direction but was still satisfactory in the tangential direction.
... L'objectif est de fiabiliser l'utilisation de ces essences pour les structures en bois justifiées selon les règles Eurocode 5 par les validations ou des corrections nécessaires de la norme NF EN 338. Bien que quelques rares études, sur quelques essences feuillues existent ( (Bahar et al., 2016;Berthelot et al., 2009;Constant et al., 2003;Hunt, 1984;Rousset, 1999),…), elles doivent être complétées par de nouvelles données scientifiques et expérimentales. Plusieurs incertitudes sont à lever, dont : ...
La conception et la vérification du dimensionnement des structures en bois, pour une utilisation en Génie Civil, est régie par la norme Européenne Eurocode 5. Elle permet de dimensionner tout élément de structure en fonction du chargement qui lui est appliqué et d’hypothèses relatives à son environnement. Coordonnée particulièrement par la filière bois des pays Nordiques, cette norme est basée sur l’emploi exclusif de résineux. Or, la France détient la plus grande forêt de feuillus d’Europe. Ces essences ont leur place dans les structures bois d’aujourd’hui en apportant, par exemple, des propriétés de durabilité naturelle. Malheureusement, ces essences ne sont pas aujourd’hui considérées dans les règles de dimensionnement.Le projet EFEUR5, financé par l’agence nationale de la recherche, a pour objectif de répondre à cette problématique en effectuant des travaux similaires à ceux déjà réalisés pour les résineux. Ainsi, les travaux présentés ont pour objectifs de comprendre et de modéliser le comportement mécanique différé des essences de feuillus français que sont le chêne, le hêtre et le peuplier. A ces trois essences, est rajouté le douglas comme essence de référence des résineux.Nous commençons l’étude par l’analyse microscopique de la structure anatomique des essences étudiées. Cette analyse a pour objectif de mettre en avant les différences entre ces quatre essences. Le comportement mécanique différé du bois étant sensible à l’humidité de celui-ci, nous devons dans un premier temps déterminer l’évolution de l’humidité dans la section d’un élément de structure. Pour cela, il est présenté dans le second chapitre un protocole expérimental innovant de détermination des propriétés de diffusion d’une éprouvette située en extérieur. L’identification des propriétés de diffusion effectuée, nous proposons une étude de sensibilité des écarts de comportement hydrique sur un élément de structure.Le comportement mécanique long terme est ensuite étudié par une approche couplée entre expérimentation et modélisation. Différents comportements mécaniques dépendant du temps et de l’humidité sont pris en compte et hiérarchisés. L’implémentation, dans un logiciel aux éléments finis, de l’évolution du champ hydrique 3D au sein d’un élément de structure, ainsi que des différents phénomènes physiques mis en jeu lors du fluage sont discutés. Par la suite, la conception de deux bancs de flexion quatre points à l’échelle métrique des poutres de feuillus et de douglas est détaillée. Les mesures réalisées sur plus d’une année permettent dans un premier temps d’identifier les principaux comportements. Dans un second temps, ces données permettent de déterminer les propriétés viscoélastiques hygro activées par analyse inverse. Ainsi une comparaison des propriétés viscoélastiques entre ces quatre essences est présentée.Enfin, dans la dernière partie, les modèles développés et validés sont mis en forme pour une approche technologique. De nouveaux abaques d’équilibre d’humidité interne sont proposés essence par essence. Une méthodologie de détermination du coefficient kdef est proposée, et permet, par une relation très simple, d’affiner sa détermination en fonction de l’environnement réel, de la section, et de l’humidité initiale.
... A similar experience is described in the literature, where the FEM results underestimate the distortion of LVPs. Constant et al. (2003) developed a FE model that predicted distortion in plywood and compared the results with empirical data observed under different climate conditions. Two types of simulation models were used; one describing the properties of each veneer according to the longitudinal and tangential swelling, as well as the wood density before the moulding, and the other using the mean density and average swelling coefficients for the LVP. ...
A shortcoming of the laminated bending process is that the product may become distorted after moulding. This study focused on the influence of fibre orientation deviation for individual veneers on the distortion of a moulded shell. The distortion of 90 cross-laminated shells of the same geometrical shape, consisting of seven peeled birch veneers, were studied under relative humidity variation. All the veneers were straight-grained in the longitudinal-tangential plane, but to simulate a deviation in fibre orientation, some of the individual veneers were oriented at an angle of 7° relative to the main orientation of the other veneers in the laminate. A finite element model (FEM) was applied to study the possibility of predicting the results of a practical experiment. The study confirms the well-known fact that deviation in fibre orientation influences shape stability. The results also show how the placement of the abnormal veneer influences the degree of distortion. From this basic knowledge, some improvements in the industrial production were suggested. However, the FE model significantly underestimated the results, according to the empirical experiment, and it did not show full coherence. The survey shows the complexity of modelling the behaviour of laminated veneer products under changing climate conditions and that there is a great need to improve the material and process data to achieve accurate simulations. Examples of such parameters that may lead to distortion are density, annual ring orientation in the cross section of the veneer, the orientation of the loose and tight sides of the veneer, and parameters related to the design of the moulding tool.
... Panels used in the experiment were fabricated with rotary-peeled veneers, by using the ureaformaldehyde glue as binder and three different core veneer joints. Furthermore, the dimensional stability of Douglas fir and mixed beech-poplar 3-ply plywood of 5.7 mm in thickness were carried out by experimental measurements and simulations by Constant et al. (2003). ...
Three-layered plywood panels are widely used for packing cases, furniture parts and other applications. Veneers of 1 mm and 1.5 mm nominal thickness, obtained by rotary cut of tree-of-heaven trunk, were used in the construction of 3-ply plywood. The mean thicknesses of the two plywood, used in this study, were 3.13 mm for the thinner one and 4.58 mm for the thicker one, respectively. According to the test results, the highest mean Bending strength value proved to have derived from the thin plywood (91.22 N/mm2) and the thick one presented a little lower value of the respective strength (88.19 N/mm2), while the thick plywood recorded higher mean Modulus of Elasticity value, compared to the corresponding value of the thin one. Referring to the Impact Bending strength, the thin plywood presented a little higher mean value (2.93 J/cm2), than the thick plywood (2.60 J/cm2), whereas the thick plywood was proved to have quite higher Screw Withdrawal capacity (33.35 N/mm2), compared to the thin one (29.01 N/mm2). Additionally, the thin plywood resulted in high Shear strength, both in the case when the plywood was tested in dry conditions and also, when it was tested after the immersion of the specimens in water. Finally, the thin plywood recorded higher Swelling percentage value, lower Absorption percentage value and lower percentage value of Permanent Swelling, compared to the thick plywood. Generally, according to these results it could be claimed that the thin plywood presents quite satisfying properties, compared to the corresponding thick plywood.
... The main factors influencing to plywood panels properties are: wood species (Biblis 1999, Constant et al. 2003 Baldassino et al. (1998), considered the influence of plywood lay-up to its mechanical properties in two directions (according to wood fibers orientation in plywoods outer layers). ...
In this paper the influence of composition and number of layers for ten poplar plywood boards of different thicknesses to bending strength and modulus of elasticity has been investigated. Results showed that with both increasing of plywood thickness and number of layers, density, bending strength and modulus of elasticity in lengthwise direction have been decreed, while in crosswise direction, bending strength and modulus of elasticity increased until 18 mm thickness and after that decreased. Analysis showed that with increasing both of number of layers and thickness in plywood composition, the difference of examined properties decreased in both directions. The plywood composition decreasing also influenced on anisotropy: as the difference in percentage part of veneer thickness in crosswise v.s. lengthwise direction decreased, so bending strength and modulus of elasticity decreased.
Key words: poplar plywood, bending strength, modulus of elasticity, anisotropy.
... Efficient software makes virtual sawing or veneer cutting of a log described by 3D tomography a reality (Longuetaud et al. 2007(Longuetaud et al. , 2012Mothe et al. 2002;Roussel et al. 2014), opening the door to better sawing patterns in a sawmill for a known resource and a given market (Berglund et al. 2014). By using a simple geometrical transformation to simulate the veneering process, the tree stem description given by growth and yield simulation software (Dufour-Kowalski et al. 2012) allows the delivery of virtual maps of wood properties in veneers (grain angle and wood density) and prediction of the properties of secondary transformation products (Constant et al. 2003;Mothe et al. 2002). ...
Key message Wood machining is compulsory both for timber separation and the surfacing of wooden objects. The anisotropy, cellular nature and multi-scale level organisation of wood make its cutting complicated to study. During the last 50 years, most of the wood machining subjects were covered by French teams. Context Woodcutting is a very old technology but scientific research is scarce on the subject. In the last 50 years, much work on basic mechanisms as well as on industrial processes has been done in France. Aims The specific nature of wood introduces strong differences between wood and metal cutting processes. The paper focuses on French teams’ contributions. Results The basic aspects of the tool–material interaction for different basic modes in woodcutting are highlighted. In primary conversion such as sawing, veneer cutting or green wood chipping, huge progress comes from automation and the possibility of linking the process to log and product quality through new sensors. In secondary processing, much has been done on the links between the cutting process, surface qualification and the properties of these surfaces for further processing, such as gluing or coating. Tool wear depends on the cutting process, timber quality and species. Trade-offs are required in tool technology and coating technologies may improve tool life. Conclusion A large amount of knowledge and innovation has come from 50 years of worldwide research effort, with France being particularly active in this period. The transfer of skills from metals cutting industry was often a key, but much is needed to move closer to both metal cutting sector and woodcutting skills among craftsmen.
... Dimensional changes in different types of wood-based materials have been investigated for many years now. Research studies conducted on the subject pertain first of all to changes in length caused by changes in relative humidity at room or elevated temperature (Constant et al. 2003, Pugel et al. 1990, Sekino and Shibusawa 2002). ...
The study investigated dimensional changes of OSB/4 caused by cyclical changes in relative humidity at a constant ambient temperature. As a result of conducted analyses it was found that relative changes in length of OSB/4 to a much higher degree depend on the course of changes in relative humidity than on their thickness. Cyclically changing relative humidity causes an increase in board length in relation to its original length, irrespective of the direction of their changes. In turn, a relative change in thickness of OSB/4 calculated for individual stages is inversely proportional to board thickness.
Deux outils de simulation (1) de la coupe du bois par déroulage et (2) de la stabilité dimensionnelle des produits sont présentés. Les entrées sont constituées de la distribution spatiale des propriétés du bois dans la tige (forme des limites de cerne, branchaison) délivrée par le simulateur de croissance et de qualité du bois Winepifin. Un exemple d’application est présenté sur 2 épicéas communs typiques ; les simulateurs permettent de visualiser l’aspect des placages produits et de prédire la stabilité dimensionnelle de panneaux contreplaqués
Tezin çalışma konusu döşeme kaplaması olarak en sık kullanılan malzemelerden biri olan masif ahşap parkelerin boyutsal ve geometrik kararlılığının araştırılmasıdır. İç mekânda kullanılan masif ahşap parke kullanım sırasında doğal yapısı nedeniyle iç mekândaki denge nem oranına bağlı olarak su ve su buharı ile etkileşim içinde bulunur, bunun sonucunda da çalışır. Ahşabın çalışmasını denetlemek ve kontrol altında tutabilmek için buna neden olan etmenlerin belirlenmesi ve bu etmenler karşısında ahşabın davranışı irdelenmelidir. Bu araştırma kapsamında genel olarak ahşap malzeme, performansı, çalışması, çalışmasındaki etmenler ile oluşabilecek hasarlar göz önünde bulundurularak bunun en aza indirilmesi incelenmektedir.
Çalışma literatür araştırması ve deneysel çalışmalar olmak üzere iki aşamadan oluşmaktadır. Deneysel çalışma; meşe ve sapelli ağacından elde edilen numuneler üzerinde gerçekleştirilen ön deney ve deneylerden oluşmaktadır. Ön deneyler kapsamında; meşe ve sapelli ağaç türlerinden elde edilen (20x20x30) mm boyutundaki numunelerin yoğunluk, radyal ve teğet doğrultuda daralma ile genişleme tayini deneyleri bulunmaktadır.
Deneyler kısmında; meşe ve sapelli ağaç türlerinden elde edilen (250x50x15) ve (250x90x15) mm boyutlarındaki masif parke taslakları %85 ve %35 bağıl nem değerlerinde şartlandırılarak boyutsal ve geometrik kararlılık değişimleri belirlenmiştir. Bu aşamada numunelerin bir kısmına farklı oranlarda oluk açılmış, bir kısmı da sistire edilip her sistirenin ardından tekrar şartlandırılmıştır. Burada amaç olukların ve sistire etmenin boyutsal ve geometrik kararlılıktaki etkisini araştırmaktır. Ayrıca hiçbir uygulamaya maruz kalmayan bir numune grubu da karşılaştırma yapmak amacıyla aynı şekilde şartlandırılmıştır. Çalışmanın son aşamasında elde edilen sonuçlar literatür bilgileri ve yapılan istatistiki test bulguları ile birlikte değerlendirilmiştir.
Sonuç olarak; masif ahşap parkelere oluk açılmasının geometrik ve boyutsal kararlılığın iyileştirilmesinde fayda sağlamadığı belirlenmiştir. Ancak yine de oluk açma işlemi alt yüzün belirlenebilmesi ve tutkallama sırasında yapışma yüzeyini artırması açısından tavsiye edilmektedir. Parkenin üst yüzeyinin verniklenmesi de ufak bağıl nem değişikliklerine karşı malzemenin boyutsal kararlılığının belirli bir derecede artırılabilmesi için uygulanmalıdır.
Parkenin sistire edilip ardışık şartlandırmalara tabi tutulmasında sakınca olmadığı görülmektedir. Histerezin de etkisiyle geometrik kararlılığın kısmen ve boyutsal kararlılığın da belirgin şekilde artmasına etki ettiği de bulunan bir diğer sonuçtur.
Ayrıca ardışık olarak farklı iklim koşullarında kalan malzemenin boyutsal ve geometrik kararlılıklarında genel olarak artış olduğu, bu çerçevede masif parke taslaklarının daha stabil olması açısından döşeneceği yerlerin iklim koşullarında bir süre bekletilmesi önerilebilir.
Yine araştırma sonucunda; radyal kesitten elde edilen parkelerin tercih edilmesinin gerekliliği belirlenmiştir. Ayrıca parke uygulamalarının yüzer sistem olarak gerçekleştirilmesinin serbest hareket eden malzemenin çarpılma ve boyutsal değişime bağlı kusurlarının artması nedeniyle tavsiye edilmediği de eklenmelidir.
Bowing and swelling of LVL made of alternate laminations of rubber wood (Hevea brasiliensis) and falcata (Albizzia falcata) were measured while increasing the moisture content. The results were compared to the corresponding properties of LVL of all rubber wood or all falcata. As the moisture contents was raised from 11 to 16%, the bowing of rubber wood LVL was the greatest and that of falcata LVL the least, and bowing of the alternately laminated mixed LVL was similar to the falcata LVL. When center and edge parts of the specimens were analyzed using a digital image correlation method, the distributions of swelling and shear strains along the thickness direction (laminated direction) followed complicated curves regardless of the type of LVL. Even though the swelling distribution was complicated, the bowing of falcata LVL was less which probably resulted from its lower density. The bowing of alternately laminated mixed LVL was similar to that of the falcata LVL. This is interpreted to mean that the low density falcata layer will not transmit swelling stresses from one rubber wood layer to the next.
A new type of laminated veneer lumber (LVL) was developed by the alternate piling of the rotary lathe veneer of rubberwood and falcata. This mixed-species LVL is of moderate hardness, and like solid falcata timber, it does not display a tendency to warp. The rubberwood LVL warped almost eight times more than the falcata LVL. The alternately laminated LVL made of rubberwood and falcata veneer warped to the same extent as the falcata LVL. The soft layer of the falcata veneer probably mitigates the warping of the hard layer of the rubberwood veneer. Further, the mechanical properties of the LVL were also tested. The shear strength and transverse compression strength of the mixed-species LVL were similar to those of the falcata LVL, but its bending strength was identical to that of the rubberwood LVL. The soft layer of the falcata veneer appears to increase the amount of energy required to rupture the mixed-species LVL.
Coat ruptures are particularly disturbing if the fold runs through a continuously printed area. The defect impairs the visual homogeneity of the surface and the overall optical appearance of the print product. This affects the inside and the outside of the fold and broken-out coat particles can reach the product surface and disturb the print image in areas further away from the fold if the rupture occurs on the inside. The forces applied during folding frequently cause the coat layer to rupture at the fold, the starting point being the outermost layer of the paper. The elasticity of the paper determines whether the rupture affects the paper itself, destroying its microstructure. The paper is placed between two plates that are moving towards each other to apply an increasing mechanical load on the paper to simulate the folding process.
Nowadays, under growing environmental pressure, it is necessary to find alternatives to reduce the environmental impact of chemically treated wood and wood-based panels. In respect to this necessity we have evaluated the natural durability of mixed plywood composed of naturally durable plies (cypress, cedar and chestnut heartwood) and non-durables plies (poplar, beech, cypress and cedar sapwood) against wood destroying fungi (Coriolus versicolor and Coniophora puteana) and termites (Reticulitermes santonensis). The thickness of plies (1.3 and 2.6 mm), the percentage of durable plies and non durable plies (40% and 60%), the number of plies (5 and 9) and finally the modality of integration of durable plies to non durable plies (11 different symmetrical combinations) in each mixed panel were studied as variable factors. Ultrasonic (non destructive) and indentation (semi destructive) methods were assessed to evaluate the presence and the extend of decay in pure and mixed plywood both as rapid methods in comparison with the biological assessment based on mass loss (according to the ENV12038). Shear test was also performed before and after exposition to fungi as a reference mechanical property of plywood in order to evaluate the relationship between the mass loss and the level of shear strength decrease after fungal degradation. The results obtained have shown that mixing durable plies with non durable plies improves the natural durability of plywood against fungi. Nevertheless, the shear test after fungal degradation revealed that the decrease of shear strength in mixed plywoods is closer to the decrease observed for the non durable pure plywoods than for the pure durable plywoods. These results signify that there is a localised attack in the non durable plies of the mixed plywoods. The localised mass loss of non durable plies of mixed plywoods is better correlated to the decrease of shear strength than the classical mass loss (total) of the plywoods. The plywoods the most resistant against fungi were found amongst the different mixed plywoods made of cypress sapwood and heartwood, and some models were found resistant to termite attack as well. The resistant panels to both fungi and termites had durable plies in exterior faces. The presence of intact thick sapwood plies in mixed cypress plywoods shows a protective effect (against termite attack) from the durable plies. The indentation test (the measurement of energy before and after degradation) in comparison with the ultrasonic test (the measurement of velocity before and after degradation) showed the better correlation to the mass loss. In addition the indentation test can also be used to determine the gradient of mass loss and the threshold of rupture when decay takes place in the non durable plies situated under the durable exterior plies.
Laminated veneer products (LVPs) consist of veneers bonded together with an adhesive into a predetermined shape. Deviation from the intended shape is a problem for both manufacturers and customers of the final products. The shape stability of LVP depends on a variety of material and process parameters, and this review identifies several important factors that influence shape stability, the purpose being to find ways of decreasing distortion and improving shape stability so that product quality can be improved.
The review ends to the following statements:
The fibre orientation of the veneer strongly influences the shape stability of LVP. The products can show considerable distortion (particularly twist) if the fibre orientation of the veneer is oriented in an unsuitable way in the assembly before moulding.
The effect of fibre orientation on shape stability can be very small directly after moulding, but it can increase considerably when the moulded product is subjected to a change in moisture content (MC). In general, a change in moisture leads to distortion. Moisture changes alone, however, result in a controlled distortion.
The orientation of the loose and tight sides of the veneers in LVP affects the shape stability.
An asymmetrical construction, coupled with different levels of moisture in the different veneers in an assembly before moulding, will result in poor shape stability of the product.
The choice of wood species affects shape stability. Beech shows more distortion and poorer shape stability than birch.
Based on existing knowledge to produce shape-stable LVP, the veneers should be conditioned to uniform MC and sorted with regard to fibre orientation and loose or tight side of the veneer before bonding. End-user climates should govern the veneers’ MC and added moisture from the adhesive during the process. Straight grain veneers and symmetry should always be sought.
This review reveals several areas that need to be further clarified in order to achieve shape-stable LVP, for example the influence of adhesive, the distribution of pressure, temperature, stresses and strains during moulding and the development of numerical methods to better predict the final shape.
This paper gives a bibliographical review of the finite element methods (FEMs) applied in the analysis of wood. The added
bibliography at the end of this article contains 300 references to papers and conference proceedings on the subject that were
published between 1995 and 2004. The following topics are included: Wood as a construction material—material and mechanical
properties; wood joining and fastening; fracture mechanics problems; drying process, thermal properties; other topics. Wood
products and structures—lumber; glulam, panels, wood composites; trusses and frames; floors, roofs; bridges; other products/structures.
Two apparatuses developed in our laboratory and based on the same principle are described and tested. They semi-automatically acquire the 3D location of dots making up a network at the surface of wood-products in order to measure the resulting displacement field. The location of each dot is achieved by means of an image analyser coupled with a 2-axis robot to determine the location of the barycentre in the corresponding plan. The third co-ordinate is obtained through a displacement sensor. The accuracy of each device is studied and discussed through the analysis of repeated measurements performed on three products such as: small oak plank (15012027mm), commercial size oak plank (193015030mm), 3ply cross-laminated douglas fir panel (3903906mm). Two kinds of testing are performed: A-Test without motion of the sample to assess the accuracy in dot detection, and B-Test corresponding to a real measurement where no variation of moisture content has occurred and which theoretically leads to displacements equal to zero.
The ring width profile from pith to bark in a tree population for the basic wood used by the grading rules was discussed. Sessile Oak was taken as an example to predict the final production coming from a forest resource when two data sets were available. A linear mixed model was used to simulate two basic wooden properties which were volumetric swelling coefficient and wood density. Modeling jointly several properties with a mixed model consisted of defining a covariance structure between the random effects of the model. It was observed that the best model was the one with a moderated covariance structure between the properties.
Nowadays, under growing environmental pressure, it is necessary to find alternatives to reduce the environmental impact of chemically treated wood and wood-based panels. In respect to this necessity we have evaluated the natural durability of mixed plywood composed of naturally durable plies (cypress, cedar and chestnut heartwood) and non-durables plies (poplar, beech, cypress and cedar sapwood) against wood destroying fungi (Coriolus versicolor and Coniophora puteana) and termites (Reticulitermes santonensis). The thickness of plies (1.3 and 2.6 mm), the percentage of durable plies and non durable plies (40% and 60%), the number of plies (5 and 9) and finally the modality of integration of durable plies to non durable plies (11 different symmetrical combinations) in each mixed panel were studied as variable factors. Ultrasonic (non destructive) and indentation (semi destructive) methods were assessed to evaluate the presence and the extend of decay in pure and mixed plywood both as rapid methods in comparison with the biological assessment based on mass loss (according to the ENV12038). Shear test was also performed before and after exposition to fungi as a reference mechanical property of plywood in order to evaluate the relationship between the mass loss and the level of shear strength decrease after fungal degradation. The results obtained have shown that mixing durable plies with non durable plies improves the natural durability of plywood against fungi. Nevertheless, the shear test after fungal degradation revealed that the decrease of shear strength in mixed plywoods is closer to the decrease observed for the non durable pure plywoods than for the pure durable plywoods. These results signify that there is a localised attack in the non durable plies of the mixed plywoods. The localised mass loss of non durable plies of mixed plywoods is better correlated to the decrease of shear strength than the classical mass loss (total) of the plywoods. The plywoods the most resistant against fungi were found amongst the different mixed plywoods made of cypress sapwood and heartwood, and some models were found resistant to termite attack as well. The resistant panels to both fungi and termites had durable plies in exterior faces. The presence of intact thick sapwood plies in mixed cypress plywoods shows a protective effect (against termite attack) from the durable plies. The indentation test (the measurement of energy before and after degradation) in comparison with the ultrasonic test (the measurement of velocity before and after degradation) showed the better correlation to the mass loss. In addition the indentation test can also be used to determine the gradient of mass loss and the threshold of rupture when decay takes place in the non durable plies situated under the durable exterior plies.
The warping of laminated panels in response to moisture content changes is simulated by a computer model based on the assumption that the laminas respond elastically to the developing forces. The computer model is capable of constructing large numbers of laminated panels in which face layers are selected from a large pool of naturally variable veneer sheets. The strong effect of this natural variability on warping is clearly demonstrated. The model is applied to loblolly pine plywood and veneered furniture panels. The sometimes significant warping along the face grain of construction plywood is ascribed to the presence of cross grain, reaction wood, or juvenile wood. Graphs and nomographs illustrate the effect of variables such as veneer thickness, core thickness, core modulus of elasticity, and amount of face grain deviation.
This paper presents an investigation of the effects of moisture-induced swelling upon the out-of-plane warping of general anti-symmetric cross-ply composite laminates. The analy sis assumes linear elastic material behavior and includes non-linear geometric effects. Ap proximate solutions are obtained by assuming kinematically admissible expressions for the mid-plane displacements in terms of several generalized coordinates which are subse quently found by requiring a minimal value for the total potential energy of the laminate. It is shown that any specific anti-symmetric cross-ply lay-up will assume either a unique "saddle" shape or one of two possible approximately cylindrical shapes depending upon the in-plane dimensions of the laminate and the amount and distribution of moisture within the laminate.
The objective of this research is to study the moisture deformation of densified in technological pressing wooden composite materials. By generalizing Cosserat's couple-stress theory a structural model for determination of mid-surface strains and curvatures of composite board taking into account asymmetric structure and nonuniform distribution of moisture as well as nonlinear swelling functions of the wooden stuff and dependence of strain characteristics on moisture content is developed. On the basis of a laminate analogy the optimum flake or veneer alignment in board has been estimated for the real composite production process. According to analysis performed, the optimum alignment for hygromechanical properties may not coincide with that for mechanical properties, and for every type of loading, material with a definite structure should be used.
Numerical simulations were performed to investigate how the annual ring orientation affects the shape stability of sawn timber.
The influence of radial variations in the basic properties and the spiral grain is also studied. The knowledge obtained can
contribute to more effective use of the raw material through allowing boards having properties that would yield bad shape
stability to be sorted out. Possibilities for improving shape stability through gluing pieces of wood together are examined
as well.
The aim of this paper is to propose a consistent framework for analyzing the influence of silviculture, site quality and, to some extent, genetics on the wood production of Norway spruce from both a quantitative and a qualitative point of view. Tree and stand volume, stem taper, wood basic density, proportion of juvenile wood as well as knottiness are considered as the result of growth processes.Two complementary applications are presented. (1) An average-tree growth model which is built of several interrelated processes: site quality has an effect on height growth and hence on all other tree and stand characteristics; crown development is driven by height growth and controlled by stand density; stand basal area increment is predicted from empirical rules; tree basal area increment is then distributed along the stem. (2) A model that aims at assessing timber quality of a standing tree from usual inventory measurements such as tree age, height and diameter at breast height: growth equations are used to reconstruct the past growth of a tree and to predict its current internal structure, namely ring distribution. Both models are linked to allometric equations that estimate the characteristics of branchiness, to densitometric models that predict wood basic density from ring distribution and to a software that simulates the grading of any board located in a stem whose morphology is known in detail.The aim of these models is not to make precise quantitative predictions but to show how different pieces of knowledge of silviculturists, forest biometricians and wood scientists may be brought together in simulation software in order to help forest managers and wood industrialists to make decisions. This framework could be extended to other fast-growing coniferous species.
The coupled and highly non-linear nature of the transport equations that govern the drying process highlights the applicability of numerical simulation in this field. This work is devoted to presenting the latest version of TransPore: a unique 3-D drying model that is able to deal with the heat and mass transfer in porous media. The set of equations used in the present work is among the more comprehensive physical descriptions for the drying of a porous medium modelled at the macroscopic scale.Several simulation results are presented that depict the new possibilities offered by such a tool. In particular, the effect of the number of exchange faces is studied both for an isotropic medium (a cube of light concrete) and for an anisotropic medium (a board of wood) where the anisotropy ratios and geometrical factors are large. These simulations prove that only three spatial dimensions are able to capture the effect of the width of the medium in the case of high temperature drying. Indeed, with such conditions, the longitudinal direction is required to highlight the effect of overpressure in a strongly anisotropic medium, while both thickness and width are necessary to account for the coupling between the thermal field and the pressure driven flow.
Une banque de donnees informatisee, reproduite ici, comprend les proprietes physiques, masse volumique p, et taux d'humidite H, et les proprietes elastiques tridimensionnelles de 80 bois. Une analyse statistique a permis d'etablir des regressions multiples significatives entre ces deux groupes de parametres; elle conduit a la proposition de deux modeles previsionnels de comportement elastique tridimensionnel l'un pour les bois feuillus et l'autre pour les bois resineux, et permet de prevoir de facon satisfaisante la matrice complete des complaisances elastiques pour un bois dont la masse volumique p est connue a un taux d'humidite voisin de 12%. L'efficacite du modele bois feuillus est discutee a partir de resultats experimentaux obtenus sur le bois d'un meme hetre par neuf laboratoires
Les retraits longitudinal et tangentiel, la densité moyenne et l'épaisseur de séries radiales de placages déroulés de douglas ont été mesurés. On observe une forte corrélation positive (r >0,9) entre la densité et le retrait tangentiel. Le retrait longitudinal est lié négativement à la densité. La liaison densité-retrait est perturbée dans 2 cas particuliers : - dans le bois le plus dense, pour lequel le retrait tangentiel est inférieur au retrait attendu; on admet que ce comportement est dû aux fissures qui apparaissent préférentiellement dans le bois dense, et limitent la déformation en s'ouvrant au séchage ; - dans les zones de bois de compression qui présentent un retrait longitudinal anormalement élevé et un retrait tangentiel faible dans le bois final. L'une des méthodes expérimentées pour mesurer les retraits permet d'enregistrer à tout moment au cours du séchage l'état de la déformation longitudinale et tangentielle et d'étudier ainsi la cinétique du retrait. Le retrait longitudinal commence avant et s'achève après le retrait tangentiel. On distingue 2 phases sur la courbe temps-retrait; dans la première phase (T1), le placage perd de l'humidité sans se déformer, le retrait ne s'effectuant qu'au cours de la seconde phase (T 2). La longueur de la phase T2 augmente dans les 2 directions (L, T) avec la densité, alors que la phase T1 se réduit lorsque la densité augmente. En définitive, la durée totale du retrait tangentiel (T1+T2) est indépendante de la densité. Dans la direction longitudinale, la déformation passe en général par une phase de gonflement, mais la dimension obtenue à la fin du séchage est presque systématiquement inférieure à la dimension initiale (à l'état saturé). Study on shrinkage-structure relationships. I. Radial variation of longitudinal and tangential shrinkage on rotary cut Douglas veneers. Longitudinal and tangential shrinkage, average density and thickness have been measured on a 8 radial series of rotary cut veneers (2 × 2 cm) using the experimental procedure described in figure 1. The results are presented on radial profiles (fig 3). There was a strong positive correlation (r>0.9) between tangential shrinkage and density. The phenomenogical relationship between longitudinal shrinkage and density was negative and lower than the one mentioned above (fig 4). Two cases deviate from this density/shrinkage relationship: - zones characterised by high density present lower than expected tangential shrinkage. It is assumed that this phenomenon is due to cracks which mostly appear in late-wood and reduce the strain by opening during drying; - compression wood (marked BC in fig 3) shows abnormally high longitudinal and low tangential shrinkage in late-wood. The method of shrinkage measurement described in figure 2 can give the tangential and longitudinal deformations at any time during drying, thus allowing the study of shrinkage kinetics. Table IV shows the mean values of the kinetic parameters as defined in figure 5. Longitudinal shrinkage is induced first, and then, tangential shrinkage acts simultaneously with the longitudinal one. The longitudinal shrinkage subsequently completes the deformation. Two phases were distinguished on the shrinkage vs time curve: during the first (T1), the veneer loses humidity without warping, then shrinkage occurs during the second phase (T2). As the density increases, duration of the T2-phase increases for both directions (L, T) whereas the duration of the T1 phase is reduced. Finally, the total duration of tangential shrinkage (t∞ = T1+T2) is not dependent on the density (table IV). In the longitudinal direction, the strain usually exhibits a swelling phase but the final dimensions of a seasoned specimen are nearly always lower than a green specimen.
Microdensitometric analysis of wood samples: data computation method used at Inra-ERQB (CERD program). The procedure used at Inra for computing data derived from the microdensitometric analysis of a wood sample is described and discussed. The initial data are in the form of one or several radial density profiles. The first step of the treatment, which is the most time consuming and the only one requiring manual intervention, consists in locating the annual ring limits on each profile of each ring. The intermediate data - which can be corrected using an independent measurement of density (ratio weight/volume of the whole sample) - are then computed for each ring profile. Finally, each ring is described by several 'synthetic' values: the ring width, the mean density, the minimal and maximal density, the earlywood and latewood widths and ratios, and a set of density values (named 'quantiles') describing the shape of the ring profile. (© Inra/Elsevier, Paris.)
Résumé
Le traitement des données obtenues par l'analyse microdensitométrique d'éprouvettes en bois tel qu'il est pratiqué à l'Inra est détaillé et discuté. Les données initiales se présentent sous la forme d'un ou plusieurs profils de densité radiaux. La première étape du traitement, la plus longue et la seule à nécessiter une intervention manuelle, consiste à indiquer la position des limites de chaque cerne sur chaque profil. Des données intermédiaires - éventuellement corrigées par une mesure indépendante de densité de contrôle (rapport poids / volume de l'éprouvette) - sont ensuite calculées sur chaque profil de cerne pour servir de base à la suite du traitement. Chaque cerne est finalement décrit par des variables « synthétiques » : sa largeur, les densités moyenne, minimale et maximale, les largeurs, proportions et densités des bois initial et final ainsi qu'une série de densités moyennes de fraction de cernes (appelées « quantiles ») permettant de reconstituer le profil simplifié du cerne. (© Inra/Elsevier, Paris.)
An Introduction to MODULEF. In: MODULEF user guide no 1, INRIA
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Bowing and cupping due to imbalance in plywood
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