Letteratura scientifica selezionata sul tema "Compressed Wood"

<p>The increasing demand on wood and wood products, and the simultaneously decreasing quality of wood as a raw material leads to the increasing significance of wood·based composites such as particleboard or flakeboard. The resulting mechanical and physical properties are to the large extend dictated by the densification of the wood component. To be able to predict the density of the material, the behavior of structural elements must be known. A theory developed for rigid plastic foams was modified and applied to the deformation of wood in transverse compression. A testing procedure for high strain compression over a range of temperatures was developed. In addition, a stochastic model for prediction of high strain behavior was developed. Wood of yellow poplar (<i>Liriodendron tulipuera</i>) was used as the experimental material.</p><br>Master of Science

Wood composites industry has been growing for decades. However, wood composites have been associated with some health concerns due to the presence of formaldehyde. A promising bio-based resin Epoxidized Sucrose Soyate (ESS) was investigated as a potential primary binder in particleboards. The goal of this research was to find a strong and durable resin for wood composites. Several ESS-MDI based formulas were found that were able to match the performance criteria for particleboard.<br>North Dakota Soybean Council

As an engineered wood product, glued laminated timber (Glulam) is produced to minimise several natural weaknesses such as knots and non-uniform strength in order to enhance its stiffness and load carrying capacity. Many reinforcement methods have been tested, with promising results in increasing the strength and stiffness of beam. Recently, with increasing concern on environmental protection there is a growing trend in construction to use as many natural materials as possible. Therefore, conventional types of reinforcement using metals and/or synthetic materials do not fit well with this trend. A new approach to strengthen Glulam beams using pure natural materials has been established through this research. Glulam beams were strengthened by inserting compressed wood (CW) blocks into the pre-cut rectangular holes on the top of the beams. This practice was to make use of moisture-dependent swelling nature of the compressed wood. As a result, a pre-camber was produced in the mid-span of the beam reinforced. Significant initial tensile and compressive stresses were generated at both the top and the bottom extreme fibres of the beam, respectively. Material tests have been done to investigate material properties of compressed Japanese cedar with different compression ratios. Structural tests were undertaken for short and long Glulam beams strengthened by CW blocks after gained the pre-camber. 3-D finite element models have also been developed by using commercial code ABAQUS to simulate the pre-stressing behaviour of Glulam beams reinforced using compressed wood blocks. The finite element models were used to simulate the structural behaviour of the pre-stressed beams subjected to subsequent destructive bending tests. All computer models developed were validated against corresponding experimental results of the free moisture-dependent swelling, pre-stressing processes and the destructive tests for both the short and long beams. Reasonably good correlation was obtained in terms of the free swelling, the pre-camber, initial stress state of the Glulam beams reinforced and load-deflection relationships. Using validated computer models, parametric studies were further carried out to provide design information for such a reinforcing technique. The results have indicated that the reinforcing techniques using compressed wood blocks are very effective means to enhance the initial stiffness, bending strength and load carrying capacity of Glulam beams. The technology is ready to be applied in practice.

Cette thèse présente une étude expérimentale et numérique sur les aptitudes en service en ce qui concerne le comportement vibratoire de planchers réalisés en bois d’ingénierie sans colle (AFEWP), à savoir les poutres en bois lamellé sans adhésif (AFLB) et les panneaux en bois lamellé-croisé sans adhésif (AFCLT), assemblés par des tourillons en bois densifié (CWD). Les analyses modales expérimentales ont été réalisées en conditions libres-libre à l'aide d'un marteau instrumenté. Les fréquences naturelles, les modes propres et les coefficients d'amortissement ont été évalués expérimentalement. De plus, des poutres en bois lamellé-collé similaires ont été fabriquées et testées à des fins de comparaison. Un modèle EF 3D a été développé et validé par comparaison avec des données expérimentales, puis utilisé pour prédire le comportement vibratoire d'un système de plancher à grande échelle réalisé avec un panneau AFCLT mesurant 4,5 m x 5,5 m. Une étude paramétrique a été réalisée en utilisant le modèle EF pour identifier les paramètres qui influent sur la rigidité et les fréquences propres du plancher. Les résultats obtenus ont été analysés au regard des critères de conception exigés par Eurocode 5 en ce qui concerne le confort vibratoire des planchers bois. Un modèle EF simplifié a ensuite été développé pour réduire les coûts de calcul. Le niveau de variabilité des résultats des AFEWP a été également étudié et discuté. La variabilité numérique des fréquences des AFEWP a été étudiée en utilisant la méthode MSP (Modal Stability Procedure). Les résultats de la méthode MSP ont d'abord été comparés aux résultats EF dans le cas nominal. Les résultats statistiques (valeur moyenne, écart type, coefficient de variation et distribution) obtenus par la méthode MSP ont été confrontés aux résultats de simulation directe de Monte Carlo ainsi qu’aux données expérimentales. Une estimation rapide des erreurs entre la méthode MSP et la simulation directe de Monte Carlo a été développée. Enfin, la variabilité (moyenne et écart-type des fréquences) du comportement vibratoire du système de plancher à grande échelle a été étudiée en utilisant la méthode MSP<br>This study presents an experimental and numerical investigation on the vibrational serviceability performance of novel adhesive free engineered wood products (AFEWPs), namely adhesive free laminated timber beams (AFLB) and adhesive free cross-laminated timber panels (AFCLT), assembled through thermo-mechanically compressed wood dowels (CWD). The experimental modal analyses were carried out under free-free conditions using a hammer impact. Natural frequencies, mode shapes, and damping ratio were assessed experimentally. In addition, similar glued timber beams (conventional glulam) were manufactured and tested for comparison purpose. A 3D FE model was developed and validated by comparison against experimental data and then used to predict the vibrational behavior of a realistic flooring system made with AFCLT panel measuring 4.5 m x 5.5 m. A parametric study was performed on the FE model to maximize the floor stiffness. The predicted FE results were discussed with regard to the Eurocode 5 vibrational serviceability design requirements showing acceptable vibrational performance. A simplified FE model was then developed to reduce computational cost. The variability level of the results for the AFEWPs was also studied and discussed. The numerical variability of frequencies of the AFEWPs was investigated based on the development of the Modal Stability Procedure (MSP). The MSP result was first compared with the FE result in the nominal case. Then, the statistic results (mean value, standard deviation, coefficient of variation and distribution) obtained from the MSP were compared with the results from the classical method (direct Monte Carlo simulation) and experimental results. A quick error estimation between the MSP and the direct Monte Carlo simulation was developed. Finally, the mean and standard deviation of the frequencies of the realistic AFCLT flooring system were predicted by the MSP

Large scale self-heating of wood pellets in storage is a major concern worldwide during long term storage due to the potential for fire with safety concerns for both personnel and plant. Additionally, any adverse publicity resulting from an incident involving biomass is likely to result in negative effects to the biomass industry as a whole. Self-heating could lead to serious accidental fires, causing enormous damage and danger to workers. The aim of the research in this thesis was to provide underpinning data and understanding in order to implement safe storage of wood pellets at Drax Power Ltd. Thus, the self-heating rates at different temperatures, and under different levels of inerting, were experimentally determined, and the thermal properties were measured for wood pellets produced for Drax Power Ltd. In addition, the thesis covers measurements at three scales; laboratory, temporary storage of approximately 40,000 tonnes through to largescale storage at approximately 280,000 tonnes spread equally over 4 domes… the largest storage of pellets ever investigated. The factors such as moisture content, pellet age and environment temperature were investigated and their impacts on the self-heating process were analysed. Moisture content has a significant effect on effective on self-heating but ventilation of the store was found to be more critical. Pellets age and environment temperature are two major factors impacting the self-heating and off-gassing process. The self-heating rate is significantly increased at higher a temperature and eventually will lead to a thermal runaway when the ambient temperature is high enough. Based on all measured properties, a self-heating model was developed to predict the self-heating process and thermal runaway in large wood pellet dome of ~120,000m3. This work contributed to the inerting of nitrogen gas to maintain an atmospheric (and therefore assumed within biomass) oxygen content less than 10% and monitoring protocols with the focus on carbon monoxide along with the understanding of temperature tracking for the large-scale dome storage of wood pellets, which is now used by Drax Power Ltd. Safety of all personnel working with biomass is paramount concern for Drax Power Ltd., which this thesis contributed to personal exposure monitoring and standard setting for gas exposure, mainly carbon monoxide and dioxide along with dust exposure. The handling of pellets through the supply chain and on site can cause the pellets to degrade and fine wood particles and dust is present within bulk pellets. Any release of wood dust into the atmosphere can pose a risk to health. Wood dust can cause serious health problems. Wood dust is a recognised respiratory irritant, sensitizer, asthmagen and for a limited number of species, a potential carcinogen (hard woods). Contact with wood dust can also lead to skin sensitisation and dermatitis. Drax Power Ltd, recognises it’s duties under Health and Safety and is committed to minimising the risk to health from wood dust in its premises. Both hardwood and softwoods dusts have a Workplace Exposure Limit (WEL) of 5mg/m3, which must not be exceeded. These are limits placed on the amount of dust in the air over an eight–hour shift. However the Control of Substance Hazardous to Health Regulations 2002 (as amended) requires exposure to wood dust to be “As low as reasonably practicable” (ALARP). Drax Power Ltd will manage exposure to wood dust to as low as reasonably practicable. The primary means of preventing exposure, is by containment of wood dust within the materials handling and processing plant. Dust control equipment such as Local Exhaust Equipment (LEV), will be used in conjunction with handling and processing plant to ensure that dust levels are kept to as low as is reasonably practicable. For certain operational and maintenance tasks it is necessary to use additional procedural controls to ensure that the risk of exposure to wood dust are managed effectively.

Este trabalho apresenta um estudo teórico e experimental a respeito da solicitação de compressão normal às fibras da madeira, para a qual não é previsto ensaio para caracterização, atualmente, na NBR-6230 (Ensaios físicos e mecânicos de madeiras). Foram abordados aspectos referentes à geometria do corpo-de-prova, velocidade de aplicação de carga, direção do carregamento em relação aos anéis de crescimento e condições de umidade. Com base nos resultados obtidos, é proposto método de ensaio para caracterização da madeira solicitada por compressão normal às fibras.<br>This work presents an experimental study about compression perpendicular to the grain in wood, for which characterization test is not indicated by NBR-6230 (Physical and mechanical test in wood). Several points were discussed: specimen geometry, speed of testing, direction of loading referred to the growth rings and moisture conditions. Based on the experimental data, a testing procedure is proposed in order to be employed in wood characterization in compression perpendicular to the grain.

Wir beschäftigen uns mit Problemen der algorithmischen Gruppentheorie und untersuchen dabei die Komplexität von komprimierten Versionen des Wortproblems und des Konjugationsproblems für endlich erzeugte Gruppen. Das Wortproblem fragt für eine feste, endlich erzeugte Gruppe ob ein gegebenes Wort über der Erzeugermenge das neutrale Element der Gruppe repräsentiert. Wir betrachten das gegebene Wort jedoch in einer komprimierten Form, als Straight-line Program (SLP) und untersuchen die Komplexität dieses Problems, das wir \'komprimiertes Wortproblem\' nennen. SLPs sind kontextfreie Grammatiken, die genau einen String erzeugen. Die Eingabegröße ist dabei stets die Größe des gegebenen SLPs. Eine Hauptmotivation ist dabei, dass für eine feste endlich erzeugte Gruppe das Wortproblem ihrer Automorphismengruppe durch eine Turingmaschine in Polynomialzeit auf das komprimierte Wortproblem der Gruppe selbst reduzierbar ist. Wir untersuchen das komprimierte Wortproblem für die verbreiteten Gruppenerweiterungen HNN-Erweiterungen (amalgamierte Produkte und Graphprodukte) und können zeigen, dass sich Instanzen des komprimierten Wortproblems von einer Turingmaschine in Polynomialzeit auf Instanzen des komprimierten Wortproblems der Basisgruppe (respektive Basisgruppen und Knotengruppen) reduzieren lassen. Weiterhin zeigen wir, dass das komprimierte Wortproblem für endlich erzeugte nilpotente Gruppen von einer Turingmaschine in Polynomialzeit entscheidbar ist. Wir betrachten außerdem eine komprimierte Variante des Konjugationsproblems. Das unkomprimierte Konjugationsproblem fragt für zwei gegebene Wörter über den Erzeugern einer festen endlich erzeugten Gruppe, ob sie in dieser Gruppe konjugiert sind. Beim komprimierten Konjugationsproblem besteht die Eingabe aus zwei SLPs und es wird gefragt, ob die beiden Wörter die von den SLPs erzeugt werden in der Gruppe konjugierte Elemente präsentieren. Wir konnten zeigen, dass sich das komprimierte Konjugationsproblem für Graphgruppen in Polynomialzeit entscheiden lässt. Weiterhin haben wir das Wortproblem der äußeren Automorphismengruppen von Graphprodukten endlich erzeugter Gruppen untersucht. Durch den engen Zusammenhang des komprimierten Konjugationsproblems einer Gruppe mit dem Wortproblem der äußeren Automorphismengruppe konnten wir zeigen, dass sich das Wortproblem der äußeren Automorphismengruppe eines Graphprodukts von endlich erzeugten Gruppen durch eine Turingmaschine in Polynomialzeit auf Instanzen von simultanen komprimierten Konjugationsproblemen der Knotengruppen und Instanzen von komprimierten Wortproblemen der Knotengruppen reduzieren lässt. Als Anwendung gelten obige Resultate auch für right-angled Coxetergruppen und Graphgruppen, da beide spezielle Graphprodukte sind. So folgt beispielsweise, dass das komprimierte Wortproblem einer right-angled Coxetergruppe in Polynomialzeit entscheidbar ist.

<p>Hårdgörning av trä syftar till att göra materialet hårdare. Detta går att åstadkomma genom komprimering, det komprimerade materialet tenderar dock att återgå mot sin ursprungsform då materialet utsätts för fukt om ingen låsning sker. Låsningen kan ske mekaniskt i en treskiktskonstruktion som motverkar träets fuktrörelser. Dessa skivor har dock visat sig vara instabila och tidigare försök har uppvisat deformationer i form av kupning. Denna undersökning syftar till att utreda förutsättningarna för att använda komprimerad asp samt att ta fram underlag för framtagning av en kommersiell produkt.</p><p>Praktiska tester har genomförts för att studera hårdhet, återfjädring och densitet hos det komprimerade virket. Vidare har treskiktsskivor tagits fram för att testa om det går att få en stabil konstruktion genom att variera tjockleken på spärrskiktet. Även skillnader i acklimatiseringstid och fuktkvotens inverkan för virkets återfjädring har studerats.</p><p>Resultaten visar på att aspens hårdhet efter komprimering blir i samma nivå som bok och ask, en låg fuktkkvot vid komprimering av materialet ger en låg återfjädring, komprimerat material har en längre acklimatiseringstid till jämviktsfuktkvot och materialet går att låsa tvärs fiberriktningen i en treskiktskonstruktion.</p><br><p>To make wood harder it can be densified. This can be achieved by compression, the compressed material tends to return to the original shape when it is exposed to moisture fluxations and if no form of fixation occurs. The fixation can be done mechanically in a three-layer panel construction which reduce movements in the timber. These panels have been found to be unstable and previous attempts have shown deformations in form of cupping. This study aims to evaluate the conditions for using compressed aspen wood, and to provide a basis for development of a commercial product.</p><p>Practical tests have been conducted to study hardness, springback and density of the compressed wood. In addition, three-layer panels have been developed to test whether it is possible to have a stable construction by varying the thickness of the buttom layer. Differences in acclimatisation time and the moisture contents effect on the springback has aslo been studied.</p><p>The results show that the hardness of aspen wood after compression is in equal levels with beech and ashes, a low moisture contet provides a low springback, compressed timber have a longer time for acclimatisation and it is possible to fixate the material in the direction across the grain in a three-layer panel construction.</p>